libpng.txt - A description on how to use and modify libpng
- libpng version 1.0.3 - January 14, 1999
+ libpng version 1.2.7 - September 12, 2004
Updated and distributed by Glenn Randers-Pehrson
- <randeg@alumni.rpi.edu>
- Copyright (c) 1998, 1999 Glenn Randers-Pehrson
+ <glennrp@users.sourceforge.net>
+ Copyright (c) 1998-2004 Glenn Randers-Pehrson
For conditions of distribution and use, see copyright
notice in png.h.
Schalnat, Group 42, Inc.
Updated/rewritten per request in the libpng FAQ
- Copyright (c) 1995 Frank J. T. Wojcik
- December 18, 1995 && January 20, 1996
+ Copyright (c) 1995, 1996 Frank J. T. Wojcik
+ December 18, 1995 & January 20, 1996
I. Introduction
Libpng was written as a companion to the PNG specification, as a way
of reducing the amount of time and effort it takes to support the PNG
-file format in application programs. The PNG specification is available
-as RFC 2083 <ftp://ftp.uu.net/graphics/png/documents/> and as a
+file format in application programs.
+
+The PNG specification (second edition), November 2003, is available as
+a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2003 (E)) at
+<http://www.w3.org/TR/2003/REC-PNG-20031110/
+The W3C and ISO documents have identical technical content.
+
+The PNG-1.2 specification is available at
+<http://www.libpng.org/pub/png/documents/>
+
+The PNG-1.0 specification is available
+as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a
W3C Recommendation <http://www.w3.org/TR/REC.png.html>. Some
additional chunks are described in the special-purpose public chunks
-documents at <ftp://ftp.uu.net/graphics/png/documents/>. Other information
+documents at <http://www.libpng.org/pub/png/documents/>.
+
+Other information
about PNG, and the latest version of libpng, can be found at the PNG home
-page, <http://www.cdrom.com/pub/png/>.
+page, <http://www.libpng.org/pub/png/>.
Most users will not have to modify the library significantly; advanced
users may want to modify it more. All attempts were made to make it as
Libpng uses zlib for its compression and decompression of PNG files.
Further information about zlib, and the latest version of zlib, can
-be found at the zlib home page, <http://www.cdrom.com/pub/infozip/zlib/>.
+be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
The zlib compression utility is a general purpose utility that is
useful for more than PNG files, and can be used without libpng.
See the documentation delivered with zlib for more details.
instances of the structures. Each thread should have its own
png_struct and png_info instances, and thus its own image.
Libpng does not protect itself against two threads using the
-same instance of a structure.
-
+same instance of a structure. Note: thread safety may be defeated
+by use of some of the MMX assembler code in pnggccrd.c, which is only
+compiled when the user defines PNG_THREAD_UNSAFE_OK.
II. Structures
PNG file. At one time, the fields of png_info were intended to be
directly accessible to the user. However, this tended to cause problems
with applications using dynamically loaded libraries, and as a result
-a set of interface functions for png_info was developed. The fields
-of png_info are still available for older applications, but it is
-suggested that applications use the new interfaces if at all possible.
+a set of interface functions for png_info (the png_get_*() and png_set_*()
+functions) was developed. The fields of png_info are still available for
+older applications, but it is suggested that applications use the new
+interfaces if at all possible.
+
+Applications that do make direct access to the members of png_struct (except
+for png_ptr->jmpbuf) must be recompiled whenever the library is updated,
+and applications that make direct access to the members of png_info must
+be recompiled if they were compiled or loaded with libpng version 1.0.6,
+in which the members were in a different order. In version 1.0.7, the
+members of the png_info structure reverted to the old order, as they were
+in versions 0.97c through 1.0.5. Starting with version 2.0.0, both
+structures are going to be hidden, and the contents of the structures will
+only be accessible through the png_get/png_set functions.
The png.h header file is an invaluable reference for programming with libpng.
And while I'm on the topic, make sure you include the libpng header file:
III. Reading
-Reading PNG files:
-
We'll now walk you through the possible functions to call when reading
-in a PNG file, briefly explaining the syntax and purpose of each one.
-See example.c and png.h for more detail. While Progressive reading
-is covered in the next section, you will still need some of the
-functions discussed in this section to read a PNG file.
+in a PNG file sequentially, briefly explaining the syntax and purpose
+of each one. See example.c and png.h for more detail. While
+progressive reading is covered in the next section, you will still
+need some of the functions discussed in this section to read a PNG
+file.
+
+Setup
You will want to do the I/O initialization(*) before you get into libpng,
so if it doesn't work, you don't have much to undo. Of course, you
will also want to insure that you are, in fact, dealing with a PNG
file. Libpng provides a simple check to see if a file is a PNG file.
-To use it, pass in the first 1 to 8 bytes of the file, and it will
-return true or false (1 or 0) depending on whether the bytes could be
-part of a PNG file. Of course, the more bytes you pass in, the
-greater the accuracy of the prediction.
+To use it, pass in the first 1 to 8 bytes of the file to the function
+png_sig_cmp(), and it will return 0 if the bytes match the corresponding
+bytes of the PNG signature, or nonzero otherwise. Of course, the more bytes
+you pass in, the greater the accuracy of the prediction.
If you are intending to keep the file pointer open for use in libpng,
you must ensure you don't read more than 8 bytes from the beginning
FILE *fp = fopen(file_name, "rb");
if (!fp)
{
- return;
+ return (ERROR);
}
fread(header, 1, number, fp);
is_png = !png_sig_cmp(header, 0, number);
if (!is_png)
{
- return;
+ return (NOT_PNG);
}
use by the error functions, if necessary (the pointer and functions can
be NULL if the default error handlers are to be used). See the section
on Changes to Libpng below regarding the old initialization functions.
+The structure allocation functions quietly return NULL if they fail to
+create the structure, so your application should check for that.
png_structp png_ptr = png_create_read_struct
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
- return;
+ return (ERROR);
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_read_struct(&png_ptr,
(png_infopp)NULL, (png_infopp)NULL);
- return;
+ return (ERROR);
}
png_infop end_info = png_create_info_struct(png_ptr);
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
- return;
+ return (ERROR);
}
If you want to use your own memory allocation routines,
When libpng encounters an error, it expects to longjmp back
to your routine. Therefore, you will need to call setjmp and pass
-your png_ptr->jmpbuf. If you read the file from different
+your png_jmpbuf(png_ptr). If you read the file from different
routines, you will need to update the jmpbuf field every time you enter
-a new routine that will call a png_ function.
+a new routine that will call a png_*() function.
See your documentation of setjmp/longjmp for your compiler for more
-handling in the Customizing Libpng section below for more information on
-the libpng error handling. If an error occurs, and libpng longjmp's
+information on setjmp/longjmp. See the discussion on libpng error
+handling in the Customizing Libpng section below for more information
+on the libpng error handling. If an error occurs, and libpng longjmp's
back to your setjmp, you will want to call png_destroy_read_struct() to
free any memory.
- if (setjmp(png_ptr->jmpbuf))
+ if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
&end_info);
fclose(fp);
- return;
+ return (ERROR);
}
+If you would rather avoid the complexity of setjmp/longjmp issues,
+you can compile libpng with PNG_SETJMP_NOT_SUPPORTED, in which case
+errors will result in a call to PNG_ABORT() which defaults to abort().
+
Now you need to set up the input code. The default for libpng is to
use the C function fread(). If you use this, you will need to pass a
valid FILE * in the function png_init_io(). Be sure that the file is
png_set_sig_bytes(png_ptr, number);
+Setting up callback code
+
+You can set up a callback function to handle any unknown chunks in the
+input stream. You must supply the function
+
+ read_chunk_callback(png_ptr ptr,
+ png_unknown_chunkp chunk);
+ {
+ /* The unknown chunk structure contains your
+ chunk data: */
+ png_byte name[5];
+ png_byte *data;
+ png_size_t size;
+ /* Note that libpng has already taken care of
+ the CRC handling */
+
+ /* put your code here. Return one of the
+ following: */
+
+ return (-n); /* chunk had an error */
+ return (0); /* did not recognize */
+ return (n); /* success */
+ }
+
+(You can give your function another name that you like instead of
+"read_chunk_callback")
+
+To inform libpng about your function, use
+
+ png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
+ read_chunk_callback);
+
+This names not only the callback function, but also a user pointer that
+you can retrieve with
+
+ png_get_user_chunk_ptr(png_ptr);
+
At this point, you can set up a callback function that will be
called after each row has been read, which you can use to control
a progress meter or the like. It's demonstrated in pngtest.c.
You must supply a function
- void read_row_callback(png_ptr, png_uint_32 row, int pass);
+ void read_row_callback(png_ptr ptr, png_uint_32 row,
+ int pass);
{
/* put your code here */
}
png_set_read_status_fn(png_ptr, read_row_callback);
-In PNG files, the alpha channel in an image is the level of opacity.
-If you need the alpha channel in an image to be the level of transparency
-instead of opacity, you can invert the alpha channel (or the tRNS chunk
-data) after it's read, so that 0 is fully opaque and 255 (in 8-bit or
-paletted images) or 65535 (in 16-bit images) is fully transparent, with
+Width and height limits
+
+The PNG specification allows the width and height of an image to be as
+large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
+Since very few applications really need to process such large images,
+we have imposed an arbitrary 1-million limit on rows and columns.
+Larger images will be rejected immediately with a png_error() call. If
+you wish to override this limit, you can use
+
+ png_set_user_limits(png_ptr, width_max, height_max);
+
+to set your own limits, or use width_max = height_max = 0x7fffffffL
+to allow all valid dimensions (libpng may reject some very large images
+anyway because of potential buffer overflow conditions).
+
+You should put this statement after you create the PNG structure and
+before calling png_read_info(), png_read_png(), or png_process_data().
+If you need to retrieve the limits that are being applied, use
+
+ width_max = png_get_user_width_max(png_ptr);
+ height_max = png_get_user_height_max(png_ptr);
+
+Unknown-chunk handling
+
+Now you get to set the way the library processes unknown chunks in the
+input PNG stream. Both known and unknown chunks will be read. Normal
+behavior is that known chunks will be parsed into information in
+various info_ptr members; unknown chunks will be discarded. To change
+this, you can call:
+
+ png_set_keep_unknown_chunks(png_ptr, keep,
+ chunk_list, num_chunks);
+ keep - 0: do not handle as unknown
+ 1: do not keep
+ 2: keep only if safe-to-copy
+ 3: keep even if unsafe-to-copy
+ You can use these definitions:
+ PNG_HANDLE_CHUNK_AS_DEFAULT 0
+ PNG_HANDLE_CHUNK_NEVER 1
+ PNG_HANDLE_CHUNK_IF_SAFE 2
+ PNG_HANDLE_CHUNK_ALWAYS 3
+ chunk_list - list of chunks affected (a byte string,
+ five bytes per chunk, NULL or '\0' if
+ num_chunks is 0)
+ num_chunks - number of chunks affected; if 0, all
+ unknown chunks are affected. If nonzero,
+ only the chunks in the list are affected
+
+Unknown chunks declared in this way will be saved as raw data onto a
+list of png_unknown_chunk structures. If a chunk that is normally
+known to libpng is named in the list, it will be handled as unknown,
+according to the "keep" directive. If a chunk is named in successive
+instances of png_set_keep_unknown_chunks(), the final instance will
+take precedence. The IHDR and IEND chunks should not be named in
+chunk_list; if they are, libpng will process them normally anyway.
+
+The high-level read interface
+
+At this point there are two ways to proceed; through the high-level
+read interface, or through a sequence of low-level read operations.
+You can use the high-level interface if (a) you are willing to read
+the entire image into memory, and (b) the input transformations
+you want to do are limited to the following set:
+
+ PNG_TRANSFORM_IDENTITY No transformation
+ PNG_TRANSFORM_STRIP_16 Strip 16-bit samples to
+ 8 bits
+ PNG_TRANSFORM_STRIP_ALPHA Discard the alpha channel
+ PNG_TRANSFORM_PACKING Expand 1, 2 and 4-bit
+ samples to bytes
+ PNG_TRANSFORM_PACKSWAP Change order of packed
+ pixels to LSB first
+ PNG_TRANSFORM_EXPAND Perform set_expand()
+ PNG_TRANSFORM_INVERT_MONO Invert monochrome images
+ PNG_TRANSFORM_SHIFT Normalize pixels to the
+ sBIT depth
+ PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
+ to BGRA
+ PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
+ to AG
+ PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
+ to transparency
+ PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
+
+(This excludes setting a background color, doing gamma transformation,
+dithering, and setting filler.) If this is the case, simply do this:
+
+ png_read_png(png_ptr, info_ptr, png_transforms, NULL)
+
+where png_transforms is an integer containing the logical OR of
+some set of transformation flags. This call is equivalent to png_read_info(),
+followed the set of transformations indicated by the transform mask,
+then png_read_image(), and finally png_read_end().
+
+(The final parameter of this call is not yet used. Someday it might point
+to transformation parameters required by some future input transform.)
+
+You must use png_transforms and not call any png_set_transform() functions
+when you use png_read_png().
+
+After you have called png_read_png(), you can retrieve the image data
+with
- png_set_invert_alpha(png_ptr);
+ row_pointers = png_get_rows(png_ptr, info_ptr);
-This has to appear here rather than later with the other transformations
-because the tRNS chunk data must be modified in the case of paletted images.
-If your image is not a paletted image, the tRNS data (which in such cases
-represents a single color to be rendered as transparent) won't be changed.
+where row_pointers is an array of pointers to the pixel data for each row:
-Finally, you can write your own transformation function if none of
-the existing ones meets your needs. This is done by setting a callback
-with
+ png_bytep row_pointers[height];
- png_set_read_user_transform_fn(png_ptr,
- read_transform_fn);
+If you know your image size and pixel size ahead of time, you can allocate
+row_pointers prior to calling png_read_png() with
-You must supply the function
+ if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
+ png_error (png_ptr,
+ "Image is too tall to process in memory");
+ if (width > PNG_UINT_32_MAX/pixel_size)
+ png_error (png_ptr,
+ "Image is too wide to process in memory");
+ row_pointers = png_malloc(png_ptr,
+ height*png_sizeof(png_bytep));
+ for (int i=0; i<height, i++)
+ row_pointers[i]=png_malloc(png_ptr,
+ width*pixel_size);
+ png_set_rows(png_ptr, info_ptr, &row_pointers);
- void read_transform_fn(png_ptr ptr, row_info_ptr
- row_info, png_bytep data)
+Alternatively you could allocate your image in one big block and define
+row_pointers[i] to point into the proper places in your block.
-See pngtest.c for a working example. Your function will be called
-after all of the other transformations have been processed.
+If you use png_set_rows(), the application is responsible for freeing
+row_pointers (and row_pointers[i], if they were separately allocated).
+
+If you don't allocate row_pointers ahead of time, png_read_png() will
+do it, and it'll be free'ed when you call png_destroy_*().
-You are now ready to read all the file information up to the actual
-image data. You do this with a call to png_read_info().
+The low-level read interface
+
+If you are going the low-level route, you are now ready to read all
+the file information up to the actual image data. You do this with a
+call to png_read_info().
png_read_info(png_ptr, info_ptr);
-Functions are used to get the information from the info_ptr:
+This will process all chunks up to but not including the image data.
+
+Querying the info structure
+
+Functions are used to get the information from the info_ptr once it
+has been read. Note that these fields may not be completely filled
+in until png_read_end() has read the chunk data following the image.
png_get_IHDR(png_ptr, info_ptr, &width, &height,
&bit_depth, &color_type, &interlace_type,
- &compression_type, &filter_type);
+ &compression_type, &filter_method);
width - holds the width of the image
in pixels (up to 2^31).
PNG_COLOR_MASK_COLOR
PNG_COLOR_MASK_ALPHA
- filter_type - (must be PNG_FILTER_TYPE_BASE
- for PNG 1.0)
+ filter_method - (must be PNG_FILTER_TYPE_BASE
+ for PNG 1.0, and can also be
+ PNG_INTRAPIXEL_DIFFERENCING if
+ the PNG datastream is embedded in
+ a MNG-1.0 datastream)
compression_type - (must be PNG_COMPRESSION_TYPE_BASE
for PNG 1.0)
interlace_type - (PNG_INTERLACE_NONE or
PNG_INTERLACE_ADAM7)
Any or all of interlace_type, compression_type, of
- filter_type can be
- NULL if you are not interested in their values.
+ filter_method can be NULL if you are
+ not interested in their values.
channels = png_get_channels(png_ptr, info_ptr);
channels - number of channels of info for the
info_ptr);
color_type = png_get_color_type(png_ptr,
info_ptr);
- filter_type = png_get_filter_type(png_ptr,
+ filter_method = png_get_filter_type(png_ptr,
info_ptr);
compression_type = png_get_compression_type(png_ptr,
info_ptr);
implies specific values of gAMA and
cHRM.
+ png_get_iCCP(png_ptr, info_ptr, &name,
+ &compression_type, &profile, &proflen);
+ name - The profile name.
+ compression - The compression type; always
+ PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
+ You may give NULL to this argument to
+ ignore it.
+ profile - International Color Consortium color
+ profile data. May contain NULs.
+ proflen - length of profile data in bytes.
+
png_get_sBIT(png_ptr, info_ptr, &sig_bit);
sig_bit - the number of significant bits for
(PNG_INFO_sBIT) each of the gray,
&trans_values);
trans - array of transparent entries for
palette (PNG_INFO_tRNS)
- trans_values - transparent pixel for non-paletted
- images (PNG_INFO_tRNS)
+ trans_values - graylevel or color sample values of
+ the single transparent color for
+ non-paletted images (PNG_INFO_tRNS)
num_trans - number of transparent entries
(PNG_INFO_tRNS)
png_get_hIST(png_ptr, info_ptr, &hist);
(PNG_INFO_hIST)
hist - histogram of palette (array of
- png_color_16)
+ png_uint_16)
png_get_tIME(png_ptr, info_ptr, &mod_time);
mod_time - time image was last modified
png_get_bKGD(png_ptr, info_ptr, &background);
background - background color (PNG_VALID_bKGD)
+ valid 16-bit red, green and blue
+ values, regardless of color_type
- num_text = png_get_text(png_ptr, info_ptr, &text_ptr);
+ num_comments = png_get_text(png_ptr, info_ptr,
+ &text_ptr, &num_text);
+ num_comments - number of comments
text_ptr - array of png_text holding image
comments
- text_ptr[i]->key - keyword for comment.
- text_ptr[i]->text - text comments for current
- keyword.
- text_ptr[i]->compression - type of compression used
- on "text" PNG_TEXT_COMPRESSION_NONE
- or PNG_TEXT_COMPRESSION_zTXt
- num_text - number of comments
+ text_ptr[i].compression - type of compression used
+ on "text" PNG_TEXT_COMPRESSION_NONE
+ PNG_TEXT_COMPRESSION_zTXt
+ PNG_ITXT_COMPRESSION_NONE
+ PNG_ITXT_COMPRESSION_zTXt
+ text_ptr[i].key - keyword for comment. Must contain
+ 1-79 characters.
+ text_ptr[i].text - text comments for current
+ keyword. Can be empty.
+ text_ptr[i].text_length - length of text string,
+ after decompression, 0 for iTXt
+ text_ptr[i].itxt_length - length of itxt string,
+ after decompression, 0 for tEXt/zTXt
+ text_ptr[i].lang - language of comment (empty
+ string for unknown).
+ text_ptr[i].lang_key - keyword in UTF-8
+ (empty string for unknown).
+ num_text - number of comments (same as
+ num_comments; you can put NULL here
+ to avoid the duplication)
+ Note while png_set_text() will accept text, language,
+ and translated keywords that can be NULL pointers, the
+ structure returned by png_get_text will always contain
+ regular zero-terminated C strings. They might be
+ empty strings but they will never be NULL pointers.
+
+ num_spalettes = png_get_sPLT(png_ptr, info_ptr,
+ &palette_ptr);
+ palette_ptr - array of palette structures holding
+ contents of one or more sPLT chunks
+ read.
+ num_spalettes - number of sPLT chunks read.
png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
- &unit_type);
+ &unit_type);
offset_x - positive offset from the left edge
of the screen
offset_y - positive offset from the top edge
unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
- &unit_type);
+ &unit_type);
res_x - pixels/unit physical resolution in
x direction
res_y - pixels/unit physical resolution in
unit_type - PNG_RESOLUTION_UNKNOWN,
PNG_RESOLUTION_METER
+ png_get_sCAL(png_ptr, info_ptr, &unit, &width,
+ &height)
+ unit - physical scale units (an integer)
+ width - width of a pixel in physical scale units
+ height - height of a pixel in physical scale units
+ (width and height are doubles)
+
+ png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
+ &height)
+ unit - physical scale units (an integer)
+ width - width of a pixel in physical scale units
+ height - height of a pixel in physical scale units
+ (width and height are strings like "2.54")
+
+ num_unknown_chunks = png_get_unknown_chunks(png_ptr,
+ info_ptr, &unknowns)
+ unknowns - array of png_unknown_chunk
+ structures holding unknown chunks
+ unknowns[i].name - name of unknown chunk
+ unknowns[i].data - data of unknown chunk
+ unknowns[i].size - size of unknown chunk's data
+ unknowns[i].location - position of chunk in file
+
+ The value of "i" corresponds to the order in which the
+ chunks were read from the PNG file or inserted with the
+ png_set_unknown_chunks() function.
+
The data from the pHYs chunk can be retrieved in several convenient
forms:
res_x = png_get_x_pixels_per_meter(png_ptr,
- info_ptr)
+ info_ptr)
res_y = png_get_y_pixels_per_meter(png_ptr,
- info_ptr)
+ info_ptr)
res_x_and_y = png_get_pixels_per_meter(png_ptr,
- info_ptr)
+ info_ptr)
+ res_x = png_get_x_pixels_per_inch(png_ptr,
+ info_ptr)
+ res_y = png_get_y_pixels_per_inch(png_ptr,
+ info_ptr)
+ res_x_and_y = png_get_pixels_per_inch(png_ptr,
+ info_ptr)
aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
- info_ptr)
+ info_ptr)
(Each of these returns 0 [signifying "unknown"] if
the data is not present or if res_x is 0;
res_x_and_y is 0 if res_x != res_y)
+The data from the oFFs chunk can be retrieved in several convenient
+forms:
+
+ x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
+ y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
+ x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
+ y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
+
+ (Each of these returns 0 [signifying "unknown" if both
+ x and y are 0] if the data is not present or if the
+ chunk is present but the unit is the pixel)
+
For more information, see the png_info definition in png.h and the
PNG specification for chunk contents. Be careful with trusting
rowbytes, as some of the transformations could increase the space
Keywords should be limited to 79 Latin-1 characters without leading or
trailing spaces, but non-consecutive spaces are allowed within the
keyword. It is possible to have the same keyword any number of times.
-The text_ptr is an array of png_text structures, each holding pointer
-to a keyword and a pointer to a text string. Only the text string may
-be null. The keyword/text pairs are put into the array in the order
-that they are received. However, some or all of the text chunks may be
-after the image, so, to make sure you have read all the text chunks,
-don't mess with these until after you read the stuff after the image.
-This will be mentioned again below in the discussion that goes with
-png_read_end().
+The text_ptr is an array of png_text structures, each holding a
+pointer to a language string, a pointer to a keyword and a pointer to
+a text string. The text string, language code, and translated
+keyword may be empty or NULL pointers. The keyword/text
+pairs are put into the array in the order that they are received.
+However, some or all of the text chunks may be after the image, so, to
+make sure you have read all the text chunks, don't mess with these
+until after you read the stuff after the image. This will be
+mentioned again below in the discussion that goes with png_read_end().
+
+Input transformations
After you've read the header information, you can set up the library
to handle any special transformations of the image data. The various
For example, 4 bit/pixel paletted or grayscale data will be returned
2 pixels/byte with the leftmost pixel in the high-order bits of the
byte, unless png_set_packing() is called. 8-bit RGB data will be stored
-in RGB RGB RGB format unless png_set_filler() is called to insert filler
-bytes, either before or after each RGB triplet. 16-bit RGB data will
-be returned RRGGBB RRGGBB, with the most significant byte of the color
-value first, unless png_set_strip_16() is called to transform it to
-regular RGB RGB triplets, or png_set_filler() is called to insert
-filler bytes, either before or after each RRGGBB triplet. Similarly,
-8-bit or 16-bit grayscale data can be modified with png_set_filler()
-or png_set_strip_16().
+in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
+is called to insert filler bytes, either before or after each RGB triplet.
+16-bit RGB data will be returned RRGGBB RRGGBB, with the most significant
+byte of the color value first, unless png_set_strip_16() is called to
+transform it to regular RGB RGB triplets, or png_set_filler|add alpha()
+is called to insert filler bytes, either before or after each RRGGBB
+triplet. Similarly, 8-bit or 16-bit grayscale data can be modified with
+png_set_filler(), png_set_add_alpha(), or png_set_strip_16().
The following code transforms grayscale images of less than 8 to 8 bits,
changes paletted images to RGB, and adds a full alpha channel if there is
grayscale images with bit depths of 2 or 4 or if there is a multiple-image
viewing application that wishes to treat all images in the same way.
- if (color_type == PNG_COLOR_TYPE_PALETTE &&
- bit_depth <= 8) png_set_expand(png_ptr);
+ if (color_type == PNG_COLOR_TYPE_PALETTE)
+ png_set_palette_to_rgb(png_ptr);
if (color_type == PNG_COLOR_TYPE_GRAY &&
- bit_depth < 8) png_set_expand(png_ptr);
+ bit_depth < 8) png_set_gray_1_2_4_to_8(png_ptr);
if (png_get_valid(png_ptr, info_ptr,
- PNG_INFO_tRNS)) png_set_expand(png_ptr);
+ PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
+
+These three functions are actually aliases for png_set_expand(), added
+in libpng version 1.0.4, with the function names expanded to improve code
+readability. In some future version they may actually do different
+things.
PNG can have files with 16 bits per channel. If you only can handle
8 bits per channel, this will strip the pixels down to 8 bit.
if (bit_depth == 16)
png_set_strip_16(png_ptr);
-The png_set_background() function tells libpng to composite images
-with alpha or simple transparency against the supplied background
-color. If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
-you may use this color, or supply another color more suitable for
-the current display (e.g., the background color from a web page). You
-need to tell libpng whether the color is in the gamma space of the
-display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file
-(PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one
-that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't
-know why anyone would use this, but it's here).
-
If, for some reason, you don't need the alpha channel on an image,
and you want to remove it rather than combining it with the background
(but the image author certainly had in mind that you *would* combine
if (color_type & PNG_COLOR_MASK_ALPHA)
png_set_strip_alpha(png_ptr);
+In PNG files, the alpha channel in an image
+is the level of opacity. If you need the alpha channel in an image to
+be the level of transparency instead of opacity, you can invert the
+alpha channel (or the tRNS chunk data) after it's read, so that 0 is
+fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
+images) is fully transparent, with
+
+ png_set_invert_alpha(png_ptr);
+
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
they can, resulting in, for example, 8 pixels per byte for 1 bit
files. This code expands to 1 pixel per byte without changing the
convert the PNG pixel data back to the original bit depth of the image.
This call reduces the pixels back down to the original bit depth:
- png_color_16p sig_bit;
+ png_color_8p sig_bit;
if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
png_set_shift(png_ptr, sig_bit);
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
png_set_bgr(png_ptr);
-PNG files store RGB pixels packed into 3 bytes. This code expands them
-into 4 bytes for windowing systems that need them in this format:
+PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
+into 4 or 8 bytes for windowing systems that need them in this format:
- if (bit_depth == 8 && color_type ==
- PNG_COLOR_TYPE_RGB) png_set_filler(png_ptr,
- filler, PNG_FILLER_BEFORE);
+ if (color_type == PNG_COLOR_TYPE_RGB)
+ png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
where "filler" is the 8 or 16-bit number to fill with, and the location is
either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
you want the filler before the RGB or after. This transformation
-does not affect images that already have full alpha channels.
+does not affect images that already have full alpha channels. To add an
+opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
+will generate RGBA pixels.
+
+Note that png_set_filler() does not change the color type. If you want
+to do that, you can add a true alpha channel with
+
+ if (color_type == PNG_COLOR_TYPE_RGB ||
+ color_type == PNG_COLOR_TYPE_GRAY)
+ png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
+
+where "filler" contains the alpha value to assign to each pixel.
+This function became available in libpng-1.2.7.
If you are reading an image with an alpha channel, and you need the
data as ARGB instead of the normal PNG format RGBA:
png_set_gray_to_rgb(png_ptr);
Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
-with alpha. This is intended for conversion of images that really are
-gray (red == green == blue), so the function simply strips out the red
-and blue channels, leaving the green channel in the gray position.
+with alpha.
if (color_type == PNG_COLOR_TYPE_RGB ||
color_type == PNG_COLOR_TYPE_RGB_ALPHA)
- png_set_rgb_to_gray(png_ptr, error_action,
- float red_weight, float green_weight);
+ png_set_rgb_to_gray_fixed(png_ptr, error_action,
+ int red_weight, int green_weight);
error_action = 1: silently do the conversion
error_action = 2: issue a warning if the original
image has any pixel where
red != green or red != blue
- red_weight: weight of red component
- (NULL -> default 54/256)
- green_weight: weight of green component
- (NULL -> default 183/256)
+ red_weight: weight of red component times 100000
+ green_weight: weight of green component times 100000
+ If either weight is negative, default
+ weights (21268, 71514) are used.
If you have set error_action = 1 or 2, you can
later check whether the image really was gray, after processing
will be silently converted to grayscale, using the green channel
data, regardless of the error_action setting.
-With 0.0<=red_weight+green_weight<=1.0,
+With red_weight+green_weight<=100000,
the normalized graylevel is computed:
- int rw = red_weight * 256;
- int gw = green_weight * 256;
- int bw = 256 - (rw + gw);
- gray = (rw*red + gw*green + bw*blue)/256;
+ int rw = red_weight * 65536;
+ int gw = green_weight * 65536;
+ int bw = 65536 - (rw + gw);
+ gray = (rw*red + gw*green + bw*blue)/65536;
The default values approximate those recommended in the Charles
Poynton's Color FAQ, <http://www.inforamp.net/~poynton/>
Libpng approximates this with
- Y = 0.211 * R + 0.715 * G + 0.074 * B
+ Y = 0.21268 * R + 0.7151 * G + 0.07217 * B
which can be expressed with integers as
- Y = (54 * R + 183 * G + 19 * B)/256
+ Y = (6969 * R + 23434 * G + 2365 * B)/32768
The calculation is done in a linear colorspace, if the image gamma
is known.
-If you have a grayscale and you are using png_set_expand() to change to
+If you have a grayscale and you are using png_set_expand_depth(),
+png_set_expand(), or png_set_gray_to_rgb to change to truecolor or to
a higher bit-depth, you must either supply the background color as a gray
value at the original file bit-depth (need_expand = 1) or else supply the
background color as an RGB triplet at the final, expanded bit depth
png_set_background(png_ptr, &my_background,
PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0);
+The png_set_background() function tells libpng to composite images
+with alpha or simple transparency against the supplied background
+color. If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
+you may use this color, or supply another color more suitable for
+the current display (e.g., the background color from a web page). You
+need to tell libpng whether the color is in the gamma space of the
+display (PNG_BACKGROUND_GAMMA_SCREEN for colors you supply), the file
+(PNG_BACKGROUND_GAMMA_FILE for colors from the bKGD chunk), or one
+that is neither of these gammas (PNG_BACKGROUND_GAMMA_UNIQUE - I don't
+know why anyone would use this, but it's here).
+
To properly display PNG images on any kind of system, the application needs
to know what the display gamma is. Ideally, the user will know this, and
the application will allow them to set it. One method of allowing the user
-to set the display gamma separately for each system is to check for the
-DISPLAY_GAMMA and VIEWING_GAMMA environment variables or for a SCREEN_GAMMA
-environment variable, which will hopefully be correctly set.
-
-Note that display_gamma is the gamma of your display, while screen_gamma is
-the overall gamma correction required to produce pleasing results,
-which depends on the lighting conditions in the surrounding environment.
-Screen_gamma is display_gamma/viewing_gamma, where viewing_gamma is
-the amount of additional gamma correction needed to compensate for
-a (viewing_gamma=1.25) environment. In a dim or brightly lit room, no
-compensation other than the display_gamma is needed (viewing_gamma=1.0).
+to set the display gamma separately for each system is to check for a
+SCREEN_GAMMA or DISPLAY_GAMMA environment variable, which will hopefully be
+correctly set.
+
+Note that display_gamma is the overall gamma correction required to produce
+pleasing results, which depends on the lighting conditions in the surrounding
+environment. In a dim or brightly lit room, no compensation other than
+the physical gamma exponent of the monitor is needed, while in a dark room
+a slightly smaller exponent is better.
+
+ double gamma, screen_gamma;
if (/* We have a user-defined screen
gamma value */)
else if ((gamma_str = getenv("SCREEN_GAMMA"))
!= NULL)
{
- screen_gamma = atof(gamma_str);
+ screen_gamma = (double)atof(gamma_str);
}
/* If we don't have another value */
else
if (png_get_valid(png_ptr, info_ptr,
PNG_INFO_PLTE))
{
- png_color_16p histogram;
+ png_uint_16p histogram = NULL;
png_get_hIST(png_ptr, info_ptr,
&histogram);
The following code will reverse this (make black be one and white be
zero):
- if (bit_depth == 1 && color_type == PNG_COLOR_GRAY)
+ if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
+ png_set_invert_mono(png_ptr);
+
+This function can also be used to invert grayscale and gray-alpha images:
+
+ if (color_type == PNG_COLOR_TYPE_GRAY ||
+ color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
png_set_invert_mono(png_ptr);
PNG files store 16 bit pixels in network byte order (big-endian,
if (bit_depth < 8)
png_set_packswap(png_ptr);
+Finally, you can write your own transformation function if none of
+the existing ones meets your needs. This is done by setting a callback
+with
+
+ png_set_read_user_transform_fn(png_ptr,
+ read_transform_fn);
+
+You must supply the function
+
+ void read_transform_fn(png_ptr ptr, row_info_ptr
+ row_info, png_bytep data)
+
+See pngtest.c for a working example. Your function will be called
+after all of the other transformations have been processed.
+
+You can also set up a pointer to a user structure for use by your
+callback function, and you can inform libpng that your transform
+function will change the number of channels or bit depth with the
+function
+
+ png_set_user_transform_info(png_ptr, user_ptr,
+ user_depth, user_channels);
+
+The user's application, not libpng, is responsible for allocating and
+freeing any memory required for the user structure.
+
+You can retrieve the pointer via the function
+png_get_user_transform_ptr(). For example:
+
+ voidp read_user_transform_ptr =
+ png_get_user_transform_ptr(png_ptr);
+
The last thing to handle is interlacing; this is covered in detail below,
but you must call the function here if you want libpng to handle expansion
of the interlaced image.
array of pointers to each row, as it will be needed for some
of the functions below.
+Reading image data
+
After you've allocated memory, you can read the image data.
The simplest way to do this is in one function call. If you are
allocating enough memory to hold the whole image, you can just
where row_pointers is the same as in the png_read_image() call.
If you are doing this just one row at a time, you can do this with
-row_pointers:
+a single row_pointer instead of an array of row_pointers:
- png_bytep row_pointers = row;
- png_read_row(png_ptr, &row_pointers, NULL);
+ png_bytep row_pointer = row;
+ png_read_row(png_ptr, row_pointer, NULL);
-If the file is interlaced (info_ptr->interlace_type != 0), things get
-somewhat harder. The only current (PNG Specification version 1.0)
+If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
+get somewhat harder. The only current (PNG Specification version 1.2)
interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7)
is a somewhat complicated 2D interlace scheme, known as Adam7, that
breaks down an image into seven smaller images of varying size, based
png_read_rows(png_ptr, NULL, row_pointers,
number_of_rows);
-After you are finished reading the image, you can finish reading
-the file. If you are interested in comments or time, which may be
-stored either before or after the image data, you should pass the
-separate png_info struct if you want to keep the comments from
-before and after the image separate. If you are not interested, you
-can pass NULL.
+Finishing a sequential read
+
+After you are finished reading the image through either the high- or
+low-level interfaces, you can finish reading the file. If you are
+interested in comments or time, which may be stored either before or
+after the image data, you should pass the separate png_info struct if
+you want to keep the comments from before and after the image
+separate. If you are not interested, you can pass NULL.
png_read_end(png_ptr, end_info);
png_destroy_read_struct(&png_ptr, &info_ptr,
&end_info);
-For a more compact example of reading a PNG image, see the file example.c.
+It is also possible to individually free the info_ptr members that
+point to libpng-allocated storage with the following function:
+
+ png_free_data(png_ptr, info_ptr, mask, seq)
+ mask - identifies data to be freed, a mask
+ containing the logical OR of one or
+ more of
+ PNG_FREE_PLTE, PNG_FREE_TRNS,
+ PNG_FREE_HIST, PNG_FREE_ICCP,
+ PNG_FREE_PCAL, PNG_FREE_ROWS,
+ PNG_FREE_SCAL, PNG_FREE_SPLT,
+ PNG_FREE_TEXT, PNG_FREE_UNKN,
+ or simply PNG_FREE_ALL
+ seq - sequence number of item to be freed
+ (-1 for all items)
+
+This function may be safely called when the relevant storage has
+already been freed, or has not yet been allocated, or was allocated
+by the user and not by libpng, and will in those
+cases do nothing. The "seq" parameter is ignored if only one item
+of the selected data type, such as PLTE, is allowed. If "seq" is not
+-1, and multiple items are allowed for the data type identified in
+the mask, such as text or sPLT, only the n'th item in the structure
+is freed, where n is "seq".
+
+The default behavior is only to free data that was allocated internally
+by libpng. This can be changed, so that libpng will not free the data,
+or so that it will free data that was allocated by the user with png_malloc()
+or png_zalloc() and passed in via a png_set_*() function, with
+
+ png_data_freer(png_ptr, info_ptr, freer, mask)
+ mask - which data elements are affected
+ same choices as in png_free_data()
+ freer - one of
+ PNG_DESTROY_WILL_FREE_DATA
+ PNG_SET_WILL_FREE_DATA
+ PNG_USER_WILL_FREE_DATA
+
+This function only affects data that has already been allocated.
+You can call this function after reading the PNG data but before calling
+any png_set_*() functions, to control whether the user or the png_set_*()
+function is responsible for freeing any existing data that might be present,
+and again after the png_set_*() functions to control whether the user
+or png_destroy_*() is supposed to free the data. When the user assumes
+responsibility for libpng-allocated data, the application must use
+png_free() to free it, and when the user transfers responsibility to libpng
+for data that the user has allocated, the user must have used png_malloc()
+or png_zalloc() to allocate it.
+
+If you allocated your row_pointers in a single block, as suggested above in
+the description of the high level read interface, you must not transfer
+responsibility for freeing it to the png_set_rows or png_read_destroy function,
+because they would also try to free the individual row_pointers[i].
+
+If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
+separately, do not transfer responsibility for freeing text_ptr to libpng,
+because when libpng fills a png_text structure it combines these members with
+the key member, and png_free_data() will free only text_ptr.key. Similarly,
+if you transfer responsibility for free'ing text_ptr from libpng to your
+application, your application must not separately free those members.
+
+The png_free_data() function will turn off the "valid" flag for anything
+it frees. If you need to turn the flag off for a chunk that was freed by your
+application instead of by libpng, you can use
+
+ png_set_invalid(png_ptr, info_ptr, mask);
+ mask - identifies the chunks to be made invalid,
+ containing the logical OR of one or
+ more of
+ PNG_INFO_gAMA, PNG_INFO_sBIT,
+ PNG_INFO_cHRM, PNG_INFO_PLTE,
+ PNG_INFO_tRNS, PNG_INFO_bKGD,
+ PNG_INFO_hIST, PNG_INFO_pHYs,
+ PNG_INFO_oFFs, PNG_INFO_tIME,
+ PNG_INFO_pCAL, PNG_INFO_sRGB,
+ PNG_INFO_iCCP, PNG_INFO_sPLT,
+ PNG_INFO_sCAL, PNG_INFO_IDAT
+For a more compact example of reading a PNG image, see the file example.c.
-Reading PNG files progressively:
+Reading PNG files progressively
The progressive reader is slightly different then the non-progressive
reader. Instead of calling png_read_info(), png_read_rows(), and
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
- return -1;
+ return (ERROR);
info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_read_struct(&png_ptr, (png_infopp)NULL,
(png_infopp)NULL);
- return -1;
+ return (ERROR);
}
- if (setjmp(png_ptr->jmpbuf))
+ if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
- return -1;
+ return (ERROR);
}
/* This one's new. You can provide functions
int
process_data(png_bytep buffer, png_uint_32 length)
{
- if (setjmp(png_ptr->jmpbuf))
+ if (setjmp(png_jmpbuf(png_ptr)))
{
png_destroy_read_struct(&png_ptr, &info_ptr,
(png_infopp)NULL);
- return -1;
+ return (ERROR);
}
/* This one's new also. Simply give it a chunk
importance is repeated here, so you won't have to constantly look
back up in the reading section to understand writing.
+Setup
+
You will want to do the I/O initialization before you get into libpng,
so if it doesn't work, you don't have anything to undo. If you are not
using the standard I/O functions, you will need to replace them with
FILE *fp = fopen(file_name, "wb");
if (!fp)
{
- return;
+ return (ERROR);
}
Next, png_struct and png_info need to be allocated and initialized.
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
user_error_fn, user_warning_fn);
if (!png_ptr)
- return;
+ return (ERROR);
png_infop info_ptr = png_create_info_struct(png_ptr);
if (!info_ptr)
{
png_destroy_write_struct(&png_ptr,
(png_infopp)NULL);
- return;
+ return (ERROR);
}
If you want to use your own memory allocation routines,
define PNG_USER_MEM_SUPPORTED and use
-png_create_write_struct_2() instead of png_create_read_struct():
+png_create_write_struct_2() instead of png_create_write_struct():
png_structp png_ptr = png_create_write_struct_2
(PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
After you have these structures, you will need to set up the
error handling. When libpng encounters an error, it expects to
longjmp() back to your routine. Therefore, you will need to call
-setjmp() and pass the png_ptr->jmpbuf. If you
+setjmp() and pass the png_jmpbuf(png_ptr). If you
write the file from different routines, you will need to update
-the jmpbuf field every time you enter a new routine that will
-call a png_ function. See your documentation of setjmp/longjmp
+the png_jmpbuf(png_ptr) every time you enter a new routine that will
+call a png_*() function. See your documentation of setjmp/longjmp
for your compiler for more information on setjmp/longjmp. See
the discussion on libpng error handling in the Customizing Libpng
section below for more information on the libpng error handling.
- if (setjmp(png_ptr->jmpbuf))
+ if (setjmp(png_jmpbuf(png_ptr)))
{
- png_destroy_write_struct(&png_ptr, &info_ptr);
- fclose(fp);
- return;
+ png_destroy_write_struct(&png_ptr, &info_ptr);
+ fclose(fp);
+ return (ERROR);
}
...
return;
+If you would rather avoid the complexity of setjmp/longjmp issues,
+you can compile libpng with PNG_SETJMP_NOT_SUPPORTED, in which case
+errors will result in a call to PNG_ABORT() which defaults to abort().
+
Now you need to set up the output code. The default for libpng is to
use the C function fwrite(). If you use this, you will need to pass a
valid FILE * in the function png_init_io(). Be sure that the file is
png_init_io(png_ptr, fp);
+Write callbacks
+
At this point, you can set up a callback function that will be
called after each row has been written, which you can use to control
a progress meter or the like. It's demonstrated in pngtest.c.
You must supply a function
- void write_row_callback(png_ptr, png_uint_32 row, int pass);
+ void write_row_callback(png_ptr, png_uint_32 row,
+ int pass);
{
/* put your code here */
}
have no special needs in this area, let the library do what it wants by
not calling this function at all, as it has been tuned to deliver a good
speed/compression ratio. The second parameter to png_set_filter() is
-the filter method, for which the only valid value is '0' (as of the
-October 1996 PNG specification, version 1.0). The third parameter is a
-flag that indicates which filter type(s) are to be tested for each
-scanline. See the Compression Library for details on the specific filter
+the filter method, for which the only valid values are 0 (as of the
+July 1999 PNG specification, version 1.2) or 64 (if you are writing
+a PNG datastream that is to be embedded in a MNG datastream). The third
+parameter is a flag that indicates which filter type(s) are to be tested
+for each scanline. See the PNG specification for details on the specific filter
types.
/* turn on or off filtering, and/or choose
- specific filters */
+ specific filters. You can use either a single
+ PNG_FILTER_VALUE_NAME or the logical OR of one
+ or more PNG_FILTER_NAME masks. */
png_set_filter(png_ptr, 0,
- PNG_FILTER_NONE | PNG_FILTER_SUB |
- PNG_FILTER_PAETH);
-
-The png_set_compression_???() functions interface to the zlib compression
+ PNG_FILTER_NONE | PNG_FILTER_VALUE_NONE |
+ PNG_FILTER_SUB | PNG_FILTER_VALUE_SUB |
+ PNG_FILTER_UP | PNG_FILTER_VALUE_UP |
+ PNG_FILTER_AVE | PNG_FILTER_VALUE_AVE |
+ PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
+ PNG_ALL_FILTERS);
+
+If an application
+wants to start and stop using particular filters during compression,
+it should start out with all of the filters (to ensure that the previous
+row of pixels will be stored in case it's needed later), and then add
+and remove them after the start of compression.
+
+If you are writing a PNG datastream that is to be embedded in a MNG
+datastream, the second parameter can be either 0 or 64.
+
+The png_set_compression_*() functions interface to the zlib compression
library, and should mostly be ignored unless you really know what you are
doing. The only generally useful call is png_set_compression_level()
which changes how much time zlib spends on trying to compress the image
-data. See the Compression Library for details on the compression levels.
+data. See the Compression Library (zlib.h and algorithm.txt, distributed
+with zlib) for details on the compression levels.
/* set the zlib compression level */
png_set_compression_level(png_ptr,
Z_DEFAULT_STRATEGY);
png_set_compression_window_bits(png_ptr, 15);
png_set_compression_method(png_ptr, 8);
+ png_set_compression_buffer_size(png_ptr, 8192)
+
+extern PNG_EXPORT(void,png_set_zbuf_size)
+
+Setting the contents of info for output
You now need to fill in the png_info structure with all the data you
wish to write before the actual image. Note that the only thing you
are allowed to write after the image is the text chunks and the time
-chunk (as of PNG Specification 1.0, anyway). See png_write_end() and
+chunk (as of PNG Specification 1.2, anyway). See png_write_end() and
the latest PNG specification for more information on that. If you
wish to write them before the image, fill them in now, and flag that
data as being valid. If you want to wait until after the data, don't
png_set_IHDR(png_ptr, info_ptr, width, height,
bit_depth, color_type, interlace_type,
- compression_type, filter_type)
+ compression_type, filter_method)
width - holds the width of the image
in pixels (up to 2^31).
height - holds the height of the image
PNG_INTERLACE_ADAM7
compression_type - (must be
PNG_COMPRESSION_TYPE_DEFAULT)
- filter_type - (must be PNG_FILTER_TYPE_DEFAULT)
+ filter_method - (must be PNG_FILTER_TYPE_DEFAULT
+ or, if you are writing a PNG to
+ be embedded in a MNG datastream,
+ can also be
+ PNG_INTRAPIXEL_DIFFERENCING)
png_set_PLTE(png_ptr, info_ptr, palette,
num_palette);
Color Consortium
(http://www.color.org).
It can be one of
- PNG_SRGB_INTENT_SATURATION,
- PNG_SRGB_INTENT_PERCEPTUAL,
- PNG_SRGB_INTENT_ABSOLUTE, or
- PNG_SRGB_INTENT_RELATIVE.
+ PNG_sRGB_INTENT_SATURATION,
+ PNG_sRGB_INTENT_PERCEPTUAL,
+ PNG_sRGB_INTENT_ABSOLUTE, or
+ PNG_sRGB_INTENT_RELATIVE.
png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
that are consistent with sRGB to be
written.
+ png_set_iCCP(png_ptr, info_ptr, name, compression_type,
+ profile, proflen);
+ name - The profile name.
+ compression - The compression type; always
+ PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
+ You may give NULL to this argument to
+ ignore it.
+ profile - International Color Consortium color
+ profile data. May contain NULs.
+ proflen - length of profile data in bytes.
+
png_set_sBIT(png_ptr, info_ptr, sig_bit);
sig_bit - the number of significant bits for
(PNG_INFO_sBIT) each of the gray, red,
trans_values);
trans - array of transparent entries for
palette (PNG_INFO_tRNS)
- trans_values - transparent pixel for non-paletted
- images (PNG_INFO_tRNS)
+ trans_values - graylevel or color sample values of
+ the single transparent color for
+ non-paletted images (PNG_INFO_tRNS)
num_trans - number of transparent entries
(PNG_INFO_tRNS)
png_set_hIST(png_ptr, info_ptr, hist);
(PNG_INFO_hIST)
hist - histogram of palette (array of
- png_color_16)
+ png_uint_16)
png_set_tIME(png_ptr, info_ptr, mod_time);
mod_time - time image was last modified
png_set_text(png_ptr, info_ptr, text_ptr, num_text);
text_ptr - array of png_text holding image
comments
- text_ptr[i]->key - keyword for comment.
- text_ptr[i]->text - text comments for current
- keyword.
- text_ptr[i]->compression - type of compression used
- on "text" PNG_TEXT_COMPRESSION_NONE or
- PNG_TEXT_COMPRESSION_zTXt
- num_text - number of comments in text_ptr
+ text_ptr[i].compression - type of compression used
+ on "text" PNG_TEXT_COMPRESSION_NONE
+ PNG_TEXT_COMPRESSION_zTXt
+ PNG_ITXT_COMPRESSION_NONE
+ PNG_ITXT_COMPRESSION_zTXt
+ text_ptr[i].key - keyword for comment. Must contain
+ 1-79 characters.
+ text_ptr[i].text - text comments for current
+ keyword. Can be NULL or empty.
+ text_ptr[i].text_length - length of text string,
+ after decompression, 0 for iTXt
+ text_ptr[i].itxt_length - length of itxt string,
+ after decompression, 0 for tEXt/zTXt
+ text_ptr[i].lang - language of comment (NULL or
+ empty for unknown).
+ text_ptr[i].translated_keyword - keyword in UTF-8 (NULL
+ or empty for unknown).
+ num_text - number of comments
+
+ png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
+ num_spalettes);
+ palette_ptr - array of png_sPLT_struct structures
+ to be added to the list of palettes
+ in the info structure.
+ num_spalettes - number of palette structures to be
+ added.
png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
unit_type);
unit_type - PNG_RESOLUTION_UNKNOWN,
PNG_RESOLUTION_METER
-In PNG files, the alpha channel in an image is the level of opacity.
-If your data is supplied as a level of transparency, you can invert the
-alpha channel before you write it, so that 0 is fully transparent and 255
-(in 8-bit or paletted images) or 65535 (in 16-bit images) is fully opaque,
-with
-
- png_set_invert_alpha(png_ptr);
-
-This must appear here instead of later with the other transformations
-because in the case of paletted images the tRNS chunk data has to
-be inverted before the tRNS chunk is written. If your image is not a
-paletted image, the tRNS data (which in such cases represents a single
-color to be rendered as transparent) won't be changed.
+ png_set_sCAL(png_ptr, info_ptr, unit, width, height)
+ unit - physical scale units (an integer)
+ width - width of a pixel in physical scale units
+ height - height of a pixel in physical scale units
+ (width and height are doubles)
+
+ png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
+ unit - physical scale units (an integer)
+ width - width of a pixel in physical scale units
+ height - height of a pixel in physical scale units
+ (width and height are strings like "2.54")
+
+ png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
+ num_unknowns)
+ unknowns - array of png_unknown_chunk
+ structures holding unknown chunks
+ unknowns[i].name - name of unknown chunk
+ unknowns[i].data - data of unknown chunk
+ unknowns[i].size - size of unknown chunk's data
+ unknowns[i].location - position to write chunk in file
+ 0: do not write chunk
+ PNG_HAVE_IHDR: before PLTE
+ PNG_HAVE_PLTE: before IDAT
+ PNG_AFTER_IDAT: after IDAT
+
+The "location" member is set automatically according to
+what part of the output file has already been written.
+You can change its value after calling png_set_unknown_chunks()
+as demonstrated in pngtest.c. Within each of the "locations",
+the chunks are sequenced according to their position in the
+structure (that is, the value of "i", which is the order in which
+the chunk was either read from the input file or defined with
+png_set_unknown_chunks).
A quick word about text and num_text. text is an array of png_text
structures. num_text is the number of valid structures in the array.
-If you want, you can use max_text to hold the size of the array, but
-libpng ignores it for writing (it does use it for reading). Each
-png_text structure holds a keyword-text value, and a compression type.
+Each png_text structure holds a language code, a keyword, a text value,
+and a compression type.
+
The compression types have the same valid numbers as the compression
types of the image data. Currently, the only valid number is zero.
However, you can store text either compressed or uncompressed, unlike
images, which always have to be compressed. So if you don't want the
text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
+Because tEXt and zTXt chunks don't have a language field, if you
+specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
+any language code or translated keyword will not be written out.
+
Until text gets around 1000 bytes, it is not worth compressing it.
After the text has been written out to the file, the compression type
is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
is compressed anyway, so the compression would be meaningless.
PNG supports modification time via the png_time structure. Two
-conversion routines are proved, png_convert_from_time_t() for
+conversion routines are provided, png_convert_from_time_t() for
time_t and png_convert_from_struct_tm() for struct tm. The
time_t routine uses gmtime(). You don't have to use either of
these, but if you wish to fill in the png_time structure directly,
png_convert_to_rfc1123(png_timep) is provided to convert from PNG
time to an RFC 1123 format string.
-You are now ready to write all the file information up to the actual
-image data. You do this with a call to png_write_info().
+Writing unknown chunks
+
+You can use the png_set_unknown_chunks function to queue up chunks
+for writing. You give it a chunk name, raw data, and a size; that's
+all there is to it. The chunks will be written by the next following
+png_write_info_before_PLTE, png_write_info, or png_write_end function.
+Any chunks previously read into the info structure's unknown-chunk
+list will also be written out in a sequence that satisfies the PNG
+specification's ordering rules.
+
+The high-level write interface
+
+At this point there are two ways to proceed; through the high-level
+write interface, or through a sequence of low-level write operations.
+You can use the high-level interface if your image data is present
+in the info structure. All defined output
+transformations are permitted, enabled by the following masks.
+
+ PNG_TRANSFORM_IDENTITY No transformation
+ PNG_TRANSFORM_PACKING Pack 1, 2 and 4-bit samples
+ PNG_TRANSFORM_PACKSWAP Change order of packed
+ pixels to LSB first
+ PNG_TRANSFORM_INVERT_MONO Invert monochrome images
+ PNG_TRANSFORM_SHIFT Normalize pixels to the
+ sBIT depth
+ PNG_TRANSFORM_BGR Flip RGB to BGR, RGBA
+ to BGRA
+ PNG_TRANSFORM_SWAP_ALPHA Flip RGBA to ARGB or GA
+ to AG
+ PNG_TRANSFORM_INVERT_ALPHA Change alpha from opacity
+ to transparency
+ PNG_TRANSFORM_SWAP_ENDIAN Byte-swap 16-bit samples
+ PNG_TRANSFORM_STRIP_FILLER Strip out filler bytes.
+
+If you have valid image data in the info structure (you can use
+png_set_rows() to put image data in the info structure), simply do this:
+
+ png_write_png(png_ptr, info_ptr, png_transforms, NULL)
+
+where png_transforms is an integer containing the logical OR of some set of
+transformation flags. This call is equivalent to png_write_info(),
+followed the set of transformations indicated by the transform mask,
+then png_write_image(), and finally png_write_end().
+
+(The final parameter of this call is not yet used. Someday it might point
+to transformation parameters required by some future output transform.)
+
+You must use png_transforms and not call any png_set_transform() functions
+when you use png_write_png().
+
+The low-level write interface
+
+If you are going the low-level route instead, you are now ready to
+write all the file information up to the actual image data. You do
+this with a call to png_write_info().
png_write_info(png_ptr, info_ptr);
+Note that there is one transformation you may need to do before
+png_write_info(). In PNG files, the alpha channel in an image is the
+level of opacity. If your data is supplied as a level of
+transparency, you can invert the alpha channel before you write it, so
+that 0 is fully transparent and 255 (in 8-bit or paletted images) or
+65535 (in 16-bit images) is fully opaque, with
+
+ png_set_invert_alpha(png_ptr);
+
+This must appear before png_write_info() instead of later with the
+other transformations because in the case of paletted images the tRNS
+chunk data has to be inverted before the tRNS chunk is written. If
+your image is not a paletted image, the tRNS data (which in such cases
+represents a single color to be rendered as transparent) won't need to
+be changed, and you can safely do this transformation after your
+png_write_info() call.
+
+If you need to write a private chunk that you want to appear before
+the PLTE chunk when PLTE is present, you can write the PNG info in
+two steps, and insert code to write your own chunk between them:
+
+ png_write_info_before_PLTE(png_ptr, info_ptr);
+ png_set_unknown_chunks(png_ptr, info_ptr, ...);
+ png_write_info(png_ptr, info_ptr);
+
After you've written the file information, you can set up the library
to handle any special transformations of the image data. The various
ways to transform the data will be described in the order that they
data. For example, don't swap red and blue on grayscale data.
PNG files store RGB pixels packed into 3 or 6 bytes. This code tells
-the library to expand the input data to 4 or 8 bytes per pixel
-(or expand 1 or 2-byte grayscale data to 2 or 4 bytes per pixel).
+the library to strip input data that has 4 or 8 bytes per pixel down
+to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
+bytes per pixel).
png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
-where the 0 is the value that will be put in the 4th byte, and the
-location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending
-upon whether the filler byte is stored XRGB or RGBX.
+where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
+PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
+is stored XRGB or RGBX.
PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
they can, resulting in, for example, 8 pixels per byte for 1 bit files.
PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your
data is of another bit depth, you can write an sBIT chunk into the
-file so that decoders can get the original data if desired.
+file so that decoders can recover the original data if desired.
/* Set the true bit depth of the image data */
if (color_type & PNG_COLOR_MASK_COLOR)
row_info, png_bytep data)
See pngtest.c for a working example. Your function will be called
-before any of the other transformations have been processed.
+before any of the other transformations are processed.
+
+You can also set up a pointer to a user structure for use by your
+callback function.
+
+ png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
+
+The user_channels and user_depth parameters of this function are ignored
+when writing; you can set them to zero as shown.
+
+You can retrieve the pointer via the function png_get_user_transform_ptr().
+For example:
+
+ voidp write_user_transform_ptr =
+ png_get_user_transform_ptr(png_ptr);
It is possible to have libpng flush any pending output, either manually,
or automatically after a certain number of lines have been written. To
only degrade the compression performance by a few percent over images
that do not use flushing.
+Writing the image data
+
That's it for the transformations. Now you can write the image data.
-The simplest way to do this is in one function call. If have the
+The simplest way to do this is in one function call. If you have the
whole image in memory, you can just call png_write_image() and libpng
will write the image. You will need to pass in an array of pointers to
each row. This function automatically handles interlacing, so you don't
row_pointers is the same as in the png_write_image() call.
If you are just writing one row at a time, you can do this with
-row_pointers:
+a single row_pointer instead of an array of row_pointers:
png_bytep row_pointer = row;
- png_write_row(png_ptr, &row_pointer);
+ png_write_row(png_ptr, row_pointer);
When the file is interlaced, things can get a good deal more
-complicated. The only currently (as of February 1998 -- PNG Specification
-version 1.0, dated October 1996) defined interlacing scheme for PNG files
+complicated. The only currently (as of the PNG Specification
+version 1.2, dated July 1999) defined interlacing scheme for PNG files
is the "Adam7" interlace scheme, that breaks down an
image into seven smaller images of varying size. libpng will build
these images for you, or you can do them yourself. If you want to
you may want to read about interlacing in the PNG specification,
and only update the rows that are actually used.
+Finishing a sequential write
+
After you are finished writing the image, you should finish writing
the file. If you are interested in writing comments or time, you should
pass an appropriately filled png_info pointer. If you are not interested,
png_destroy_write_struct(&png_ptr, &info_ptr);
-You must free any data you allocated for info_ptr, such as comments,
-palette, or histogram, before the call to png_destroy_write_struct();
-
+It is also possible to individually free the info_ptr members that
+point to libpng-allocated storage with the following function:
+
+ png_free_data(png_ptr, info_ptr, mask, seq)
+ mask - identifies data to be freed, a mask
+ containing the logical OR of one or
+ more of
+ PNG_FREE_PLTE, PNG_FREE_TRNS,
+ PNG_FREE_HIST, PNG_FREE_ICCP,
+ PNG_FREE_PCAL, PNG_FREE_ROWS,
+ PNG_FREE_SCAL, PNG_FREE_SPLT,
+ PNG_FREE_TEXT, PNG_FREE_UNKN,
+ or simply PNG_FREE_ALL
+ seq - sequence number of item to be freed
+ (-1 for all items)
+
+This function may be safely called when the relevant storage has
+already been freed, or has not yet been allocated, or was allocated
+by the user and not by libpng, and will in those
+cases do nothing. The "seq" parameter is ignored if only one item
+of the selected data type, such as PLTE, is allowed. If "seq" is not
+-1, and multiple items are allowed for the data type identified in
+the mask, such as text or sPLT, only the n'th item in the structure
+is freed, where n is "seq".
+
+If you allocated data such as a palette that you passed
+in to libpng with png_set_*, you must not free it until just before the call to
+png_destroy_write_struct().
+
+The default behavior is only to free data that was allocated internally
+by libpng. This can be changed, so that libpng will not free the data,
+or so that it will free data that was allocated by the user with png_malloc()
+or png_zalloc() and passed in via a png_set_*() function, with
+
+ png_data_freer(png_ptr, info_ptr, freer, mask)
+ mask - which data elements are affected
+ same choices as in png_free_data()
+ freer - one of
+ PNG_DESTROY_WILL_FREE_DATA
+ PNG_SET_WILL_FREE_DATA
+ PNG_USER_WILL_FREE_DATA
+
+For example, to transfer responsibility for some data from a read structure
+to a write structure, you could use
+
+ png_data_freer(read_ptr, read_info_ptr,
+ PNG_USER_WILL_FREE_DATA,
+ PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
+ png_data_freer(write_ptr, write_info_ptr,
+ PNG_DESTROY_WILL_FREE_DATA,
+ PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
+
+thereby briefly reassigning responsibility for freeing to the user but
+immediately afterwards reassigning it once more to the write_destroy
+function. Having done this, it would then be safe to destroy the read
+structure and continue to use the PLTE, tRNS, and hIST data in the write
+structure.
+
+This function only affects data that has already been allocated.
+You can call this function before calling after the png_set_*() functions
+to control whether the user or png_destroy_*() is supposed to free the data.
+When the user assumes responsibility for libpng-allocated data, the
+application must use
+png_free() to free it, and when the user transfers responsibility to libpng
+for data that the user has allocated, the user must have used png_malloc()
+or png_zalloc() to allocate it.
+
+If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
+separately, do not transfer responsibility for freeing text_ptr to libpng,
+because when libpng fills a png_text structure it combines these members with
+the key member, and png_free_data() will free only text_ptr.key. Similarly,
+if you transfer responsibility for free'ing text_ptr from libpng to your
+application, your application must not separately free those members.
For a more compact example of writing a PNG image, see the file example.c.
-
V. Modifying/Customizing libpng:
-There are two issues here. The first is changing how libpng does
+There are three issues here. The first is changing how libpng does
standard things like memory allocation, input/output, and error handling.
The second deals with more complicated things like adding new chunks,
adding new transformations, and generally changing how libpng works.
+Both of those are compile-time issues; that is, they are generally
+determined at the time the code is written, and there is rarely a need
+to provide the user with a means of changing them. The third is a
+run-time issue: choosing between and/or tuning one or more alternate
+versions of computationally intensive routines; specifically, optimized
+assembly-language (and therefore compiler- and platform-dependent)
+versions.
+
+Memory allocation, input/output, and error handling
All of the memory allocation, input/output, and error handling in libpng
-goes through callbacks that are user settable. The default routines are
-in pngmem.c, pngrio.c, pngwio.c, and pngerror.c respectively. To change
-these functions, call the appropriate png_set_???_fn() function.
-
-Memory allocation is done through the functions png_large_malloc(),
-png_malloc(), png_realloc(), png_large_free(), and png_free(). These
-currently just call the standard C functions. The large functions must
-handle exactly 64K, but they don't have to handle more than that. If
+goes through callbacks that are user-settable. The default routines are
+in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively. To change
+these functions, call the appropriate png_set_*_fn() function.
+
+Memory allocation is done through the functions png_malloc()
+and png_free(). These currently just call the standard C functions. If
your pointers can't access more then 64K at a time, you will want to set
MAXSEG_64K in zlib.h. Since it is unlikely that the method of handling
memory allocation on a platform will change between applications, these
-functions must be modified in the library at compile time.
+functions must be modified in the library at compile time. If you prefer
+to use a different method of allocating and freeing data, you can use
+png_create_read_struct_2() or png_create_write_struct_2() to register
+your own functions as described above.
+These functions also provide a void pointer that can be retrieved via
+
+ mem_ptr=png_get_mem_ptr(png_ptr);
+
+Your replacement memory functions must have prototypes as follows:
+
+ png_voidp malloc_fn(png_structp png_ptr,
+ png_size_t size);
+ void free_fn(png_structp png_ptr, png_voidp ptr);
+
+Your malloc_fn() must return NULL in case of failure. The png_malloc()
+function will normally call png_error() if it receives a NULL from the
+system memory allocator or from your replacement malloc_fn().
Input/Output in libpng is done through png_read() and png_write(),
which currently just call fread() and fwrite(). The FILE * is stored in
voidp read_io_ptr = png_get_io_ptr(read_ptr);
voidp write_io_ptr = png_get_io_ptr(write_ptr);
-The replacement I/O functions should have prototypes as follows:
+The replacement I/O functions must have prototypes as follows:
void user_read_data(png_structp png_ptr,
- png_bytep data, png_uint_32 length);
+ png_bytep data, png_size_t length);
void user_write_data(png_structp png_ptr,
- png_bytep data, png_uint_32 length);
+ png_bytep data, png_size_t length);
void user_flush_data(png_structp png_ptr);
Supplying NULL for the read, write, or flush functions sets them back
Error handling in libpng is done through png_error() and png_warning().
Errors handled through png_error() are fatal, meaning that png_error()
should never return to its caller. Currently, this is handled via
-setjmp() and longjmp(), but you could change this to do things like
-exit() if you should wish. On non-fatal errors, png_warning() is called
+setjmp() and longjmp() (unless you have compiled libpng with
+PNG_SETJMP_NOT_SUPPORTED, in which case it is handled via PNG_ABORT()),
+but you could change this to do things like exit() if you should wish.
+
+On non-fatal errors, png_warning() is called
to print a warning message, and then control returns to the calling code.
By default png_error() and png_warning() print a message on stderr via
-fprintf() unless the library is compiled with PNG_NO_STDIO defined. If
-you wish to change the behavior of the error functions, you will need to
-set up your own message callbacks. These functions are normally supplied
-at the time that the png_struct is created. It is also possible to change
-these functions after png_create_???_struct() has been called by calling:
+fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
+(because you don't want the messages) or PNG_NO_STDIO defined (because
+fprintf() isn't available). If you wish to change the behavior of the error
+functions, you will need to set up your own message callbacks. These
+functions are normally supplied at the time that the png_struct is created.
+It is also possible to redirect errors and warnings to your own replacement
+functions after png_create_*_struct() has been called by calling:
png_set_error_fn(png_structp png_ptr,
png_voidp error_ptr, png_error_ptr error_fn,
However, there are some uncertainties about the status of local variables
after a longjmp, so the user may want to be careful about doing anything after
setjmp returns non-zero besides returning itself. Consult your compiler
-documentation for more details.
-
-If you need to read or write custom chunks, you will need to get deeper
-into the libpng code, as a mechanism has not yet been supplied for user
-callbacks with custom chunks. First, read the PNG specification, and have
-a first level of understanding of how it works. Pay particular attention
-to the sections that describe chunk names, and look at how other chunks
-were designed, so you can do things similarly. Second, check out the
-sections of libpng that read and write chunks. Try to find a chunk that
-is similar to yours and copy off of it. More details can be found in the
-comments inside the code. A way of handling unknown chunks in a generic
-method, potentially via callback functions, would be best.
+documentation for more details. For an alternative approach, you may wish
+to use the "cexcept" facility (see http://cexcept.sourceforge.net).
+
+Custom chunks
+
+If you need to read or write custom chunks, you may need to get deeper
+into the libpng code. The library now has mechanisms for storing
+and writing chunks of unknown type; you can even declare callbacks
+for custom chunks. Hoewver, this may not be good enough if the
+library code itself needs to know about interactions between your
+chunk and existing `intrinsic' chunks.
+
+If you need to write a new intrinsic chunk, first read the PNG
+specification. Acquire a first level of
+understanding of how it works. Pay particular attention to the
+sections that describe chunk names, and look at how other chunks were
+designed, so you can do things similarly. Second, check out the
+sections of libpng that read and write chunks. Try to find a chunk
+that is similar to yours and use it as a template. More details can
+be found in the comments inside the code. It is best to handle unknown
+chunks in a generic method, via callback functions, instead of by
+modifying libpng functions.
If you wish to write your own transformation for the data, look through
the part of the code that does the transformations, and check out some of
transformation to the one you want to add and copy off of it. More details
can be found in the comments inside the code itself.
-Configuring for 16 bit platforms:
+Configuring for 16 bit platforms
-You may need to change the png_large_malloc() and png_large_free()
-routines in pngmem.c, as these are required to allocate 64K, although
-there is already support for many of the common DOS compilers. Also,
-you will want to look into zconf.h to tell zlib (and thus libpng) that
+You will want to look into zconf.h to tell zlib (and thus libpng) that
it cannot allocate more then 64K at a time. Even if you can, the memory
won't be accessible. So limit zlib and libpng to 64K by defining MAXSEG_64K.
-Configuring for DOS:
+Configuring for DOS
For DOS users who only have access to the lower 640K, you will
have to limit zlib's memory usage via a png_set_compression_mem_level()
call. See zlib.h or zconf.h in the zlib library for more information.
-Configuring for Medium Model:
+Configuring for Medium Model
Libpng's support for medium model has been tested on most of the popular
compilers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
all set. Everything in the library (except for zlib's structure) is
expecting far data. You must use the typedefs with the p or pp on
the end for pointers (or at least look at them and be careful). Make
-note that the row's of data are defined as png_bytepp, which is an
+note that the rows of data are defined as png_bytepp, which is an
unsigned char far * far *.
Configuring for gui/windowing platforms:
You will need to write new error and warning functions that use the GUI
interface, as described previously, and set them to be the error and
-warning functions at the time that png_create_???_struct() is called,
+warning functions at the time that png_create_*_struct() is called,
in order to have them available during the structure initialization.
They can be changed later via png_set_error_fn(). On some compilers,
you may also have to change the memory allocators (png_malloc, etc.).
Another useful one is to reduce the memory level used by the library.
The memory level defaults to 8, but it can be lowered if you are
short on memory (running DOS, for example, where you only have 640K).
+Note that the memory level does have an effect on compression; among
+other things, lower levels will result in sections of incompressible
+data being emitted in smaller stored blocks, with a correspondingly
+larger relative overhead of up to 15% in the worst case.
png_set_compression_mem_level(png_ptr, level);
png_set_compression_window_bits(png_ptr,
window_bits);
png_set_compression_method(png_ptr, method);
+ png_set_compression_buffer_size(png_ptr, size);
-Controlling row filtering:
+Controlling row filtering
If you want to control whether libpng uses filtering or not, which
filters are used, and how it goes about picking row filters, you
for any images with bit depths less than 8 bits/pixel.
The 'method' parameter sets the main filtering method, which is
-currently only '0' in the PNG 1.0 specification. The 'filters'
+currently only '0' in the PNG 1.2 specification. The 'filters'
parameter sets which filter(s), if any, should be used for each
scanline. Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
to turn filtering on and off, respectively.
Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
-ORed together '|' to specify one or more filters to use. These
-filters are described in more detail in the PNG specification. If
-you intend to change the filter type during the course of writing
+ORed together with '|' to specify one or more filters to use.
+These filters are described in more detail in the PNG specification.
+If you intend to change the filter type during the course of writing
the image, you should start with flags set for all of the filters
you intend to use so that libpng can initialize its internal
-structures appropriately for all of the filter types.
+structures appropriately for all of the filter types. (Note that this
+means the first row must always be adaptively filtered, because libpng
+currently does not allocate the filter buffers until png_write_row()
+is called for the first time.)
filters = PNG_FILTER_NONE | PNG_FILTER_SUB
- | PNG_FILTER_UP;
+ PNG_FILTER_UP | PNG_FILTER_AVE |
+ PNG_FILTER_PAETH | PNG_ALL_FILTERS;
+
png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
filters);
+ The second parameter can also be
+ PNG_INTRAPIXEL_DIFFERENCING if you are
+ writing a PNG to be embedded in a MNG
+ datastream. This parameter must be the
+ same as the value of filter_method used
+ in png_set_IHDR().
It is also possible to influence how libpng chooses from among the
-available filters. This is done in two ways - by telling it how
-important it is to keep the same filter for successive rows, and
-by telling it the relative computational costs of the filters.
+available filters. This is done in one or both of two ways - by
+telling it how important it is to keep the same filter for successive
+rows, and by telling it the relative computational costs of the filters.
double weights[3] = {1.5, 1.3, 1.1},
costs[PNG_FILTER_VALUE_LAST] =
{1.0, 1.3, 1.3, 1.5, 1.7};
- png_set_filter_selection(png_ptr,
- PNG_FILTER_SELECTION_WEIGHTED, 3,
+ png_set_filter_heuristics(png_ptr,
+ PNG_FILTER_HEURISTIC_WEIGHTED, 3,
weights, costs);
The weights are multiplying factors that indicate to libpng that the
are given only to help explain the function usage. Little testing has
been done to find optimum values for either the costs or the weights.
-Removing unwanted object code:
+Removing unwanted object code
There are a bunch of #define's in pngconf.h that control what parts of
libpng are compiled. All the defines end in _SUPPORTED. If you are
PNG_NO_.
You can also turn all of the transforms and ancillary chunk capabilities
-off en masse with compiler directives that define
+off en masse with compiler directives that define
PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS,
or all four,
along with directives to turn on any of the capabilities that you do
want. The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable
the extra transformations but still leave the library fully capable of reading
-and writing PNG files with all known public chunks [except for sPLT].
+and writing PNG files with all known public chunks
Use of the PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive
produces a library that is incapable of reading or writing ancillary chunks.
If you are not using the progressive reading capability, you can
The size of the library itself should not be an issue, because only
those sections that are actually used will be loaded into memory.
-Requesting debug printout:
+Requesting debug printout
The macro definition PNG_DEBUG can be used to request debugging
printout. Set it to an integer value in the range 0 to 3. Higher
having level = 0 will be printed. There aren't any such statements in
this version of libpng, but if you insert some they will be printed.
-VI. Changes to Libpng from version 0.88
+VI. Runtime optimization
+
+A new feature in libpng 1.2.0 is the ability to dynamically switch between
+standard and optimized versions of some routines. Currently these are
+limited to three computationally intensive tasks when reading PNG files:
+decoding row filters, expanding interlacing, and combining interlaced or
+transparent row data with previous row data. Currently the optimized
+versions are available only for x86 (Intel, AMD, etc.) platforms with
+MMX support, though this may change in future versions. (For example,
+the non-MMX assembler optimizations for zlib might become similarly
+runtime-selectable in future releases, in which case libpng could be
+extended to support them. Alternatively, the compile-time choice of
+floating-point versus integer routines for gamma correction might become
+runtime-selectable.)
+
+Because such optimizations tend to be very platform- and compiler-dependent,
+both in how they are written and in how they perform, the new runtime code
+in libpng has been written to allow programs to query, enable, and disable
+either specific optimizations or all such optimizations. For example, to
+enable all possible optimizations (bearing in mind that some "optimizations"
+may actually run more slowly in rare cases):
+
+ #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
+ png_uint_32 mask, flags;
+
+ flags = png_get_asm_flags(png_ptr);
+ mask = png_get_asm_flagmask(PNG_SELECT_READ | PNG_SELECT_WRITE);
+ png_set_asm_flags(png_ptr, flags | mask);
+ #endif
+
+To enable only optimizations relevant to reading PNGs, use PNG_SELECT_READ
+by itself when calling png_get_asm_flagmask(); similarly for optimizing
+only writing. To disable all optimizations:
+
+ #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
+ flags = png_get_asm_flags(png_ptr);
+ mask = png_get_asm_flagmask(PNG_SELECT_READ | PNG_SELECT_WRITE);
+ png_set_asm_flags(png_ptr, flags & ~mask);
+ #endif
+
+To enable or disable only MMX-related features, use png_get_mmx_flagmask()
+in place of png_get_asm_flagmask(). The mmx version takes one additional
+parameter:
+
+ #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
+ int selection = PNG_SELECT_READ | PNG_SELECT_WRITE;
+ int compilerID;
+
+ mask = png_get_mmx_flagmask(selection, &compilerID);
+ #endif
+
+On return, compilerID will indicate which version of the MMX assembler
+optimizations was compiled. Currently two flavors exist: Microsoft
+Visual C++ (compilerID == 1) and GNU C (a.k.a. gcc/gas, compilerID == 2).
+On non-x86 platforms or on systems compiled without MMX optimizations, a
+value of -1 is used.
+
+Note that both png_get_asm_flagmask() and png_get_mmx_flagmask() return
+all valid, settable optimization bits for the version of the library that's
+currently in use. In the case of shared (dynamically linked) libraries,
+this may include optimizations that did not exist at the time the code was
+written and compiled. It is also possible, of course, to enable only known,
+specific optimizations; for example:
+
+ #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
+ flags = PNG_ASM_FLAG_MMX_READ_COMBINE_ROW \
+ | PNG_ASM_FLAG_MMX_READ_INTERLACE \
+ | PNG_ASM_FLAG_MMX_READ_FILTER_SUB \
+ | PNG_ASM_FLAG_MMX_READ_FILTER_UP \
+ | PNG_ASM_FLAG_MMX_READ_FILTER_AVG \
+ | PNG_ASM_FLAG_MMX_READ_FILTER_PAETH ;
+ png_set_asm_flags(png_ptr, flags);
+ #endif
+
+This method would enable only the MMX read-optimizations available at the
+time of libpng 1.2.0's release, regardless of whether a later version of
+the DLL were actually being used. (Also note that these functions did not
+exist in versions older than 1.2.0, so any attempt to run a dynamically
+linked app on such an older version would fail.)
+
+To determine whether the processor supports MMX instructions at all, use
+the png_mmx_support() function:
+
+ #if defined(PNG_LIBPNG_VER) && (PNG_LIBPNG_VER >= 10200)
+ mmxsupport = png_mmx_support();
+ #endif
+
+It returns -1 if MMX support is not compiled into libpng, 0 if MMX code
+is compiled but MMX is not supported by the processor, or 1 if MMX support
+is fully available. Note that png_mmx_support(), png_get_mmx_flagmask(),
+and png_get_asm_flagmask() all may be called without allocating and ini-
+tializing any PNG structures (for example, as part of a usage screen or
+"about" box).
+
+The following code can be used to prevent an application from using the
+thread_unsafe features, even if libpng was built with PNG_THREAD_UNSAFE_OK
+defined:
+
+#if defined(PNG_USE_PNGGCCRD) && defined(PNG_ASSEMBLER_CODE_SUPPORTED) \
+ && defined(PNG_THREAD_UNSAFE_OK)
+ /* Disable thread-unsafe features of pnggccrd */
+ if (png_access_version() >= 10200)
+ {
+ png_uint_32 mmx_disable_mask = 0;
+ png_uint_32 asm_flags;
+
+ mmx_disable_mask |= ( PNG_ASM_FLAG_MMX_READ_COMBINE_ROW \
+ | PNG_ASM_FLAG_MMX_READ_FILTER_SUB \
+ | PNG_ASM_FLAG_MMX_READ_FILTER_AVG \
+ | PNG_ASM_FLAG_MMX_READ_FILTER_PAETH );
+ asm_flags = png_get_asm_flags(png_ptr);
+ png_set_asm_flags(png_ptr, asm_flags & ~mmx_disable_mask);
+ }
+#endif
+
+For more extensive examples of runtime querying, enabling and disabling
+of optimized features, see contrib/gregbook/readpng2.c in the libpng
+source-code distribution.
+
+VII. MNG support
+
+The MNG specification (available at http://www.libpng.org/pub/mng) allows
+certain extensions to PNG for PNG images that are embedded in MNG datastreams.
+Libpng can support some of these extensions. To enable them, use the
+png_permit_mng_features() function:
+
+ feature_set = png_permit_mng_features(png_ptr, mask)
+ mask is a png_uint_32 containing the logical OR of the
+ features you want to enable. These include
+ PNG_FLAG_MNG_EMPTY_PLTE
+ PNG_FLAG_MNG_FILTER_64
+ PNG_ALL_MNG_FEATURES
+ feature_set is a png_uint_32 that is the logical AND of
+ your mask with the set of MNG features that is
+ supported by the version of libpng that you are using.
+
+It is an error to use this function when reading or writing a standalone
+PNG file with the PNG 8-byte signature. The PNG datastream must be wrapped
+in a MNG datastream. As a minimum, it must have the MNG 8-byte signature
+and the MHDR and MEND chunks. Libpng does not provide support for these
+or any other MNG chunks; your application must provide its own support for
+them. You may wish to consider using libmng (available at
+http://www.libmng.com) instead.
+
+VIII. Changes to Libpng from version 0.88
It should be noted that versions of libpng later than 0.96 are not
distributed by the original libpng author, Guy Schalnat, nor by
still alive and well, but they have moved on to other things.
The old libpng functions png_read_init(), png_write_init(),
-png_info_init(), png_read_destroy(), and png_write_destory() have been
-moved to PNG_INTERNAL in version 0.95 to discourage their use. The
-preferred method of creating and initializing the libpng structures is
+png_info_init(), png_read_destroy(), and png_write_destroy() have been
+moved to PNG_INTERNAL in version 0.95 to discourage their use. These
+functions will be removed from libpng version 2.0.0.
+
+The preferred method of creating and initializing the libpng structures is
via the png_create_read_struct(), png_create_write_struct(), and
png_create_info_struct() because they isolate the size of the structures
from the application, allow version error checking, and also allow the
because this caused applications that do not use custom error functions
to fail if the png_ptr was not initialized to zero. It is still possible
to set the error callbacks AFTER png_read_init(), or to change them with
-png_set_error_fn(), which is essentially the same function, but with a
-new name to force compilation errors with applications that try to use
-the old method.
+png_set_error_fn(), which is essentially the same function, but with a new
+name to force compilation errors with applications that try to use the old
+method.
+
+Starting with version 1.0.7, you can find out which version of the library
+you are using at run-time:
+
+ png_uint_32 libpng_vn = png_access_version_number();
-VII. Y2K Compliance in libpng
+The number libpng_vn is constructed from the major version, minor
+version with leading zero, and release number with leading zero,
+(e.g., libpng_vn for version 1.0.7 is 10007).
-January 13, 1999
+You can also check which version of png.h you used when compiling your
+application:
+
+ png_uint_32 application_vn = PNG_LIBPNG_VER;
+
+IX. Y2K Compliance in libpng
+
+September 12, 2004
Since the PNG Development group is an ad-hoc body, we can't make
an official declaration.
-This is your unofficial assurance that libpng from version 0.81 and
-upward are Y2K compliant. It is my belief that earlier versions were
-also Y2K compliant.
+This is your unofficial assurance that libpng from version 0.71 and
+upward through 1.2.7 are Y2K compliant. It is my belief that earlier
+versions were also Y2K compliant.
Libpng only has three year fields. One is a 2-byte unsigned integer that
will hold years up to 65535. The other two hold the date in text
There are seven time-related functions:
- png_convert_to_rfc_1123() in png.c
+ png_convert_to_rfc_1123() in png.c
(formerly png_convert_to_rfc_1152() in error)
- png_convert_from_struct_tm() in pngwrite.c, called in pngwrite.c
+ png_convert_from_struct_tm() in pngwrite.c, called
+ in pngwrite.c
png_convert_from_time_t() in pngwrite.c
png_get_tIME() in pngget.c
png_handle_tIME() in pngrutil.c, called in pngread.c
png_set_tIME() in pngset.c
png_write_tIME() in pngwutil.c, called in pngwrite.c
-All appear to handle dates properly in a Y2K environment. The
+All appear to handle dates properly in a Y2K environment. The
png_convert_from_time_t() function calls gmtime() to convert from system
clock time, which returns (year - 1900), which we properly convert to
the full 4-digit year. There is a possibility that applications using
libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
-function, or incorrectly passing only a 2-digit year instead of
-"year - 1900" into the png_convert_from_struct_tm() function, but this
-is not under our control. The libpng documentation has always stated
-that it works with 4-digit years, and the APIs have been documented as
-such.
+function, or that they are incorrectly passing only a 2-digit year
+instead of "year - 1900" into the png_convert_from_struct_tm() function,
+but this is not under our control. The libpng documentation has always
+stated that it works with 4-digit years, and the APIs have been
+documented as such.
The tIME chunk itself is also Y2K compliant. It uses a 2-byte unsigned
integer to hold the year, and can hold years as large as 65535.
+zlib, upon which libpng depends, is also Y2K compliant. It contains
+no date-related code.
+
Glenn Randers-Pehrson
libpng maintainer
projects / wxWidgets.git / blobdiff