| 1 | libpng.txt - A description on how to use and modify libpng |
| 2 | |
| 3 | libpng version 1.0.3 - January 14, 1999 |
| 4 | Updated and distributed by Glenn Randers-Pehrson |
| 5 | <randeg@alumni.rpi.edu> |
| 6 | Copyright (c) 1998, 1999 Glenn Randers-Pehrson |
| 7 | For conditions of distribution and use, see copyright |
| 8 | notice in png.h. |
| 9 | |
| 10 | based on: |
| 11 | |
| 12 | libpng 1.0 beta 6 version 0.96 May 28, 1997 |
| 13 | Updated and distributed by Andreas Dilger |
| 14 | Copyright (c) 1996, 1997 Andreas Dilger |
| 15 | |
| 16 | libpng 1.0 beta 2 - version 0.88 January 26, 1996 |
| 17 | For conditions of distribution and use, see copyright |
| 18 | notice in png.h. Copyright (c) 1995, 1996 Guy Eric |
| 19 | Schalnat, Group 42, Inc. |
| 20 | |
| 21 | Updated/rewritten per request in the libpng FAQ |
| 22 | Copyright (c) 1995 Frank J. T. Wojcik |
| 23 | December 18, 1995 && January 20, 1996 |
| 24 | |
| 25 | I. Introduction |
| 26 | |
| 27 | This file describes how to use and modify the PNG reference library |
| 28 | (known as libpng) for your own use. There are five sections to this |
| 29 | file: introduction, structures, reading, writing, and modification and |
| 30 | configuration notes for various special platforms. In addition to this |
| 31 | file, example.c is a good starting point for using the library, as |
| 32 | it is heavily commented and should include everything most people |
| 33 | will need. We assume that libpng is already installed; see the |
| 34 | INSTALL file for instructions on how to install libpng. |
| 35 | |
| 36 | Libpng was written as a companion to the PNG specification, as a way |
| 37 | of reducing the amount of time and effort it takes to support the PNG |
| 38 | file format in application programs. The PNG specification is available |
| 39 | as RFC 2083 <ftp://ftp.uu.net/graphics/png/documents/> and as a |
| 40 | W3C Recommendation <http://www.w3.org/TR/REC.png.html>. Some |
| 41 | additional chunks are described in the special-purpose public chunks |
| 42 | documents at <ftp://ftp.uu.net/graphics/png/documents/>. Other information |
| 43 | about PNG, and the latest version of libpng, can be found at the PNG home |
| 44 | page, <http://www.cdrom.com/pub/png/>. |
| 45 | |
| 46 | Most users will not have to modify the library significantly; advanced |
| 47 | users may want to modify it more. All attempts were made to make it as |
| 48 | complete as possible, while keeping the code easy to understand. |
| 49 | Currently, this library only supports C. Support for other languages |
| 50 | is being considered. |
| 51 | |
| 52 | Libpng has been designed to handle multiple sessions at one time, |
| 53 | to be easily modifiable, to be portable to the vast majority of |
| 54 | machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy |
| 55 | to use. The ultimate goal of libpng is to promote the acceptance of |
| 56 | the PNG file format in whatever way possible. While there is still |
| 57 | work to be done (see the TODO file), libpng should cover the |
| 58 | majority of the needs of its users. |
| 59 | |
| 60 | Libpng uses zlib for its compression and decompression of PNG files. |
| 61 | Further information about zlib, and the latest version of zlib, can |
| 62 | be found at the zlib home page, <http://www.cdrom.com/pub/infozip/zlib/>. |
| 63 | The zlib compression utility is a general purpose utility that is |
| 64 | useful for more than PNG files, and can be used without libpng. |
| 65 | See the documentation delivered with zlib for more details. |
| 66 | You can usually find the source files for the zlib utility wherever you |
| 67 | find the libpng source files. |
| 68 | |
| 69 | Libpng is thread safe, provided the threads are using different |
| 70 | instances of the structures. Each thread should have its own |
| 71 | png_struct and png_info instances, and thus its own image. |
| 72 | Libpng does not protect itself against two threads using the |
| 73 | same instance of a structure. |
| 74 | |
| 75 | |
| 76 | II. Structures |
| 77 | |
| 78 | There are two main structures that are important to libpng, png_struct |
| 79 | and png_info. The first, png_struct, is an internal structure that |
| 80 | will not, for the most part, be used by a user except as the first |
| 81 | variable passed to every libpng function call. |
| 82 | |
| 83 | The png_info structure is designed to provide information about the |
| 84 | PNG file. At one time, the fields of png_info were intended to be |
| 85 | directly accessible to the user. However, this tended to cause problems |
| 86 | with applications using dynamically loaded libraries, and as a result |
| 87 | a set of interface functions for png_info was developed. The fields |
| 88 | of png_info are still available for older applications, but it is |
| 89 | suggested that applications use the new interfaces if at all possible. |
| 90 | |
| 91 | The png.h header file is an invaluable reference for programming with libpng. |
| 92 | And while I'm on the topic, make sure you include the libpng header file: |
| 93 | |
| 94 | #include <png.h> |
| 95 | |
| 96 | III. Reading |
| 97 | |
| 98 | Reading PNG files: |
| 99 | |
| 100 | We'll now walk you through the possible functions to call when reading |
| 101 | in a PNG file, briefly explaining the syntax and purpose of each one. |
| 102 | See example.c and png.h for more detail. While Progressive reading |
| 103 | is covered in the next section, you will still need some of the |
| 104 | functions discussed in this section to read a PNG file. |
| 105 | |
| 106 | You will want to do the I/O initialization(*) before you get into libpng, |
| 107 | so if it doesn't work, you don't have much to undo. Of course, you |
| 108 | will also want to insure that you are, in fact, dealing with a PNG |
| 109 | file. Libpng provides a simple check to see if a file is a PNG file. |
| 110 | To use it, pass in the first 1 to 8 bytes of the file, and it will |
| 111 | return true or false (1 or 0) depending on whether the bytes could be |
| 112 | part of a PNG file. Of course, the more bytes you pass in, the |
| 113 | greater the accuracy of the prediction. |
| 114 | |
| 115 | If you are intending to keep the file pointer open for use in libpng, |
| 116 | you must ensure you don't read more than 8 bytes from the beginning |
| 117 | of the file, and you also have to make a call to png_set_sig_bytes_read() |
| 118 | with the number of bytes you read from the beginning. Libpng will |
| 119 | then only check the bytes (if any) that your program didn't read. |
| 120 | |
| 121 | (*): If you are not using the standard I/O functions, you will need |
| 122 | to replace them with custom functions. See the discussion under |
| 123 | Customizing libpng. |
| 124 | |
| 125 | |
| 126 | FILE *fp = fopen(file_name, "rb"); |
| 127 | if (!fp) |
| 128 | { |
| 129 | return; |
| 130 | } |
| 131 | fread(header, 1, number, fp); |
| 132 | is_png = !png_sig_cmp(header, 0, number); |
| 133 | if (!is_png) |
| 134 | { |
| 135 | return; |
| 136 | } |
| 137 | |
| 138 | |
| 139 | Next, png_struct and png_info need to be allocated and initialized. In |
| 140 | order to ensure that the size of these structures is correct even with a |
| 141 | dynamically linked libpng, there are functions to initialize and |
| 142 | allocate the structures. We also pass the library version, optional |
| 143 | pointers to error handling functions, and a pointer to a data struct for |
| 144 | use by the error functions, if necessary (the pointer and functions can |
| 145 | be NULL if the default error handlers are to be used). See the section |
| 146 | on Changes to Libpng below regarding the old initialization functions. |
| 147 | |
| 148 | png_structp png_ptr = png_create_read_struct |
| 149 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
| 150 | user_error_fn, user_warning_fn); |
| 151 | if (!png_ptr) |
| 152 | return; |
| 153 | |
| 154 | png_infop info_ptr = png_create_info_struct(png_ptr); |
| 155 | if (!info_ptr) |
| 156 | { |
| 157 | png_destroy_read_struct(&png_ptr, |
| 158 | (png_infopp)NULL, (png_infopp)NULL); |
| 159 | return; |
| 160 | } |
| 161 | |
| 162 | png_infop end_info = png_create_info_struct(png_ptr); |
| 163 | if (!end_info) |
| 164 | { |
| 165 | png_destroy_read_struct(&png_ptr, &info_ptr, |
| 166 | (png_infopp)NULL); |
| 167 | return; |
| 168 | } |
| 169 | |
| 170 | If you want to use your own memory allocation routines, |
| 171 | define PNG_USER_MEM_SUPPORTED and use |
| 172 | png_create_read_struct_2() instead of png_create_read_struct(): |
| 173 | |
| 174 | png_structp png_ptr = png_create_read_struct_2 |
| 175 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
| 176 | user_error_fn, user_warning_fn, (png_voidp) |
| 177 | user_mem_ptr, user_malloc_fn, user_free_fn); |
| 178 | |
| 179 | The error handling routines passed to png_create_read_struct() |
| 180 | and the memory alloc/free routines passed to png_create_struct_2() |
| 181 | are only necessary if you are not using the libpng supplied error |
| 182 | handling and memory alloc/free functions. |
| 183 | |
| 184 | When libpng encounters an error, it expects to longjmp back |
| 185 | to your routine. Therefore, you will need to call setjmp and pass |
| 186 | your png_ptr->jmpbuf. If you read the file from different |
| 187 | routines, you will need to update the jmpbuf field every time you enter |
| 188 | a new routine that will call a png_ function. |
| 189 | |
| 190 | See your documentation of setjmp/longjmp for your compiler for more |
| 191 | handling in the Customizing Libpng section below for more information on |
| 192 | the libpng error handling. If an error occurs, and libpng longjmp's |
| 193 | back to your setjmp, you will want to call png_destroy_read_struct() to |
| 194 | free any memory. |
| 195 | |
| 196 | if (setjmp(png_ptr->jmpbuf)) |
| 197 | { |
| 198 | png_destroy_read_struct(&png_ptr, &info_ptr, |
| 199 | &end_info); |
| 200 | fclose(fp); |
| 201 | return; |
| 202 | } |
| 203 | |
| 204 | Now you need to set up the input code. The default for libpng is to |
| 205 | use the C function fread(). If you use this, you will need to pass a |
| 206 | valid FILE * in the function png_init_io(). Be sure that the file is |
| 207 | opened in binary mode. If you wish to handle reading data in another |
| 208 | way, you need not call the png_init_io() function, but you must then |
| 209 | implement the libpng I/O methods discussed in the Customizing Libpng |
| 210 | section below. |
| 211 | |
| 212 | png_init_io(png_ptr, fp); |
| 213 | |
| 214 | If you had previously opened the file and read any of the signature from |
| 215 | the beginning in order to see if this was a PNG file, you need to let |
| 216 | libpng know that there are some bytes missing from the start of the file. |
| 217 | |
| 218 | png_set_sig_bytes(png_ptr, number); |
| 219 | |
| 220 | At this point, you can set up a callback function that will be |
| 221 | called after each row has been read, which you can use to control |
| 222 | a progress meter or the like. It's demonstrated in pngtest.c. |
| 223 | You must supply a function |
| 224 | |
| 225 | void read_row_callback(png_ptr, png_uint_32 row, int pass); |
| 226 | { |
| 227 | /* put your code here */ |
| 228 | } |
| 229 | |
| 230 | (You can give it another name that you like instead of "read_row_callback") |
| 231 | |
| 232 | To inform libpng about your function, use |
| 233 | |
| 234 | png_set_read_status_fn(png_ptr, read_row_callback); |
| 235 | |
| 236 | In PNG files, the alpha channel in an image is the level of opacity. |
| 237 | If you need the alpha channel in an image to be the level of transparency |
| 238 | instead of opacity, you can invert the alpha channel (or the tRNS chunk |
| 239 | data) after it's read, so that 0 is fully opaque and 255 (in 8-bit or |
| 240 | paletted images) or 65535 (in 16-bit images) is fully transparent, with |
| 241 | |
| 242 | png_set_invert_alpha(png_ptr); |
| 243 | |
| 244 | This has to appear here rather than later with the other transformations |
| 245 | because the tRNS chunk data must be modified in the case of paletted images. |
| 246 | If your image is not a paletted image, the tRNS data (which in such cases |
| 247 | represents a single color to be rendered as transparent) won't be changed. |
| 248 | |
| 249 | Finally, you can write your own transformation function if none of |
| 250 | the existing ones meets your needs. This is done by setting a callback |
| 251 | with |
| 252 | |
| 253 | png_set_read_user_transform_fn(png_ptr, |
| 254 | read_transform_fn); |
| 255 | |
| 256 | You must supply the function |
| 257 | |
| 258 | void read_transform_fn(png_ptr ptr, row_info_ptr |
| 259 | row_info, png_bytep data) |
| 260 | |
| 261 | See pngtest.c for a working example. Your function will be called |
| 262 | after all of the other transformations have been processed. |
| 263 | |
| 264 | You are now ready to read all the file information up to the actual |
| 265 | image data. You do this with a call to png_read_info(). |
| 266 | |
| 267 | png_read_info(png_ptr, info_ptr); |
| 268 | |
| 269 | Functions are used to get the information from the info_ptr: |
| 270 | |
| 271 | png_get_IHDR(png_ptr, info_ptr, &width, &height, |
| 272 | &bit_depth, &color_type, &interlace_type, |
| 273 | &compression_type, &filter_type); |
| 274 | |
| 275 | width - holds the width of the image |
| 276 | in pixels (up to 2^31). |
| 277 | height - holds the height of the image |
| 278 | in pixels (up to 2^31). |
| 279 | bit_depth - holds the bit depth of one of the |
| 280 | image channels. (valid values are |
| 281 | 1, 2, 4, 8, 16 and depend also on |
| 282 | the color_type. See also |
| 283 | significant bits (sBIT) below). |
| 284 | color_type - describes which color/alpha channels |
| 285 | are present. |
| 286 | PNG_COLOR_TYPE_GRAY |
| 287 | (bit depths 1, 2, 4, 8, 16) |
| 288 | PNG_COLOR_TYPE_GRAY_ALPHA |
| 289 | (bit depths 8, 16) |
| 290 | PNG_COLOR_TYPE_PALETTE |
| 291 | (bit depths 1, 2, 4, 8) |
| 292 | PNG_COLOR_TYPE_RGB |
| 293 | (bit_depths 8, 16) |
| 294 | PNG_COLOR_TYPE_RGB_ALPHA |
| 295 | (bit_depths 8, 16) |
| 296 | |
| 297 | PNG_COLOR_MASK_PALETTE |
| 298 | PNG_COLOR_MASK_COLOR |
| 299 | PNG_COLOR_MASK_ALPHA |
| 300 | |
| 301 | filter_type - (must be PNG_FILTER_TYPE_BASE |
| 302 | for PNG 1.0) |
| 303 | compression_type - (must be PNG_COMPRESSION_TYPE_BASE |
| 304 | for PNG 1.0) |
| 305 | interlace_type - (PNG_INTERLACE_NONE or |
| 306 | PNG_INTERLACE_ADAM7) |
| 307 | Any or all of interlace_type, compression_type, of |
| 308 | filter_type can be |
| 309 | NULL if you are not interested in their values. |
| 310 | |
| 311 | channels = png_get_channels(png_ptr, info_ptr); |
| 312 | channels - number of channels of info for the |
| 313 | color type (valid values are 1 (GRAY, |
| 314 | PALETTE), 2 (GRAY_ALPHA), 3 (RGB), |
| 315 | 4 (RGB_ALPHA or RGB + filler byte)) |
| 316 | rowbytes = png_get_rowbytes(png_ptr, info_ptr); |
| 317 | rowbytes - number of bytes needed to hold a row |
| 318 | |
| 319 | signature = png_get_signature(png_ptr, info_ptr); |
| 320 | signature - holds the signature read from the |
| 321 | file (if any). The data is kept in |
| 322 | the same offset it would be if the |
| 323 | whole signature were read (i.e. if an |
| 324 | application had already read in 4 |
| 325 | bytes of signature before starting |
| 326 | libpng, the remaining 4 bytes would |
| 327 | be in signature[4] through signature[7] |
| 328 | (see png_set_sig_bytes())). |
| 329 | |
| 330 | |
| 331 | width = png_get_image_width(png_ptr, |
| 332 | info_ptr); |
| 333 | height = png_get_image_height(png_ptr, |
| 334 | info_ptr); |
| 335 | bit_depth = png_get_bit_depth(png_ptr, |
| 336 | info_ptr); |
| 337 | color_type = png_get_color_type(png_ptr, |
| 338 | info_ptr); |
| 339 | filter_type = png_get_filter_type(png_ptr, |
| 340 | info_ptr); |
| 341 | compression_type = png_get_compression_type(png_ptr, |
| 342 | info_ptr); |
| 343 | interlace_type = png_get_interlace_type(png_ptr, |
| 344 | info_ptr); |
| 345 | |
| 346 | |
| 347 | These are also important, but their validity depends on whether the chunk |
| 348 | has been read. The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and |
| 349 | png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the |
| 350 | data has been read, or zero if it is missing. The parameters to the |
| 351 | png_get_<chunk> are set directly if they are simple data types, or a pointer |
| 352 | into the info_ptr is returned for any complex types. |
| 353 | |
| 354 | png_get_PLTE(png_ptr, info_ptr, &palette, |
| 355 | &num_palette); |
| 356 | palette - the palette for the file |
| 357 | (array of png_color) |
| 358 | num_palette - number of entries in the palette |
| 359 | |
| 360 | png_get_gAMA(png_ptr, info_ptr, &gamma); |
| 361 | gamma - the gamma the file is written |
| 362 | at (PNG_INFO_gAMA) |
| 363 | |
| 364 | png_get_sRGB(png_ptr, info_ptr, &srgb_intent); |
| 365 | srgb_intent - the rendering intent (PNG_INFO_sRGB) |
| 366 | The presence of the sRGB chunk |
| 367 | means that the pixel data is in the |
| 368 | sRGB color space. This chunk also |
| 369 | implies specific values of gAMA and |
| 370 | cHRM. |
| 371 | |
| 372 | png_get_sBIT(png_ptr, info_ptr, &sig_bit); |
| 373 | sig_bit - the number of significant bits for |
| 374 | (PNG_INFO_sBIT) each of the gray, |
| 375 | red, green, and blue channels, |
| 376 | whichever are appropriate for the |
| 377 | given color type (png_color_16) |
| 378 | |
| 379 | png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans, |
| 380 | &trans_values); |
| 381 | trans - array of transparent entries for |
| 382 | palette (PNG_INFO_tRNS) |
| 383 | trans_values - transparent pixel for non-paletted |
| 384 | images (PNG_INFO_tRNS) |
| 385 | num_trans - number of transparent entries |
| 386 | (PNG_INFO_tRNS) |
| 387 | |
| 388 | png_get_hIST(png_ptr, info_ptr, &hist); |
| 389 | (PNG_INFO_hIST) |
| 390 | hist - histogram of palette (array of |
| 391 | png_color_16) |
| 392 | |
| 393 | png_get_tIME(png_ptr, info_ptr, &mod_time); |
| 394 | mod_time - time image was last modified |
| 395 | (PNG_VALID_tIME) |
| 396 | |
| 397 | png_get_bKGD(png_ptr, info_ptr, &background); |
| 398 | background - background color (PNG_VALID_bKGD) |
| 399 | |
| 400 | num_text = png_get_text(png_ptr, info_ptr, &text_ptr); |
| 401 | text_ptr - array of png_text holding image |
| 402 | comments |
| 403 | text_ptr[i]->key - keyword for comment. |
| 404 | text_ptr[i]->text - text comments for current |
| 405 | keyword. |
| 406 | text_ptr[i]->compression - type of compression used |
| 407 | on "text" PNG_TEXT_COMPRESSION_NONE |
| 408 | or PNG_TEXT_COMPRESSION_zTXt |
| 409 | num_text - number of comments |
| 410 | |
| 411 | png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y, |
| 412 | &unit_type); |
| 413 | offset_x - positive offset from the left edge |
| 414 | of the screen |
| 415 | offset_y - positive offset from the top edge |
| 416 | of the screen |
| 417 | unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER |
| 418 | |
| 419 | png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y, |
| 420 | &unit_type); |
| 421 | res_x - pixels/unit physical resolution in |
| 422 | x direction |
| 423 | res_y - pixels/unit physical resolution in |
| 424 | x direction |
| 425 | unit_type - PNG_RESOLUTION_UNKNOWN, |
| 426 | PNG_RESOLUTION_METER |
| 427 | |
| 428 | The data from the pHYs chunk can be retrieved in several convenient |
| 429 | forms: |
| 430 | |
| 431 | res_x = png_get_x_pixels_per_meter(png_ptr, |
| 432 | info_ptr) |
| 433 | res_y = png_get_y_pixels_per_meter(png_ptr, |
| 434 | info_ptr) |
| 435 | res_x_and_y = png_get_pixels_per_meter(png_ptr, |
| 436 | info_ptr) |
| 437 | aspect_ratio = png_get_pixel_aspect_ratio(png_ptr, |
| 438 | info_ptr) |
| 439 | |
| 440 | (Each of these returns 0 [signifying "unknown"] if |
| 441 | the data is not present or if res_x is 0; |
| 442 | res_x_and_y is 0 if res_x != res_y) |
| 443 | |
| 444 | For more information, see the png_info definition in png.h and the |
| 445 | PNG specification for chunk contents. Be careful with trusting |
| 446 | rowbytes, as some of the transformations could increase the space |
| 447 | needed to hold a row (expand, filler, gray_to_rgb, etc.). |
| 448 | See png_read_update_info(), below. |
| 449 | |
| 450 | A quick word about text_ptr and num_text. PNG stores comments in |
| 451 | keyword/text pairs, one pair per chunk, with no limit on the number |
| 452 | of text chunks, and a 2^31 byte limit on their size. While there are |
| 453 | suggested keywords, there is no requirement to restrict the use to these |
| 454 | strings. It is strongly suggested that keywords and text be sensible |
| 455 | to humans (that's the point), so don't use abbreviations. Non-printing |
| 456 | symbols are not allowed. See the PNG specification for more details. |
| 457 | There is also no requirement to have text after the keyword. |
| 458 | |
| 459 | Keywords should be limited to 79 Latin-1 characters without leading or |
| 460 | trailing spaces, but non-consecutive spaces are allowed within the |
| 461 | keyword. It is possible to have the same keyword any number of times. |
| 462 | The text_ptr is an array of png_text structures, each holding pointer |
| 463 | to a keyword and a pointer to a text string. Only the text string may |
| 464 | be null. The keyword/text pairs are put into the array in the order |
| 465 | that they are received. However, some or all of the text chunks may be |
| 466 | after the image, so, to make sure you have read all the text chunks, |
| 467 | don't mess with these until after you read the stuff after the image. |
| 468 | This will be mentioned again below in the discussion that goes with |
| 469 | png_read_end(). |
| 470 | |
| 471 | After you've read the header information, you can set up the library |
| 472 | to handle any special transformations of the image data. The various |
| 473 | ways to transform the data will be described in the order that they |
| 474 | should occur. This is important, as some of these change the color |
| 475 | type and/or bit depth of the data, and some others only work on |
| 476 | certain color types and bit depths. Even though each transformation |
| 477 | checks to see if it has data that it can do something with, you should |
| 478 | make sure to only enable a transformation if it will be valid for the |
| 479 | data. For example, don't swap red and blue on grayscale data. |
| 480 | |
| 481 | The colors used for the background and transparency values should be |
| 482 | supplied in the same format/depth as the current image data. They |
| 483 | are stored in the same format/depth as the image data in a bKGD or tRNS |
| 484 | chunk, so this is what libpng expects for this data. The colors are |
| 485 | transformed to keep in sync with the image data when an application |
| 486 | calls the png_read_update_info() routine (see below). |
| 487 | |
| 488 | Data will be decoded into the supplied row buffers packed into bytes |
| 489 | unless the library has been told to transform it into another format. |
| 490 | For example, 4 bit/pixel paletted or grayscale data will be returned |
| 491 | 2 pixels/byte with the leftmost pixel in the high-order bits of the |
| 492 | byte, unless png_set_packing() is called. 8-bit RGB data will be stored |
| 493 | in RGB RGB RGB format unless png_set_filler() is called to insert filler |
| 494 | bytes, either before or after each RGB triplet. 16-bit RGB data will |
| 495 | be returned RRGGBB RRGGBB, with the most significant byte of the color |
| 496 | value first, unless png_set_strip_16() is called to transform it to |
| 497 | regular RGB RGB triplets, or png_set_filler() is called to insert |
| 498 | filler bytes, either before or after each RRGGBB triplet. Similarly, |
| 499 | 8-bit or 16-bit grayscale data can be modified with png_set_filler() |
| 500 | or png_set_strip_16(). |
| 501 | |
| 502 | The following code transforms grayscale images of less than 8 to 8 bits, |
| 503 | changes paletted images to RGB, and adds a full alpha channel if there is |
| 504 | transparency information in a tRNS chunk. This is most useful on |
| 505 | grayscale images with bit depths of 2 or 4 or if there is a multiple-image |
| 506 | viewing application that wishes to treat all images in the same way. |
| 507 | |
| 508 | if (color_type == PNG_COLOR_TYPE_PALETTE && |
| 509 | bit_depth <= 8) png_set_expand(png_ptr); |
| 510 | |
| 511 | if (color_type == PNG_COLOR_TYPE_GRAY && |
| 512 | bit_depth < 8) png_set_expand(png_ptr); |
| 513 | |
| 514 | if (png_get_valid(png_ptr, info_ptr, |
| 515 | PNG_INFO_tRNS)) png_set_expand(png_ptr); |
| 516 | |
| 517 | PNG can have files with 16 bits per channel. If you only can handle |
| 518 | 8 bits per channel, this will strip the pixels down to 8 bit. |
| 519 | |
| 520 | if (bit_depth == 16) |
| 521 | png_set_strip_16(png_ptr); |
| 522 | |
| 523 | The png_set_background() function tells libpng to composite images |
| 524 | with alpha or simple transparency against the supplied background |
| 525 | color. If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), |
| 526 | you may use this color, or supply another color more suitable for |
| 527 | the current display (e.g., the background color from a web page). You |
| 528 | need to tell libpng whether the color is in the gamma space of the |
| 529 | display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file |
| 530 | (PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one |
| 531 | that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't |
| 532 | know why anyone would use this, but it's here). |
| 533 | |
| 534 | If, for some reason, you don't need the alpha channel on an image, |
| 535 | and you want to remove it rather than combining it with the background |
| 536 | (but the image author certainly had in mind that you *would* combine |
| 537 | it with the background, so that's what you should probably do): |
| 538 | |
| 539 | if (color_type & PNG_COLOR_MASK_ALPHA) |
| 540 | png_set_strip_alpha(png_ptr); |
| 541 | |
| 542 | PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
| 543 | they can, resulting in, for example, 8 pixels per byte for 1 bit |
| 544 | files. This code expands to 1 pixel per byte without changing the |
| 545 | values of the pixels: |
| 546 | |
| 547 | if (bit_depth < 8) |
| 548 | png_set_packing(png_ptr); |
| 549 | |
| 550 | PNG files have possible bit depths of 1, 2, 4, 8, and 16. All pixels |
| 551 | stored in a PNG image have been "scaled" or "shifted" up to the next |
| 552 | higher possible bit depth (e.g. from 5 bits/sample in the range [0,31] to |
| 553 | 8 bits/sample in the range [0, 255]). However, it is also possible to |
| 554 | convert the PNG pixel data back to the original bit depth of the image. |
| 555 | This call reduces the pixels back down to the original bit depth: |
| 556 | |
| 557 | png_color_16p sig_bit; |
| 558 | |
| 559 | if (png_get_sBIT(png_ptr, info_ptr, &sig_bit)) |
| 560 | png_set_shift(png_ptr, sig_bit); |
| 561 | |
| 562 | PNG files store 3-color pixels in red, green, blue order. This code |
| 563 | changes the storage of the pixels to blue, green, red: |
| 564 | |
| 565 | if (color_type == PNG_COLOR_TYPE_RGB || |
| 566 | color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
| 567 | png_set_bgr(png_ptr); |
| 568 | |
| 569 | PNG files store RGB pixels packed into 3 bytes. This code expands them |
| 570 | into 4 bytes for windowing systems that need them in this format: |
| 571 | |
| 572 | if (bit_depth == 8 && color_type == |
| 573 | PNG_COLOR_TYPE_RGB) png_set_filler(png_ptr, |
| 574 | filler, PNG_FILLER_BEFORE); |
| 575 | |
| 576 | where "filler" is the 8 or 16-bit number to fill with, and the location is |
| 577 | either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether |
| 578 | you want the filler before the RGB or after. This transformation |
| 579 | does not affect images that already have full alpha channels. |
| 580 | |
| 581 | If you are reading an image with an alpha channel, and you need the |
| 582 | data as ARGB instead of the normal PNG format RGBA: |
| 583 | |
| 584 | if (color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
| 585 | png_set_swap_alpha(png_ptr); |
| 586 | |
| 587 | For some uses, you may want a grayscale image to be represented as |
| 588 | RGB. This code will do that conversion: |
| 589 | |
| 590 | if (color_type == PNG_COLOR_TYPE_GRAY || |
| 591 | color_type == PNG_COLOR_TYPE_GRAY_ALPHA) |
| 592 | png_set_gray_to_rgb(png_ptr); |
| 593 | |
| 594 | Conversely, you can convert an RGB or RGBA image to grayscale or grayscale |
| 595 | with alpha. This is intended for conversion of images that really are |
| 596 | gray (red == green == blue), so the function simply strips out the red |
| 597 | and blue channels, leaving the green channel in the gray position. |
| 598 | |
| 599 | if (color_type == PNG_COLOR_TYPE_RGB || |
| 600 | color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
| 601 | png_set_rgb_to_gray(png_ptr, error_action, |
| 602 | float red_weight, float green_weight); |
| 603 | |
| 604 | error_action = 1: silently do the conversion |
| 605 | error_action = 2: issue a warning if the original |
| 606 | image has any pixel where |
| 607 | red != green or red != blue |
| 608 | error_action = 3: issue an error and abort the |
| 609 | conversion if the original |
| 610 | image has any pixel where |
| 611 | red != green or red != blue |
| 612 | |
| 613 | red_weight: weight of red component |
| 614 | (NULL -> default 54/256) |
| 615 | green_weight: weight of green component |
| 616 | (NULL -> default 183/256) |
| 617 | |
| 618 | If you have set error_action = 1 or 2, you can |
| 619 | later check whether the image really was gray, after processing |
| 620 | the image rows, with the png_get_rgb_to_gray_status(png_ptr) function. |
| 621 | It will return a png_byte that is zero if the image was gray or |
| 622 | 1 if there were any non-gray pixels. bKGD and sBIT data |
| 623 | will be silently converted to grayscale, using the green channel |
| 624 | data, regardless of the error_action setting. |
| 625 | |
| 626 | With 0.0<=red_weight+green_weight<=1.0, |
| 627 | the normalized graylevel is computed: |
| 628 | |
| 629 | int rw = red_weight * 256; |
| 630 | int gw = green_weight * 256; |
| 631 | int bw = 256 - (rw + gw); |
| 632 | gray = (rw*red + gw*green + bw*blue)/256; |
| 633 | |
| 634 | The default values approximate those recommended in the Charles |
| 635 | Poynton's Color FAQ, <http://www.inforamp.net/~poynton/> |
| 636 | Copyright (c) 1998-01-04 Charles Poynton poynton@inforamp.net |
| 637 | |
| 638 | Y = 0.212671 * R + 0.715160 * G + 0.072169 * B |
| 639 | |
| 640 | Libpng approximates this with |
| 641 | |
| 642 | Y = 0.211 * R + 0.715 * G + 0.074 * B |
| 643 | |
| 644 | which can be expressed with integers as |
| 645 | |
| 646 | Y = (54 * R + 183 * G + 19 * B)/256 |
| 647 | |
| 648 | The calculation is done in a linear colorspace, if the image gamma |
| 649 | is known. |
| 650 | |
| 651 | If you have a grayscale and you are using png_set_expand() to change to |
| 652 | a higher bit-depth, you must either supply the background color as a gray |
| 653 | value at the original file bit-depth (need_expand = 1) or else supply the |
| 654 | background color as an RGB triplet at the final, expanded bit depth |
| 655 | (need_expand = 0). Similarly, if you are reading a paletted image, you |
| 656 | must either supply the background color as a palette index (need_expand = 1) |
| 657 | or as an RGB triplet that may or may not be in the palette (need_expand = 0). |
| 658 | |
| 659 | png_color_16 my_background; |
| 660 | png_color_16p image_background; |
| 661 | |
| 662 | if (png_get_bKGD(png_ptr, info_ptr, &image_background)) |
| 663 | png_set_background(png_ptr, image_background, |
| 664 | PNG_BACKGROUND_GAMMA_FILE, 1, 1.0); |
| 665 | else |
| 666 | png_set_background(png_ptr, &my_background, |
| 667 | PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0); |
| 668 | |
| 669 | To properly display PNG images on any kind of system, the application needs |
| 670 | to know what the display gamma is. Ideally, the user will know this, and |
| 671 | the application will allow them to set it. One method of allowing the user |
| 672 | to set the display gamma separately for each system is to check for the |
| 673 | DISPLAY_GAMMA and VIEWING_GAMMA environment variables or for a SCREEN_GAMMA |
| 674 | environment variable, which will hopefully be correctly set. |
| 675 | |
| 676 | Note that display_gamma is the gamma of your display, while screen_gamma is |
| 677 | the overall gamma correction required to produce pleasing results, |
| 678 | which depends on the lighting conditions in the surrounding environment. |
| 679 | Screen_gamma is display_gamma/viewing_gamma, where viewing_gamma is |
| 680 | the amount of additional gamma correction needed to compensate for |
| 681 | a (viewing_gamma=1.25) environment. In a dim or brightly lit room, no |
| 682 | compensation other than the display_gamma is needed (viewing_gamma=1.0). |
| 683 | |
| 684 | if (/* We have a user-defined screen |
| 685 | gamma value */) |
| 686 | { |
| 687 | screen_gamma = user_defined_screen_gamma; |
| 688 | } |
| 689 | /* One way that applications can share the same |
| 690 | screen gamma value */ |
| 691 | else if ((gamma_str = getenv("SCREEN_GAMMA")) |
| 692 | != NULL) |
| 693 | { |
| 694 | screen_gamma = atof(gamma_str); |
| 695 | } |
| 696 | /* If we don't have another value */ |
| 697 | else |
| 698 | { |
| 699 | screen_gamma = 2.2; /* A good guess for a |
| 700 | PC monitor in a bright office or a dim room */ |
| 701 | screen_gamma = 2.0; /* A good guess for a |
| 702 | PC monitor in a dark room */ |
| 703 | screen_gamma = 1.7 or 1.0; /* A good |
| 704 | guess for Mac systems */ |
| 705 | } |
| 706 | |
| 707 | The png_set_gamma() function handles gamma transformations of the data. |
| 708 | Pass both the file gamma and the current screen_gamma. If the file does |
| 709 | not have a gamma value, you can pass one anyway if you have an idea what |
| 710 | it is (usually 0.45455 is a good guess for GIF images on PCs). Note |
| 711 | that file gammas are inverted from screen gammas. See the discussions |
| 712 | on gamma in the PNG specification for an excellent description of what |
| 713 | gamma is, and why all applications should support it. It is strongly |
| 714 | recommended that PNG viewers support gamma correction. |
| 715 | |
| 716 | if (png_get_gAMA(png_ptr, info_ptr, &gamma)) |
| 717 | png_set_gamma(png_ptr, screen_gamma, gamma); |
| 718 | else |
| 719 | png_set_gamma(png_ptr, screen_gamma, 0.45455); |
| 720 | |
| 721 | If you need to reduce an RGB file to a paletted file, or if a paletted |
| 722 | file has more entries then will fit on your screen, png_set_dither() |
| 723 | will do that. Note that this is a simple match dither that merely |
| 724 | finds the closest color available. This should work fairly well with |
| 725 | optimized palettes, and fairly badly with linear color cubes. If you |
| 726 | pass a palette that is larger then maximum_colors, the file will |
| 727 | reduce the number of colors in the palette so it will fit into |
| 728 | maximum_colors. If there is a histogram, it will use it to make |
| 729 | more intelligent choices when reducing the palette. If there is no |
| 730 | histogram, it may not do as good a job. |
| 731 | |
| 732 | if (color_type & PNG_COLOR_MASK_COLOR) |
| 733 | { |
| 734 | if (png_get_valid(png_ptr, info_ptr, |
| 735 | PNG_INFO_PLTE)) |
| 736 | { |
| 737 | png_color_16p histogram; |
| 738 | |
| 739 | png_get_hIST(png_ptr, info_ptr, |
| 740 | &histogram); |
| 741 | png_set_dither(png_ptr, palette, num_palette, |
| 742 | max_screen_colors, histogram, 1); |
| 743 | } |
| 744 | else |
| 745 | { |
| 746 | png_color std_color_cube[MAX_SCREEN_COLORS] = |
| 747 | { ... colors ... }; |
| 748 | |
| 749 | png_set_dither(png_ptr, std_color_cube, |
| 750 | MAX_SCREEN_COLORS, MAX_SCREEN_COLORS, |
| 751 | NULL,0); |
| 752 | } |
| 753 | } |
| 754 | |
| 755 | PNG files describe monochrome as black being zero and white being one. |
| 756 | The following code will reverse this (make black be one and white be |
| 757 | zero): |
| 758 | |
| 759 | if (bit_depth == 1 && color_type == PNG_COLOR_GRAY) |
| 760 | png_set_invert_mono(png_ptr); |
| 761 | |
| 762 | PNG files store 16 bit pixels in network byte order (big-endian, |
| 763 | ie. most significant bits first). This code changes the storage to the |
| 764 | other way (little-endian, i.e. least significant bits first, the |
| 765 | way PCs store them): |
| 766 | |
| 767 | if (bit_depth == 16) |
| 768 | png_set_swap(png_ptr); |
| 769 | |
| 770 | If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you |
| 771 | need to change the order the pixels are packed into bytes, you can use: |
| 772 | |
| 773 | if (bit_depth < 8) |
| 774 | png_set_packswap(png_ptr); |
| 775 | |
| 776 | The last thing to handle is interlacing; this is covered in detail below, |
| 777 | but you must call the function here if you want libpng to handle expansion |
| 778 | of the interlaced image. |
| 779 | |
| 780 | number_of_passes = png_set_interlace_handling(png_ptr); |
| 781 | |
| 782 | After setting the transformations, libpng can update your png_info |
| 783 | structure to reflect any transformations you've requested with this |
| 784 | call. This is most useful to update the info structure's rowbytes |
| 785 | field so you can use it to allocate your image memory. This function |
| 786 | will also update your palette with the correct screen_gamma and |
| 787 | background if these have been given with the calls above. |
| 788 | |
| 789 | png_read_update_info(png_ptr, info_ptr); |
| 790 | |
| 791 | After you call png_read_update_info(), you can allocate any |
| 792 | memory you need to hold the image. The row data is simply |
| 793 | raw byte data for all forms of images. As the actual allocation |
| 794 | varies among applications, no example will be given. If you |
| 795 | are allocating one large chunk, you will need to build an |
| 796 | array of pointers to each row, as it will be needed for some |
| 797 | of the functions below. |
| 798 | |
| 799 | After you've allocated memory, you can read the image data. |
| 800 | The simplest way to do this is in one function call. If you are |
| 801 | allocating enough memory to hold the whole image, you can just |
| 802 | call png_read_image() and libpng will read in all the image data |
| 803 | and put it in the memory area supplied. You will need to pass in |
| 804 | an array of pointers to each row. |
| 805 | |
| 806 | This function automatically handles interlacing, so you don't need |
| 807 | to call png_set_interlace_handling() or call this function multiple |
| 808 | times, or any of that other stuff necessary with png_read_rows(). |
| 809 | |
| 810 | png_read_image(png_ptr, row_pointers); |
| 811 | |
| 812 | where row_pointers is: |
| 813 | |
| 814 | png_bytep row_pointers[height]; |
| 815 | |
| 816 | You can point to void or char or whatever you use for pixels. |
| 817 | |
| 818 | If you don't want to read in the whole image at once, you can |
| 819 | use png_read_rows() instead. If there is no interlacing (check |
| 820 | interlace_type == PNG_INTERLACE_NONE), this is simple: |
| 821 | |
| 822 | png_read_rows(png_ptr, row_pointers, NULL, |
| 823 | number_of_rows); |
| 824 | |
| 825 | where row_pointers is the same as in the png_read_image() call. |
| 826 | |
| 827 | If you are doing this just one row at a time, you can do this with |
| 828 | row_pointers: |
| 829 | |
| 830 | png_bytep row_pointers = row; |
| 831 | png_read_row(png_ptr, &row_pointers, NULL); |
| 832 | |
| 833 | If the file is interlaced (info_ptr->interlace_type != 0), things get |
| 834 | somewhat harder. The only current (PNG Specification version 1.0) |
| 835 | interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7) |
| 836 | is a somewhat complicated 2D interlace scheme, known as Adam7, that |
| 837 | breaks down an image into seven smaller images of varying size, based |
| 838 | on an 8x8 grid. |
| 839 | |
| 840 | libpng can fill out those images or it can give them to you "as is". |
| 841 | If you want them filled out, there are two ways to do that. The one |
| 842 | mentioned in the PNG specification is to expand each pixel to cover |
| 843 | those pixels that have not been read yet (the "rectangle" method). |
| 844 | This results in a blocky image for the first pass, which gradually |
| 845 | smooths out as more pixels are read. The other method is the "sparkle" |
| 846 | method, where pixels are drawn only in their final locations, with the |
| 847 | rest of the image remaining whatever colors they were initialized to |
| 848 | before the start of the read. The first method usually looks better, |
| 849 | but tends to be slower, as there are more pixels to put in the rows. |
| 850 | |
| 851 | If you don't want libpng to handle the interlacing details, just call |
| 852 | png_read_rows() seven times to read in all seven images. Each of the |
| 853 | images is a valid image by itself, or they can all be combined on an |
| 854 | 8x8 grid to form a single image (although if you intend to combine them |
| 855 | you would be far better off using the libpng interlace handling). |
| 856 | |
| 857 | The first pass will return an image 1/8 as wide as the entire image |
| 858 | (every 8th column starting in column 0) and 1/8 as high as the original |
| 859 | (every 8th row starting in row 0), the second will be 1/8 as wide |
| 860 | (starting in column 4) and 1/8 as high (also starting in row 0). The |
| 861 | third pass will be 1/4 as wide (every 4th pixel starting in column 0) and |
| 862 | 1/8 as high (every 8th row starting in row 4), and the fourth pass will |
| 863 | be 1/4 as wide and 1/4 as high (every 4th column starting in column 2, |
| 864 | and every 4th row starting in row 0). The fifth pass will return an |
| 865 | image 1/2 as wide, and 1/4 as high (starting at column 0 and row 2), |
| 866 | while the sixth pass will be 1/2 as wide and 1/2 as high as the original |
| 867 | (starting in column 1 and row 0). The seventh and final pass will be as |
| 868 | wide as the original, and 1/2 as high, containing all of the odd |
| 869 | numbered scanlines. Phew! |
| 870 | |
| 871 | If you want libpng to expand the images, call this before calling |
| 872 | png_start_read_image() or png_read_update_info(): |
| 873 | |
| 874 | if (interlace_type == PNG_INTERLACE_ADAM7) |
| 875 | number_of_passes |
| 876 | = png_set_interlace_handling(png_ptr); |
| 877 | |
| 878 | This will return the number of passes needed. Currently, this |
| 879 | is seven, but may change if another interlace type is added. |
| 880 | This function can be called even if the file is not interlaced, |
| 881 | where it will return one pass. |
| 882 | |
| 883 | If you are not going to display the image after each pass, but are |
| 884 | going to wait until the entire image is read in, use the sparkle |
| 885 | effect. This effect is faster and the end result of either method |
| 886 | is exactly the same. If you are planning on displaying the image |
| 887 | after each pass, the "rectangle" effect is generally considered the |
| 888 | better looking one. |
| 889 | |
| 890 | If you only want the "sparkle" effect, just call png_read_rows() as |
| 891 | normal, with the third parameter NULL. Make sure you make pass over |
| 892 | the image number_of_passes times, and you don't change the data in the |
| 893 | rows between calls. You can change the locations of the data, just |
| 894 | not the data. Each pass only writes the pixels appropriate for that |
| 895 | pass, and assumes the data from previous passes is still valid. |
| 896 | |
| 897 | png_read_rows(png_ptr, row_pointers, NULL, |
| 898 | number_of_rows); |
| 899 | |
| 900 | If you only want the first effect (the rectangles), do the same as |
| 901 | before except pass the row buffer in the third parameter, and leave |
| 902 | the second parameter NULL. |
| 903 | |
| 904 | png_read_rows(png_ptr, NULL, row_pointers, |
| 905 | number_of_rows); |
| 906 | |
| 907 | After you are finished reading the image, you can finish reading |
| 908 | the file. If you are interested in comments or time, which may be |
| 909 | stored either before or after the image data, you should pass the |
| 910 | separate png_info struct if you want to keep the comments from |
| 911 | before and after the image separate. If you are not interested, you |
| 912 | can pass NULL. |
| 913 | |
| 914 | png_read_end(png_ptr, end_info); |
| 915 | |
| 916 | When you are done, you can free all memory allocated by libpng like this: |
| 917 | |
| 918 | png_destroy_read_struct(&png_ptr, &info_ptr, |
| 919 | &end_info); |
| 920 | |
| 921 | For a more compact example of reading a PNG image, see the file example.c. |
| 922 | |
| 923 | |
| 924 | Reading PNG files progressively: |
| 925 | |
| 926 | The progressive reader is slightly different then the non-progressive |
| 927 | reader. Instead of calling png_read_info(), png_read_rows(), and |
| 928 | png_read_end(), you make one call to png_process_data(), which calls |
| 929 | callbacks when it has the info, a row, or the end of the image. You |
| 930 | set up these callbacks with png_set_progressive_read_fn(). You don't |
| 931 | have to worry about the input/output functions of libpng, as you are |
| 932 | giving the library the data directly in png_process_data(). I will |
| 933 | assume that you have read the section on reading PNG files above, |
| 934 | so I will only highlight the differences (although I will show |
| 935 | all of the code). |
| 936 | |
| 937 | png_structp png_ptr; |
| 938 | png_infop info_ptr; |
| 939 | |
| 940 | /* An example code fragment of how you would |
| 941 | initialize the progressive reader in your |
| 942 | application. */ |
| 943 | int |
| 944 | initialize_png_reader() |
| 945 | { |
| 946 | png_ptr = png_create_read_struct |
| 947 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
| 948 | user_error_fn, user_warning_fn); |
| 949 | if (!png_ptr) |
| 950 | return -1; |
| 951 | info_ptr = png_create_info_struct(png_ptr); |
| 952 | if (!info_ptr) |
| 953 | { |
| 954 | png_destroy_read_struct(&png_ptr, (png_infopp)NULL, |
| 955 | (png_infopp)NULL); |
| 956 | return -1; |
| 957 | } |
| 958 | |
| 959 | if (setjmp(png_ptr->jmpbuf)) |
| 960 | { |
| 961 | png_destroy_read_struct(&png_ptr, &info_ptr, |
| 962 | (png_infopp)NULL); |
| 963 | return -1; |
| 964 | } |
| 965 | |
| 966 | /* This one's new. You can provide functions |
| 967 | to be called when the header info is valid, |
| 968 | when each row is completed, and when the image |
| 969 | is finished. If you aren't using all functions, |
| 970 | you can specify NULL parameters. Even when all |
| 971 | three functions are NULL, you need to call |
| 972 | png_set_progressive_read_fn(). You can use |
| 973 | any struct as the user_ptr (cast to a void pointer |
| 974 | for the function call), and retrieve the pointer |
| 975 | from inside the callbacks using the function |
| 976 | |
| 977 | png_get_progressive_ptr(png_ptr); |
| 978 | |
| 979 | which will return a void pointer, which you have |
| 980 | to cast appropriately. |
| 981 | */ |
| 982 | png_set_progressive_read_fn(png_ptr, (void *)user_ptr, |
| 983 | info_callback, row_callback, end_callback); |
| 984 | |
| 985 | return 0; |
| 986 | } |
| 987 | |
| 988 | /* A code fragment that you call as you receive blocks |
| 989 | of data */ |
| 990 | int |
| 991 | process_data(png_bytep buffer, png_uint_32 length) |
| 992 | { |
| 993 | if (setjmp(png_ptr->jmpbuf)) |
| 994 | { |
| 995 | png_destroy_read_struct(&png_ptr, &info_ptr, |
| 996 | (png_infopp)NULL); |
| 997 | return -1; |
| 998 | } |
| 999 | |
| 1000 | /* This one's new also. Simply give it a chunk |
| 1001 | of data from the file stream (in order, of |
| 1002 | course). On machines with segmented memory |
| 1003 | models machines, don't give it any more than |
| 1004 | 64K. The library seems to run fine with sizes |
| 1005 | of 4K. Although you can give it much less if |
| 1006 | necessary (I assume you can give it chunks of |
| 1007 | 1 byte, I haven't tried less then 256 bytes |
| 1008 | yet). When this function returns, you may |
| 1009 | want to display any rows that were generated |
| 1010 | in the row callback if you don't already do |
| 1011 | so there. |
| 1012 | */ |
| 1013 | png_process_data(png_ptr, info_ptr, buffer, length); |
| 1014 | return 0; |
| 1015 | } |
| 1016 | |
| 1017 | /* This function is called (as set by |
| 1018 | png_set_progressive_read_fn() above) when enough data |
| 1019 | has been supplied so all of the header has been |
| 1020 | read. |
| 1021 | */ |
| 1022 | void |
| 1023 | info_callback(png_structp png_ptr, png_infop info) |
| 1024 | { |
| 1025 | /* Do any setup here, including setting any of |
| 1026 | the transformations mentioned in the Reading |
| 1027 | PNG files section. For now, you _must_ call |
| 1028 | either png_start_read_image() or |
| 1029 | png_read_update_info() after all the |
| 1030 | transformations are set (even if you don't set |
| 1031 | any). You may start getting rows before |
| 1032 | png_process_data() returns, so this is your |
| 1033 | last chance to prepare for that. |
| 1034 | */ |
| 1035 | } |
| 1036 | |
| 1037 | /* This function is called when each row of image |
| 1038 | data is complete */ |
| 1039 | void |
| 1040 | row_callback(png_structp png_ptr, png_bytep new_row, |
| 1041 | png_uint_32 row_num, int pass) |
| 1042 | { |
| 1043 | /* If the image is interlaced, and you turned |
| 1044 | on the interlace handler, this function will |
| 1045 | be called for every row in every pass. Some |
| 1046 | of these rows will not be changed from the |
| 1047 | previous pass. When the row is not changed, |
| 1048 | the new_row variable will be NULL. The rows |
| 1049 | and passes are called in order, so you don't |
| 1050 | really need the row_num and pass, but I'm |
| 1051 | supplying them because it may make your life |
| 1052 | easier. |
| 1053 | |
| 1054 | For the non-NULL rows of interlaced images, |
| 1055 | you must call png_progressive_combine_row() |
| 1056 | passing in the row and the old row. You can |
| 1057 | call this function for NULL rows (it will just |
| 1058 | return) and for non-interlaced images (it just |
| 1059 | does the memcpy for you) if it will make the |
| 1060 | code easier. Thus, you can just do this for |
| 1061 | all cases: |
| 1062 | */ |
| 1063 | |
| 1064 | png_progressive_combine_row(png_ptr, old_row, |
| 1065 | new_row); |
| 1066 | |
| 1067 | /* where old_row is what was displayed for |
| 1068 | previously for the row. Note that the first |
| 1069 | pass (pass == 0, really) will completely cover |
| 1070 | the old row, so the rows do not have to be |
| 1071 | initialized. After the first pass (and only |
| 1072 | for interlaced images), you will have to pass |
| 1073 | the current row, and the function will combine |
| 1074 | the old row and the new row. |
| 1075 | */ |
| 1076 | } |
| 1077 | |
| 1078 | void |
| 1079 | end_callback(png_structp png_ptr, png_infop info) |
| 1080 | { |
| 1081 | /* This function is called after the whole image |
| 1082 | has been read, including any chunks after the |
| 1083 | image (up to and including the IEND). You |
| 1084 | will usually have the same info chunk as you |
| 1085 | had in the header, although some data may have |
| 1086 | been added to the comments and time fields. |
| 1087 | |
| 1088 | Most people won't do much here, perhaps setting |
| 1089 | a flag that marks the image as finished. |
| 1090 | */ |
| 1091 | } |
| 1092 | |
| 1093 | |
| 1094 | |
| 1095 | IV. Writing |
| 1096 | |
| 1097 | Much of this is very similar to reading. However, everything of |
| 1098 | importance is repeated here, so you won't have to constantly look |
| 1099 | back up in the reading section to understand writing. |
| 1100 | |
| 1101 | You will want to do the I/O initialization before you get into libpng, |
| 1102 | so if it doesn't work, you don't have anything to undo. If you are not |
| 1103 | using the standard I/O functions, you will need to replace them with |
| 1104 | custom writing functions. See the discussion under Customizing libpng. |
| 1105 | |
| 1106 | FILE *fp = fopen(file_name, "wb"); |
| 1107 | if (!fp) |
| 1108 | { |
| 1109 | return; |
| 1110 | } |
| 1111 | |
| 1112 | Next, png_struct and png_info need to be allocated and initialized. |
| 1113 | As these can be both relatively large, you may not want to store these |
| 1114 | on the stack, unless you have stack space to spare. Of course, you |
| 1115 | will want to check if they return NULL. If you are also reading, |
| 1116 | you won't want to name your read structure and your write structure |
| 1117 | both "png_ptr"; you can call them anything you like, such as |
| 1118 | "read_ptr" and "write_ptr". Look at pngtest.c, for example. |
| 1119 | |
| 1120 | png_structp png_ptr = png_create_write_struct |
| 1121 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
| 1122 | user_error_fn, user_warning_fn); |
| 1123 | if (!png_ptr) |
| 1124 | return; |
| 1125 | |
| 1126 | png_infop info_ptr = png_create_info_struct(png_ptr); |
| 1127 | if (!info_ptr) |
| 1128 | { |
| 1129 | png_destroy_write_struct(&png_ptr, |
| 1130 | (png_infopp)NULL); |
| 1131 | return; |
| 1132 | } |
| 1133 | |
| 1134 | If you want to use your own memory allocation routines, |
| 1135 | define PNG_USER_MEM_SUPPORTED and use |
| 1136 | png_create_write_struct_2() instead of png_create_read_struct(): |
| 1137 | |
| 1138 | png_structp png_ptr = png_create_write_struct_2 |
| 1139 | (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr, |
| 1140 | user_error_fn, user_warning_fn, (png_voidp) |
| 1141 | user_mem_ptr, user_malloc_fn, user_free_fn); |
| 1142 | |
| 1143 | After you have these structures, you will need to set up the |
| 1144 | error handling. When libpng encounters an error, it expects to |
| 1145 | longjmp() back to your routine. Therefore, you will need to call |
| 1146 | setjmp() and pass the png_ptr->jmpbuf. If you |
| 1147 | write the file from different routines, you will need to update |
| 1148 | the jmpbuf field every time you enter a new routine that will |
| 1149 | call a png_ function. See your documentation of setjmp/longjmp |
| 1150 | for your compiler for more information on setjmp/longjmp. See |
| 1151 | the discussion on libpng error handling in the Customizing Libpng |
| 1152 | section below for more information on the libpng error handling. |
| 1153 | |
| 1154 | if (setjmp(png_ptr->jmpbuf)) |
| 1155 | { |
| 1156 | png_destroy_write_struct(&png_ptr, &info_ptr); |
| 1157 | fclose(fp); |
| 1158 | return; |
| 1159 | } |
| 1160 | ... |
| 1161 | return; |
| 1162 | |
| 1163 | Now you need to set up the output code. The default for libpng is to |
| 1164 | use the C function fwrite(). If you use this, you will need to pass a |
| 1165 | valid FILE * in the function png_init_io(). Be sure that the file is |
| 1166 | opened in binary mode. Again, if you wish to handle writing data in |
| 1167 | another way, see the discussion on libpng I/O handling in the Customizing |
| 1168 | Libpng section below. |
| 1169 | |
| 1170 | png_init_io(png_ptr, fp); |
| 1171 | |
| 1172 | At this point, you can set up a callback function that will be |
| 1173 | called after each row has been written, which you can use to control |
| 1174 | a progress meter or the like. It's demonstrated in pngtest.c. |
| 1175 | You must supply a function |
| 1176 | |
| 1177 | void write_row_callback(png_ptr, png_uint_32 row, int pass); |
| 1178 | { |
| 1179 | /* put your code here */ |
| 1180 | } |
| 1181 | |
| 1182 | (You can give it another name that you like instead of "write_row_callback") |
| 1183 | |
| 1184 | To inform libpng about your function, use |
| 1185 | |
| 1186 | png_set_write_status_fn(png_ptr, write_row_callback); |
| 1187 | |
| 1188 | You now have the option of modifying how the compression library will |
| 1189 | run. The following functions are mainly for testing, but may be useful |
| 1190 | in some cases, like if you need to write PNG files extremely fast and |
| 1191 | are willing to give up some compression, or if you want to get the |
| 1192 | maximum possible compression at the expense of slower writing. If you |
| 1193 | have no special needs in this area, let the library do what it wants by |
| 1194 | not calling this function at all, as it has been tuned to deliver a good |
| 1195 | speed/compression ratio. The second parameter to png_set_filter() is |
| 1196 | the filter method, for which the only valid value is '0' (as of the |
| 1197 | October 1996 PNG specification, version 1.0). The third parameter is a |
| 1198 | flag that indicates which filter type(s) are to be tested for each |
| 1199 | scanline. See the Compression Library for details on the specific filter |
| 1200 | types. |
| 1201 | |
| 1202 | |
| 1203 | /* turn on or off filtering, and/or choose |
| 1204 | specific filters */ |
| 1205 | png_set_filter(png_ptr, 0, |
| 1206 | PNG_FILTER_NONE | PNG_FILTER_SUB | |
| 1207 | PNG_FILTER_PAETH); |
| 1208 | |
| 1209 | The png_set_compression_???() functions interface to the zlib compression |
| 1210 | library, and should mostly be ignored unless you really know what you are |
| 1211 | doing. The only generally useful call is png_set_compression_level() |
| 1212 | which changes how much time zlib spends on trying to compress the image |
| 1213 | data. See the Compression Library for details on the compression levels. |
| 1214 | |
| 1215 | /* set the zlib compression level */ |
| 1216 | png_set_compression_level(png_ptr, |
| 1217 | Z_BEST_COMPRESSION); |
| 1218 | |
| 1219 | /* set other zlib parameters */ |
| 1220 | png_set_compression_mem_level(png_ptr, 8); |
| 1221 | png_set_compression_strategy(png_ptr, |
| 1222 | Z_DEFAULT_STRATEGY); |
| 1223 | png_set_compression_window_bits(png_ptr, 15); |
| 1224 | png_set_compression_method(png_ptr, 8); |
| 1225 | |
| 1226 | You now need to fill in the png_info structure with all the data you |
| 1227 | wish to write before the actual image. Note that the only thing you |
| 1228 | are allowed to write after the image is the text chunks and the time |
| 1229 | chunk (as of PNG Specification 1.0, anyway). See png_write_end() and |
| 1230 | the latest PNG specification for more information on that. If you |
| 1231 | wish to write them before the image, fill them in now, and flag that |
| 1232 | data as being valid. If you want to wait until after the data, don't |
| 1233 | fill them until png_write_end(). For all the fields in png_info and |
| 1234 | their data types, see png.h. For explanations of what the fields |
| 1235 | contain, see the PNG specification. |
| 1236 | |
| 1237 | Some of the more important parts of the png_info are: |
| 1238 | |
| 1239 | png_set_IHDR(png_ptr, info_ptr, width, height, |
| 1240 | bit_depth, color_type, interlace_type, |
| 1241 | compression_type, filter_type) |
| 1242 | width - holds the width of the image |
| 1243 | in pixels (up to 2^31). |
| 1244 | height - holds the height of the image |
| 1245 | in pixels (up to 2^31). |
| 1246 | bit_depth - holds the bit depth of one of the |
| 1247 | image channels. |
| 1248 | (valid values are 1, 2, 4, 8, 16 |
| 1249 | and depend also on the |
| 1250 | color_type. See also significant |
| 1251 | bits (sBIT) below). |
| 1252 | color_type - describes which color/alpha |
| 1253 | channels are present. |
| 1254 | PNG_COLOR_TYPE_GRAY |
| 1255 | (bit depths 1, 2, 4, 8, 16) |
| 1256 | PNG_COLOR_TYPE_GRAY_ALPHA |
| 1257 | (bit depths 8, 16) |
| 1258 | PNG_COLOR_TYPE_PALETTE |
| 1259 | (bit depths 1, 2, 4, 8) |
| 1260 | PNG_COLOR_TYPE_RGB |
| 1261 | (bit_depths 8, 16) |
| 1262 | PNG_COLOR_TYPE_RGB_ALPHA |
| 1263 | (bit_depths 8, 16) |
| 1264 | |
| 1265 | PNG_COLOR_MASK_PALETTE |
| 1266 | PNG_COLOR_MASK_COLOR |
| 1267 | PNG_COLOR_MASK_ALPHA |
| 1268 | |
| 1269 | interlace_type - PNG_INTERLACE_NONE or |
| 1270 | PNG_INTERLACE_ADAM7 |
| 1271 | compression_type - (must be |
| 1272 | PNG_COMPRESSION_TYPE_DEFAULT) |
| 1273 | filter_type - (must be PNG_FILTER_TYPE_DEFAULT) |
| 1274 | |
| 1275 | png_set_PLTE(png_ptr, info_ptr, palette, |
| 1276 | num_palette); |
| 1277 | palette - the palette for the file |
| 1278 | (array of png_color) |
| 1279 | num_palette - number of entries in the palette |
| 1280 | |
| 1281 | png_set_gAMA(png_ptr, info_ptr, gamma); |
| 1282 | gamma - the gamma the image was created |
| 1283 | at (PNG_INFO_gAMA) |
| 1284 | |
| 1285 | png_set_sRGB(png_ptr, info_ptr, srgb_intent); |
| 1286 | srgb_intent - the rendering intent |
| 1287 | (PNG_INFO_sRGB) The presence of |
| 1288 | the sRGB chunk means that the pixel |
| 1289 | data is in the sRGB color space. |
| 1290 | This chunk also implies specific |
| 1291 | values of gAMA and cHRM. Rendering |
| 1292 | intent is the CSS-1 property that |
| 1293 | has been defined by the International |
| 1294 | Color Consortium |
| 1295 | (http://www.color.org). |
| 1296 | It can be one of |
| 1297 | PNG_SRGB_INTENT_SATURATION, |
| 1298 | PNG_SRGB_INTENT_PERCEPTUAL, |
| 1299 | PNG_SRGB_INTENT_ABSOLUTE, or |
| 1300 | PNG_SRGB_INTENT_RELATIVE. |
| 1301 | |
| 1302 | |
| 1303 | png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr, |
| 1304 | srgb_intent); |
| 1305 | srgb_intent - the rendering intent |
| 1306 | (PNG_INFO_sRGB) The presence of the |
| 1307 | sRGB chunk means that the pixel |
| 1308 | data is in the sRGB color space. |
| 1309 | This function also causes gAMA and |
| 1310 | cHRM chunks with the specific values |
| 1311 | that are consistent with sRGB to be |
| 1312 | written. |
| 1313 | |
| 1314 | png_set_sBIT(png_ptr, info_ptr, sig_bit); |
| 1315 | sig_bit - the number of significant bits for |
| 1316 | (PNG_INFO_sBIT) each of the gray, red, |
| 1317 | green, and blue channels, whichever are |
| 1318 | appropriate for the given color type |
| 1319 | (png_color_16) |
| 1320 | |
| 1321 | png_set_tRNS(png_ptr, info_ptr, trans, num_trans, |
| 1322 | trans_values); |
| 1323 | trans - array of transparent entries for |
| 1324 | palette (PNG_INFO_tRNS) |
| 1325 | trans_values - transparent pixel for non-paletted |
| 1326 | images (PNG_INFO_tRNS) |
| 1327 | num_trans - number of transparent entries |
| 1328 | (PNG_INFO_tRNS) |
| 1329 | |
| 1330 | png_set_hIST(png_ptr, info_ptr, hist); |
| 1331 | (PNG_INFO_hIST) |
| 1332 | hist - histogram of palette (array of |
| 1333 | png_color_16) |
| 1334 | |
| 1335 | png_set_tIME(png_ptr, info_ptr, mod_time); |
| 1336 | mod_time - time image was last modified |
| 1337 | (PNG_VALID_tIME) |
| 1338 | |
| 1339 | png_set_bKGD(png_ptr, info_ptr, background); |
| 1340 | background - background color (PNG_VALID_bKGD) |
| 1341 | |
| 1342 | png_set_text(png_ptr, info_ptr, text_ptr, num_text); |
| 1343 | text_ptr - array of png_text holding image |
| 1344 | comments |
| 1345 | text_ptr[i]->key - keyword for comment. |
| 1346 | text_ptr[i]->text - text comments for current |
| 1347 | keyword. |
| 1348 | text_ptr[i]->compression - type of compression used |
| 1349 | on "text" PNG_TEXT_COMPRESSION_NONE or |
| 1350 | PNG_TEXT_COMPRESSION_zTXt |
| 1351 | num_text - number of comments in text_ptr |
| 1352 | |
| 1353 | png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y, |
| 1354 | unit_type); |
| 1355 | offset_x - positive offset from the left |
| 1356 | edge of the screen |
| 1357 | offset_y - positive offset from the top |
| 1358 | edge of the screen |
| 1359 | unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER |
| 1360 | |
| 1361 | png_set_pHYs(png_ptr, info_ptr, res_x, res_y, |
| 1362 | unit_type); |
| 1363 | res_x - pixels/unit physical resolution |
| 1364 | in x direction |
| 1365 | res_y - pixels/unit physical resolution |
| 1366 | in y direction |
| 1367 | unit_type - PNG_RESOLUTION_UNKNOWN, |
| 1368 | PNG_RESOLUTION_METER |
| 1369 | |
| 1370 | In PNG files, the alpha channel in an image is the level of opacity. |
| 1371 | If your data is supplied as a level of transparency, you can invert the |
| 1372 | alpha channel before you write it, so that 0 is fully transparent and 255 |
| 1373 | (in 8-bit or paletted images) or 65535 (in 16-bit images) is fully opaque, |
| 1374 | with |
| 1375 | |
| 1376 | png_set_invert_alpha(png_ptr); |
| 1377 | |
| 1378 | This must appear here instead of later with the other transformations |
| 1379 | because in the case of paletted images the tRNS chunk data has to |
| 1380 | be inverted before the tRNS chunk is written. If your image is not a |
| 1381 | paletted image, the tRNS data (which in such cases represents a single |
| 1382 | color to be rendered as transparent) won't be changed. |
| 1383 | |
| 1384 | A quick word about text and num_text. text is an array of png_text |
| 1385 | structures. num_text is the number of valid structures in the array. |
| 1386 | If you want, you can use max_text to hold the size of the array, but |
| 1387 | libpng ignores it for writing (it does use it for reading). Each |
| 1388 | png_text structure holds a keyword-text value, and a compression type. |
| 1389 | The compression types have the same valid numbers as the compression |
| 1390 | types of the image data. Currently, the only valid number is zero. |
| 1391 | However, you can store text either compressed or uncompressed, unlike |
| 1392 | images, which always have to be compressed. So if you don't want the |
| 1393 | text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE. |
| 1394 | Until text gets around 1000 bytes, it is not worth compressing it. |
| 1395 | After the text has been written out to the file, the compression type |
| 1396 | is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR, |
| 1397 | so that it isn't written out again at the end (in case you are calling |
| 1398 | png_write_end() with the same struct. |
| 1399 | |
| 1400 | The keywords that are given in the PNG Specification are: |
| 1401 | |
| 1402 | Title Short (one line) title or |
| 1403 | caption for image |
| 1404 | Author Name of image's creator |
| 1405 | Description Description of image (possibly long) |
| 1406 | Copyright Copyright notice |
| 1407 | Creation Time Time of original image creation |
| 1408 | (usually RFC 1123 format, see below) |
| 1409 | Software Software used to create the image |
| 1410 | Disclaimer Legal disclaimer |
| 1411 | Warning Warning of nature of content |
| 1412 | Source Device used to create the image |
| 1413 | Comment Miscellaneous comment; conversion |
| 1414 | from other image format |
| 1415 | |
| 1416 | The keyword-text pairs work like this. Keywords should be short |
| 1417 | simple descriptions of what the comment is about. Some typical |
| 1418 | keywords are found in the PNG specification, as is some recommendations |
| 1419 | on keywords. You can repeat keywords in a file. You can even write |
| 1420 | some text before the image and some after. For example, you may want |
| 1421 | to put a description of the image before the image, but leave the |
| 1422 | disclaimer until after, so viewers working over modem connections |
| 1423 | don't have to wait for the disclaimer to go over the modem before |
| 1424 | they start seeing the image. Finally, keywords should be full |
| 1425 | words, not abbreviations. Keywords and text are in the ISO 8859-1 |
| 1426 | (Latin-1) character set (a superset of regular ASCII) and can not |
| 1427 | contain NUL characters, and should not contain control or other |
| 1428 | unprintable characters. To make the comments widely readable, stick |
| 1429 | with basic ASCII, and avoid machine specific character set extensions |
| 1430 | like the IBM-PC character set. The keyword must be present, but |
| 1431 | you can leave off the text string on non-compressed pairs. |
| 1432 | Compressed pairs must have a text string, as only the text string |
| 1433 | is compressed anyway, so the compression would be meaningless. |
| 1434 | |
| 1435 | PNG supports modification time via the png_time structure. Two |
| 1436 | conversion routines are proved, png_convert_from_time_t() for |
| 1437 | time_t and png_convert_from_struct_tm() for struct tm. The |
| 1438 | time_t routine uses gmtime(). You don't have to use either of |
| 1439 | these, but if you wish to fill in the png_time structure directly, |
| 1440 | you should provide the time in universal time (GMT) if possible |
| 1441 | instead of your local time. Note that the year number is the full |
| 1442 | year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and |
| 1443 | that months start with 1. |
| 1444 | |
| 1445 | If you want to store the time of the original image creation, you should |
| 1446 | use a plain tEXt chunk with the "Creation Time" keyword. This is |
| 1447 | necessary because the "creation time" of a PNG image is somewhat vague, |
| 1448 | depending on whether you mean the PNG file, the time the image was |
| 1449 | created in a non-PNG format, a still photo from which the image was |
| 1450 | scanned, or possibly the subject matter itself. In order to facilitate |
| 1451 | machine-readable dates, it is recommended that the "Creation Time" |
| 1452 | tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"), |
| 1453 | although this isn't a requirement. Unlike the tIME chunk, the |
| 1454 | "Creation Time" tEXt chunk is not expected to be automatically changed |
| 1455 | by the software. To facilitate the use of RFC 1123 dates, a function |
| 1456 | png_convert_to_rfc1123(png_timep) is provided to convert from PNG |
| 1457 | time to an RFC 1123 format string. |
| 1458 | |
| 1459 | You are now ready to write all the file information up to the actual |
| 1460 | image data. You do this with a call to png_write_info(). |
| 1461 | |
| 1462 | png_write_info(png_ptr, info_ptr); |
| 1463 | |
| 1464 | After you've written the file information, you can set up the library |
| 1465 | to handle any special transformations of the image data. The various |
| 1466 | ways to transform the data will be described in the order that they |
| 1467 | should occur. This is important, as some of these change the color |
| 1468 | type and/or bit depth of the data, and some others only work on |
| 1469 | certain color types and bit depths. Even though each transformation |
| 1470 | checks to see if it has data that it can do something with, you should |
| 1471 | make sure to only enable a transformation if it will be valid for the |
| 1472 | data. For example, don't swap red and blue on grayscale data. |
| 1473 | |
| 1474 | PNG files store RGB pixels packed into 3 or 6 bytes. This code tells |
| 1475 | the library to expand the input data to 4 or 8 bytes per pixel |
| 1476 | (or expand 1 or 2-byte grayscale data to 2 or 4 bytes per pixel). |
| 1477 | |
| 1478 | png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE); |
| 1479 | |
| 1480 | where the 0 is the value that will be put in the 4th byte, and the |
| 1481 | location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending |
| 1482 | upon whether the filler byte is stored XRGB or RGBX. |
| 1483 | |
| 1484 | PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
| 1485 | they can, resulting in, for example, 8 pixels per byte for 1 bit files. |
| 1486 | If the data is supplied at 1 pixel per byte, use this code, which will |
| 1487 | correctly pack the pixels into a single byte: |
| 1488 | |
| 1489 | png_set_packing(png_ptr); |
| 1490 | |
| 1491 | PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your |
| 1492 | data is of another bit depth, you can write an sBIT chunk into the |
| 1493 | file so that decoders can get the original data if desired. |
| 1494 | |
| 1495 | /* Set the true bit depth of the image data */ |
| 1496 | if (color_type & PNG_COLOR_MASK_COLOR) |
| 1497 | { |
| 1498 | sig_bit.red = true_bit_depth; |
| 1499 | sig_bit.green = true_bit_depth; |
| 1500 | sig_bit.blue = true_bit_depth; |
| 1501 | } |
| 1502 | else |
| 1503 | { |
| 1504 | sig_bit.gray = true_bit_depth; |
| 1505 | } |
| 1506 | if (color_type & PNG_COLOR_MASK_ALPHA) |
| 1507 | { |
| 1508 | sig_bit.alpha = true_bit_depth; |
| 1509 | } |
| 1510 | |
| 1511 | png_set_sBIT(png_ptr, info_ptr, &sig_bit); |
| 1512 | |
| 1513 | If the data is stored in the row buffer in a bit depth other than |
| 1514 | one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG), |
| 1515 | this will scale the values to appear to be the correct bit depth as |
| 1516 | is required by PNG. |
| 1517 | |
| 1518 | png_set_shift(png_ptr, &sig_bit); |
| 1519 | |
| 1520 | PNG files store 16 bit pixels in network byte order (big-endian, |
| 1521 | ie. most significant bits first). This code would be used if they are |
| 1522 | supplied the other way (little-endian, i.e. least significant bits |
| 1523 | first, the way PCs store them): |
| 1524 | |
| 1525 | if (bit_depth > 8) |
| 1526 | png_set_swap(png_ptr); |
| 1527 | |
| 1528 | If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you |
| 1529 | need to change the order the pixels are packed into bytes, you can use: |
| 1530 | |
| 1531 | if (bit_depth < 8) |
| 1532 | png_set_packswap(png_ptr); |
| 1533 | |
| 1534 | PNG files store 3 color pixels in red, green, blue order. This code |
| 1535 | would be used if they are supplied as blue, green, red: |
| 1536 | |
| 1537 | png_set_bgr(png_ptr); |
| 1538 | |
| 1539 | PNG files describe monochrome as black being zero and white being |
| 1540 | one. This code would be used if the pixels are supplied with this reversed |
| 1541 | (black being one and white being zero): |
| 1542 | |
| 1543 | png_set_invert_mono(png_ptr); |
| 1544 | |
| 1545 | Finally, you can write your own transformation function if none of |
| 1546 | the existing ones meets your needs. This is done by setting a callback |
| 1547 | with |
| 1548 | |
| 1549 | png_set_write_user_transform_fn(png_ptr, |
| 1550 | write_transform_fn); |
| 1551 | |
| 1552 | You must supply the function |
| 1553 | |
| 1554 | void write_transform_fn(png_ptr ptr, row_info_ptr |
| 1555 | row_info, png_bytep data) |
| 1556 | |
| 1557 | See pngtest.c for a working example. Your function will be called |
| 1558 | before any of the other transformations have been processed. |
| 1559 | |
| 1560 | It is possible to have libpng flush any pending output, either manually, |
| 1561 | or automatically after a certain number of lines have been written. To |
| 1562 | flush the output stream a single time call: |
| 1563 | |
| 1564 | png_write_flush(png_ptr); |
| 1565 | |
| 1566 | and to have libpng flush the output stream periodically after a certain |
| 1567 | number of scanlines have been written, call: |
| 1568 | |
| 1569 | png_set_flush(png_ptr, nrows); |
| 1570 | |
| 1571 | Note that the distance between rows is from the last time png_write_flush() |
| 1572 | was called, or the first row of the image if it has never been called. |
| 1573 | So if you write 50 lines, and then png_set_flush 25, it will flush the |
| 1574 | output on the next scanline, and every 25 lines thereafter, unless |
| 1575 | png_write_flush() is called before 25 more lines have been written. |
| 1576 | If nrows is too small (less than about 10 lines for a 640 pixel wide |
| 1577 | RGB image) the image compression may decrease noticeably (although this |
| 1578 | may be acceptable for real-time applications). Infrequent flushing will |
| 1579 | only degrade the compression performance by a few percent over images |
| 1580 | that do not use flushing. |
| 1581 | |
| 1582 | That's it for the transformations. Now you can write the image data. |
| 1583 | The simplest way to do this is in one function call. If have the |
| 1584 | whole image in memory, you can just call png_write_image() and libpng |
| 1585 | will write the image. You will need to pass in an array of pointers to |
| 1586 | each row. This function automatically handles interlacing, so you don't |
| 1587 | need to call png_set_interlace_handling() or call this function multiple |
| 1588 | times, or any of that other stuff necessary with png_write_rows(). |
| 1589 | |
| 1590 | png_write_image(png_ptr, row_pointers); |
| 1591 | |
| 1592 | where row_pointers is: |
| 1593 | |
| 1594 | png_byte *row_pointers[height]; |
| 1595 | |
| 1596 | You can point to void or char or whatever you use for pixels. |
| 1597 | |
| 1598 | If you don't want to write the whole image at once, you can |
| 1599 | use png_write_rows() instead. If the file is not interlaced, |
| 1600 | this is simple: |
| 1601 | |
| 1602 | png_write_rows(png_ptr, row_pointers, |
| 1603 | number_of_rows); |
| 1604 | |
| 1605 | row_pointers is the same as in the png_write_image() call. |
| 1606 | |
| 1607 | If you are just writing one row at a time, you can do this with |
| 1608 | row_pointers: |
| 1609 | |
| 1610 | png_bytep row_pointer = row; |
| 1611 | |
| 1612 | png_write_row(png_ptr, &row_pointer); |
| 1613 | |
| 1614 | When the file is interlaced, things can get a good deal more |
| 1615 | complicated. The only currently (as of February 1998 -- PNG Specification |
| 1616 | version 1.0, dated October 1996) defined interlacing scheme for PNG files |
| 1617 | is the "Adam7" interlace scheme, that breaks down an |
| 1618 | image into seven smaller images of varying size. libpng will build |
| 1619 | these images for you, or you can do them yourself. If you want to |
| 1620 | build them yourself, see the PNG specification for details of which |
| 1621 | pixels to write when. |
| 1622 | |
| 1623 | If you don't want libpng to handle the interlacing details, just |
| 1624 | use png_set_interlace_handling() and call png_write_rows() the |
| 1625 | correct number of times to write all seven sub-images. |
| 1626 | |
| 1627 | If you want libpng to build the sub-images, call this before you start |
| 1628 | writing any rows: |
| 1629 | |
| 1630 | number_of_passes = |
| 1631 | png_set_interlace_handling(png_ptr); |
| 1632 | |
| 1633 | This will return the number of passes needed. Currently, this |
| 1634 | is seven, but may change if another interlace type is added. |
| 1635 | |
| 1636 | Then write the complete image number_of_passes times. |
| 1637 | |
| 1638 | png_write_rows(png_ptr, row_pointers, |
| 1639 | number_of_rows); |
| 1640 | |
| 1641 | As some of these rows are not used, and thus return immediately, |
| 1642 | you may want to read about interlacing in the PNG specification, |
| 1643 | and only update the rows that are actually used. |
| 1644 | |
| 1645 | After you are finished writing the image, you should finish writing |
| 1646 | the file. If you are interested in writing comments or time, you should |
| 1647 | pass an appropriately filled png_info pointer. If you are not interested, |
| 1648 | you can pass NULL. |
| 1649 | |
| 1650 | png_write_end(png_ptr, info_ptr); |
| 1651 | |
| 1652 | When you are done, you can free all memory used by libpng like this: |
| 1653 | |
| 1654 | png_destroy_write_struct(&png_ptr, &info_ptr); |
| 1655 | |
| 1656 | You must free any data you allocated for info_ptr, such as comments, |
| 1657 | palette, or histogram, before the call to png_destroy_write_struct(); |
| 1658 | |
| 1659 | For a more compact example of writing a PNG image, see the file example.c. |
| 1660 | |
| 1661 | |
| 1662 | V. Modifying/Customizing libpng: |
| 1663 | |
| 1664 | There are two issues here. The first is changing how libpng does |
| 1665 | standard things like memory allocation, input/output, and error handling. |
| 1666 | The second deals with more complicated things like adding new chunks, |
| 1667 | adding new transformations, and generally changing how libpng works. |
| 1668 | |
| 1669 | All of the memory allocation, input/output, and error handling in libpng |
| 1670 | goes through callbacks that are user settable. The default routines are |
| 1671 | in pngmem.c, pngrio.c, pngwio.c, and pngerror.c respectively. To change |
| 1672 | these functions, call the appropriate png_set_???_fn() function. |
| 1673 | |
| 1674 | Memory allocation is done through the functions png_large_malloc(), |
| 1675 | png_malloc(), png_realloc(), png_large_free(), and png_free(). These |
| 1676 | currently just call the standard C functions. The large functions must |
| 1677 | handle exactly 64K, but they don't have to handle more than that. If |
| 1678 | your pointers can't access more then 64K at a time, you will want to set |
| 1679 | MAXSEG_64K in zlib.h. Since it is unlikely that the method of handling |
| 1680 | memory allocation on a platform will change between applications, these |
| 1681 | functions must be modified in the library at compile time. |
| 1682 | |
| 1683 | Input/Output in libpng is done through png_read() and png_write(), |
| 1684 | which currently just call fread() and fwrite(). The FILE * is stored in |
| 1685 | png_struct and is initialized via png_init_io(). If you wish to change |
| 1686 | the method of I/O, the library supplies callbacks that you can set |
| 1687 | through the function png_set_read_fn() and png_set_write_fn() at run |
| 1688 | time, instead of calling the png_init_io() function. These functions |
| 1689 | also provide a void pointer that can be retrieved via the function |
| 1690 | png_get_io_ptr(). For example: |
| 1691 | |
| 1692 | png_set_read_fn(png_structp read_ptr, |
| 1693 | voidp read_io_ptr, png_rw_ptr read_data_fn) |
| 1694 | |
| 1695 | png_set_write_fn(png_structp write_ptr, |
| 1696 | voidp write_io_ptr, png_rw_ptr write_data_fn, |
| 1697 | png_flush_ptr output_flush_fn); |
| 1698 | |
| 1699 | voidp read_io_ptr = png_get_io_ptr(read_ptr); |
| 1700 | voidp write_io_ptr = png_get_io_ptr(write_ptr); |
| 1701 | |
| 1702 | The replacement I/O functions should have prototypes as follows: |
| 1703 | |
| 1704 | void user_read_data(png_structp png_ptr, |
| 1705 | png_bytep data, png_uint_32 length); |
| 1706 | void user_write_data(png_structp png_ptr, |
| 1707 | png_bytep data, png_uint_32 length); |
| 1708 | void user_flush_data(png_structp png_ptr); |
| 1709 | |
| 1710 | Supplying NULL for the read, write, or flush functions sets them back |
| 1711 | to using the default C stream functions. It is an error to read from |
| 1712 | a write stream, and vice versa. |
| 1713 | |
| 1714 | Error handling in libpng is done through png_error() and png_warning(). |
| 1715 | Errors handled through png_error() are fatal, meaning that png_error() |
| 1716 | should never return to its caller. Currently, this is handled via |
| 1717 | setjmp() and longjmp(), but you could change this to do things like |
| 1718 | exit() if you should wish. On non-fatal errors, png_warning() is called |
| 1719 | to print a warning message, and then control returns to the calling code. |
| 1720 | By default png_error() and png_warning() print a message on stderr via |
| 1721 | fprintf() unless the library is compiled with PNG_NO_STDIO defined. If |
| 1722 | you wish to change the behavior of the error functions, you will need to |
| 1723 | set up your own message callbacks. These functions are normally supplied |
| 1724 | at the time that the png_struct is created. It is also possible to change |
| 1725 | these functions after png_create_???_struct() has been called by calling: |
| 1726 | |
| 1727 | png_set_error_fn(png_structp png_ptr, |
| 1728 | png_voidp error_ptr, png_error_ptr error_fn, |
| 1729 | png_error_ptr warning_fn); |
| 1730 | |
| 1731 | png_voidp error_ptr = png_get_error_ptr(png_ptr); |
| 1732 | |
| 1733 | If NULL is supplied for either error_fn or warning_fn, then the libpng |
| 1734 | default function will be used, calling fprintf() and/or longjmp() if a |
| 1735 | problem is encountered. The replacement error functions should have |
| 1736 | parameters as follows: |
| 1737 | |
| 1738 | void user_error_fn(png_structp png_ptr, |
| 1739 | png_const_charp error_msg); |
| 1740 | void user_warning_fn(png_structp png_ptr, |
| 1741 | png_const_charp warning_msg); |
| 1742 | |
| 1743 | The motivation behind using setjmp() and longjmp() is the C++ throw and |
| 1744 | catch exception handling methods. This makes the code much easier to write, |
| 1745 | as there is no need to check every return code of every function call. |
| 1746 | However, there are some uncertainties about the status of local variables |
| 1747 | after a longjmp, so the user may want to be careful about doing anything after |
| 1748 | setjmp returns non-zero besides returning itself. Consult your compiler |
| 1749 | documentation for more details. |
| 1750 | |
| 1751 | If you need to read or write custom chunks, you will need to get deeper |
| 1752 | into the libpng code, as a mechanism has not yet been supplied for user |
| 1753 | callbacks with custom chunks. First, read the PNG specification, and have |
| 1754 | a first level of understanding of how it works. Pay particular attention |
| 1755 | to the sections that describe chunk names, and look at how other chunks |
| 1756 | were designed, so you can do things similarly. Second, check out the |
| 1757 | sections of libpng that read and write chunks. Try to find a chunk that |
| 1758 | is similar to yours and copy off of it. More details can be found in the |
| 1759 | comments inside the code. A way of handling unknown chunks in a generic |
| 1760 | method, potentially via callback functions, would be best. |
| 1761 | |
| 1762 | If you wish to write your own transformation for the data, look through |
| 1763 | the part of the code that does the transformations, and check out some of |
| 1764 | the simpler ones to get an idea of how they work. Try to find a similar |
| 1765 | transformation to the one you want to add and copy off of it. More details |
| 1766 | can be found in the comments inside the code itself. |
| 1767 | |
| 1768 | Configuring for 16 bit platforms: |
| 1769 | |
| 1770 | You may need to change the png_large_malloc() and png_large_free() |
| 1771 | routines in pngmem.c, as these are required to allocate 64K, although |
| 1772 | there is already support for many of the common DOS compilers. Also, |
| 1773 | you will want to look into zconf.h to tell zlib (and thus libpng) that |
| 1774 | it cannot allocate more then 64K at a time. Even if you can, the memory |
| 1775 | won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K. |
| 1776 | |
| 1777 | Configuring for DOS: |
| 1778 | |
| 1779 | For DOS users who only have access to the lower 640K, you will |
| 1780 | have to limit zlib's memory usage via a png_set_compression_mem_level() |
| 1781 | call. See zlib.h or zconf.h in the zlib library for more information. |
| 1782 | |
| 1783 | Configuring for Medium Model: |
| 1784 | |
| 1785 | Libpng's support for medium model has been tested on most of the popular |
| 1786 | compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets |
| 1787 | defined, and FAR gets defined to far in pngconf.h, and you should be |
| 1788 | all set. Everything in the library (except for zlib's structure) is |
| 1789 | expecting far data. You must use the typedefs with the p or pp on |
| 1790 | the end for pointers (or at least look at them and be careful). Make |
| 1791 | note that the row's of data are defined as png_bytepp, which is an |
| 1792 | unsigned char far * far *. |
| 1793 | |
| 1794 | Configuring for gui/windowing platforms: |
| 1795 | |
| 1796 | You will need to write new error and warning functions that use the GUI |
| 1797 | interface, as described previously, and set them to be the error and |
| 1798 | warning functions at the time that png_create_???_struct() is called, |
| 1799 | in order to have them available during the structure initialization. |
| 1800 | They can be changed later via png_set_error_fn(). On some compilers, |
| 1801 | you may also have to change the memory allocators (png_malloc, etc.). |
| 1802 | |
| 1803 | Configuring for compiler xxx: |
| 1804 | |
| 1805 | All includes for libpng are in pngconf.h. If you need to add/change/delete |
| 1806 | an include, this is the place to do it. The includes that are not |
| 1807 | needed outside libpng are protected by the PNG_INTERNAL definition, |
| 1808 | which is only defined for those routines inside libpng itself. The |
| 1809 | files in libpng proper only include png.h, which includes pngconf.h. |
| 1810 | |
| 1811 | Configuring zlib: |
| 1812 | |
| 1813 | There are special functions to configure the compression. Perhaps the |
| 1814 | most useful one changes the compression level, which currently uses |
| 1815 | input compression values in the range 0 - 9. The library normally |
| 1816 | uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests |
| 1817 | have shown that for a large majority of images, compression values in |
| 1818 | the range 3-6 compress nearly as well as higher levels, and do so much |
| 1819 | faster. For online applications it may be desirable to have maximum speed |
| 1820 | (Z_BEST_SPEED = 1). With versions of zlib after v0.99, you can also |
| 1821 | specify no compression (Z_NO_COMPRESSION = 0), but this would create |
| 1822 | files larger than just storing the raw bitmap. You can specify the |
| 1823 | compression level by calling: |
| 1824 | |
| 1825 | png_set_compression_level(png_ptr, level); |
| 1826 | |
| 1827 | Another useful one is to reduce the memory level used by the library. |
| 1828 | The memory level defaults to 8, but it can be lowered if you are |
| 1829 | short on memory (running DOS, for example, where you only have 640K). |
| 1830 | |
| 1831 | png_set_compression_mem_level(png_ptr, level); |
| 1832 | |
| 1833 | The other functions are for configuring zlib. They are not recommended |
| 1834 | for normal use and may result in writing an invalid PNG file. See |
| 1835 | zlib.h for more information on what these mean. |
| 1836 | |
| 1837 | png_set_compression_strategy(png_ptr, |
| 1838 | strategy); |
| 1839 | png_set_compression_window_bits(png_ptr, |
| 1840 | window_bits); |
| 1841 | png_set_compression_method(png_ptr, method); |
| 1842 | |
| 1843 | Controlling row filtering: |
| 1844 | |
| 1845 | If you want to control whether libpng uses filtering or not, which |
| 1846 | filters are used, and how it goes about picking row filters, you |
| 1847 | can call one of these functions. The selection and configuration |
| 1848 | of row filters can have a significant impact on the size and |
| 1849 | encoding speed and a somewhat lesser impact on the decoding speed |
| 1850 | of an image. Filtering is enabled by default for RGB and grayscale |
| 1851 | images (with and without alpha), but not for paletted images nor |
| 1852 | for any images with bit depths less than 8 bits/pixel. |
| 1853 | |
| 1854 | The 'method' parameter sets the main filtering method, which is |
| 1855 | currently only '0' in the PNG 1.0 specification. The 'filters' |
| 1856 | parameter sets which filter(s), if any, should be used for each |
| 1857 | scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS |
| 1858 | to turn filtering on and off, respectively. |
| 1859 | |
| 1860 | Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB, |
| 1861 | PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise |
| 1862 | ORed together '|' to specify one or more filters to use. These |
| 1863 | filters are described in more detail in the PNG specification. If |
| 1864 | you intend to change the filter type during the course of writing |
| 1865 | the image, you should start with flags set for all of the filters |
| 1866 | you intend to use so that libpng can initialize its internal |
| 1867 | structures appropriately for all of the filter types. |
| 1868 | |
| 1869 | filters = PNG_FILTER_NONE | PNG_FILTER_SUB |
| 1870 | | PNG_FILTER_UP; |
| 1871 | png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE, |
| 1872 | filters); |
| 1873 | |
| 1874 | It is also possible to influence how libpng chooses from among the |
| 1875 | available filters. This is done in two ways - by telling it how |
| 1876 | important it is to keep the same filter for successive rows, and |
| 1877 | by telling it the relative computational costs of the filters. |
| 1878 | |
| 1879 | double weights[3] = {1.5, 1.3, 1.1}, |
| 1880 | costs[PNG_FILTER_VALUE_LAST] = |
| 1881 | {1.0, 1.3, 1.3, 1.5, 1.7}; |
| 1882 | |
| 1883 | png_set_filter_selection(png_ptr, |
| 1884 | PNG_FILTER_SELECTION_WEIGHTED, 3, |
| 1885 | weights, costs); |
| 1886 | |
| 1887 | The weights are multiplying factors that indicate to libpng that the |
| 1888 | row filter should be the same for successive rows unless another row filter |
| 1889 | is that many times better than the previous filter. In the above example, |
| 1890 | if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a |
| 1891 | "sum of absolute differences" 1.5 x 1.3 times higher than other filters |
| 1892 | and still be chosen, while the NONE filter could have a sum 1.1 times |
| 1893 | higher than other filters and still be chosen. Unspecified weights are |
| 1894 | taken to be 1.0, and the specified weights should probably be declining |
| 1895 | like those above in order to emphasize recent filters over older filters. |
| 1896 | |
| 1897 | The filter costs specify for each filter type a relative decoding cost |
| 1898 | to be considered when selecting row filters. This means that filters |
| 1899 | with higher costs are less likely to be chosen over filters with lower |
| 1900 | costs, unless their "sum of absolute differences" is that much smaller. |
| 1901 | The costs do not necessarily reflect the exact computational speeds of |
| 1902 | the various filters, since this would unduly influence the final image |
| 1903 | size. |
| 1904 | |
| 1905 | Note that the numbers above were invented purely for this example and |
| 1906 | are given only to help explain the function usage. Little testing has |
| 1907 | been done to find optimum values for either the costs or the weights. |
| 1908 | |
| 1909 | Removing unwanted object code: |
| 1910 | |
| 1911 | There are a bunch of #define's in pngconf.h that control what parts of |
| 1912 | libpng are compiled. All the defines end in _SUPPORTED. If you are |
| 1913 | never going to use a capability, you can change the #define to #undef |
| 1914 | before recompiling libpng and save yourself code and data space, or |
| 1915 | you can turn off individual capabilities with defines that begin with |
| 1916 | PNG_NO_. |
| 1917 | |
| 1918 | You can also turn all of the transforms and ancillary chunk capabilities |
| 1919 | off en masse with compiler directives that define |
| 1920 | PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS, |
| 1921 | or all four, |
| 1922 | along with directives to turn on any of the capabilities that you do |
| 1923 | want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable |
| 1924 | the extra transformations but still leave the library fully capable of reading |
| 1925 | and writing PNG files with all known public chunks [except for sPLT]. |
| 1926 | Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive |
| 1927 | produces a library that is incapable of reading or writing ancillary chunks. |
| 1928 | If you are not using the progressive reading capability, you can |
| 1929 | turn that off with PNG_NO_PROGRESSIVE_READ (don't confuse |
| 1930 | this with the INTERLACING capability, which you'll still have). |
| 1931 | |
| 1932 | All the reading and writing specific code are in separate files, so the |
| 1933 | linker should only grab the files it needs. However, if you want to |
| 1934 | make sure, or if you are building a stand alone library, all the |
| 1935 | reading files start with pngr and all the writing files start with |
| 1936 | pngw. The files that don't match either (like png.c, pngtrans.c, etc.) |
| 1937 | are used for both reading and writing, and always need to be included. |
| 1938 | The progressive reader is in pngpread.c |
| 1939 | |
| 1940 | If you are creating or distributing a dynamically linked library (a .so |
| 1941 | or DLL file), you should not remove or disable any parts of the library, |
| 1942 | as this will cause applications linked with different versions of the |
| 1943 | library to fail if they call functions not available in your library. |
| 1944 | The size of the library itself should not be an issue, because only |
| 1945 | those sections that are actually used will be loaded into memory. |
| 1946 | |
| 1947 | Requesting debug printout: |
| 1948 | |
| 1949 | The macro definition PNG_DEBUG can be used to request debugging |
| 1950 | printout. Set it to an integer value in the range 0 to 3. Higher |
| 1951 | numbers result in increasing amounts of debugging information. The |
| 1952 | information is printed to the "stderr" file, unless another file |
| 1953 | name is specified in the PNG_DEBUG_FILE macro definition. |
| 1954 | |
| 1955 | When PNG_DEBUG > 0, the following functions (macros) become available: |
| 1956 | |
| 1957 | png_debug(level, message) |
| 1958 | png_debug1(level, message, p1) |
| 1959 | png_debug2(level, message, p1, p2) |
| 1960 | |
| 1961 | in which "level" is compared to PNG_DEBUG to decide whether to print |
| 1962 | the message, "message" is the formatted string to be printed, |
| 1963 | and p1 and p2 are parameters that are to be embedded in the string |
| 1964 | according to printf-style formatting directives. For example, |
| 1965 | |
| 1966 | png_debug1(2, "foo=%d\n", foo); |
| 1967 | |
| 1968 | is expanded to |
| 1969 | |
| 1970 | if(PNG_DEBUG > 2) |
| 1971 | fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo); |
| 1972 | |
| 1973 | When PNG_DEBUG is defined but is zero, the macros aren't defined, but you |
| 1974 | can still use PNG_DEBUG to control your own debugging: |
| 1975 | |
| 1976 | #ifdef PNG_DEBUG |
| 1977 | fprintf(stderr, ... |
| 1978 | #endif |
| 1979 | |
| 1980 | When PNG_DEBUG = 1, the macros are defined, but only png_debug statements |
| 1981 | having level = 0 will be printed. There aren't any such statements in |
| 1982 | this version of libpng, but if you insert some they will be printed. |
| 1983 | |
| 1984 | VI. Changes to Libpng from version 0.88 |
| 1985 | |
| 1986 | It should be noted that versions of libpng later than 0.96 are not |
| 1987 | distributed by the original libpng author, Guy Schalnat, nor by |
| 1988 | Andreas Dilger, who had taken over from Guy during 1996 and 1997, and |
| 1989 | distributed versions 0.89 through 0.96, but rather by another member |
| 1990 | of the original PNG Group, Glenn Randers-Pehrson. Guy and Andreas are |
| 1991 | still alive and well, but they have moved on to other things. |
| 1992 | |
| 1993 | The old libpng functions png_read_init(), png_write_init(), |
| 1994 | png_info_init(), png_read_destroy(), and png_write_destory() have been |
| 1995 | moved to PNG_INTERNAL in version 0.95 to discourage their use. The |
| 1996 | preferred method of creating and initializing the libpng structures is |
| 1997 | via the png_create_read_struct(), png_create_write_struct(), and |
| 1998 | png_create_info_struct() because they isolate the size of the structures |
| 1999 | from the application, allow version error checking, and also allow the |
| 2000 | use of custom error handling routines during the initialization, which |
| 2001 | the old functions do not. The functions png_read_destroy() and |
| 2002 | png_write_destroy() do not actually free the memory that libpng |
| 2003 | allocated for these structs, but just reset the data structures, so they |
| 2004 | can be used instead of png_destroy_read_struct() and |
| 2005 | png_destroy_write_struct() if you feel there is too much system overhead |
| 2006 | allocating and freeing the png_struct for each image read. |
| 2007 | |
| 2008 | Setting the error callbacks via png_set_message_fn() before |
| 2009 | png_read_init() as was suggested in libpng-0.88 is no longer supported |
| 2010 | because this caused applications that do not use custom error functions |
| 2011 | to fail if the png_ptr was not initialized to zero. It is still possible |
| 2012 | to set the error callbacks AFTER png_read_init(), or to change them with |
| 2013 | png_set_error_fn(), which is essentially the same function, but with a |
| 2014 | new name to force compilation errors with applications that try to use |
| 2015 | the old method. |
| 2016 | |
| 2017 | VII. Y2K Compliance in libpng |
| 2018 | |
| 2019 | January 13, 1999 |
| 2020 | |
| 2021 | Since the PNG Development group is an ad-hoc body, we can't make |
| 2022 | an official declaration. |
| 2023 | |
| 2024 | This is your unofficial assurance that libpng from version 0.81 and |
| 2025 | upward are Y2K compliant. It is my belief that earlier versions were |
| 2026 | also Y2K compliant. |
| 2027 | |
| 2028 | Libpng only has three year fields. One is a 2-byte unsigned integer that |
| 2029 | will hold years up to 65535. The other two hold the date in text |
| 2030 | format, and will hold years up to 9999. |
| 2031 | |
| 2032 | The integer is |
| 2033 | "png_uint_16 year" in png_time_struct. |
| 2034 | |
| 2035 | The strings are |
| 2036 | "png_charp time_buffer" in png_struct and |
| 2037 | "near_time_buffer", which is a local character string in png.c. |
| 2038 | |
| 2039 | There are seven time-related functions: |
| 2040 | |
| 2041 | png_convert_to_rfc_1123() in png.c |
| 2042 | (formerly png_convert_to_rfc_1152() in error) |
| 2043 | png_convert_from_struct_tm() in pngwrite.c, called in pngwrite.c |
| 2044 | png_convert_from_time_t() in pngwrite.c |
| 2045 | png_get_tIME() in pngget.c |
| 2046 | png_handle_tIME() in pngrutil.c, called in pngread.c |
| 2047 | png_set_tIME() in pngset.c |
| 2048 | png_write_tIME() in pngwutil.c, called in pngwrite.c |
| 2049 | |
| 2050 | All appear to handle dates properly in a Y2K environment. The |
| 2051 | png_convert_from_time_t() function calls gmtime() to convert from system |
| 2052 | clock time, which returns (year - 1900), which we properly convert to |
| 2053 | the full 4-digit year. There is a possibility that applications using |
| 2054 | libpng are not passing 4-digit years into the png_convert_to_rfc_1123() |
| 2055 | function, or incorrectly passing only a 2-digit year instead of |
| 2056 | "year - 1900" into the png_convert_from_struct_tm() function, but this |
| 2057 | is not under our control. The libpng documentation has always stated |
| 2058 | that it works with 4-digit years, and the APIs have been documented as |
| 2059 | such. |
| 2060 | |
| 2061 | The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned |
| 2062 | integer to hold the year, and can hold years as large as 65535. |
| 2063 | |
| 2064 | |
| 2065 | Glenn Randers-Pehrson |
| 2066 | libpng maintainer |
| 2067 | PNG Development Group |