libpng.txt - A description on how to use and modify libpng
- libpng version 1.2.4 - July 8, 2002
+ libpng version 1.2.7 - September 12, 2004
Updated and distributed by Glenn Randers-Pehrson
- <randeg@alum.rpi.edu>
- Copyright (c) 1998-2002 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.
of reducing the amount of time and effort it takes to support the PNG
file format in application programs.
-The PNG-1.2 specification is available at <http://www.libpng.org/pub/png>
-and at <ftp://ftp.uu.net/graphics/png/documents/>.
+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 <ftp://ftp.uu.net/graphics/png/documents/> and as a
+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/>.
+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.libpng.org/pub/png/>
-and at <ftp://ftp.uu.net/graphics/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
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
There are two main structures that are important to libpng, png_struct
png_set_read_status_fn(png_ptr, read_row_callback);
+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
various info_ptr members; unknown chunks will be discarded. To change
this, you can call:
- png_set_keep_unknown_chunks(png_ptr, info_ptr, keep,
+ png_set_keep_unknown_chunks(png_ptr, keep,
chunk_list, num_chunks);
- keep - 0: do not keep
- 1: keep only if safe-to-copy
- 2: keep even if unsafe-to-copy
+ 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
+ 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.
+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
(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
If you know your image size and pixel size ahead of time, you can allocate
row_pointers prior to calling png_read_png() with
+ 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*sizeof(png_bytep));
+ height*png_sizeof(png_bytep));
for (int i=0; i<height, i++)
row_pointers[i]=png_malloc(png_ptr,
width*pixel_size);
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
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:
if (png_get_valid(png_ptr, info_ptr,
PNG_INFO_PLTE))
{
- png_uint_16p histogram;
+ png_uint_16p histogram = NULL;
png_get_hIST(png_ptr, info_ptr,
&histogram);
(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
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);
png_size_t size);
void free_fn(png_structp png_ptr, png_voidp ptr);
-Your malloc_fn() should return NULL in case of failure. The png_malloc()
-function will call png_error() if it receives a NULL from the system
-memory allocator or from your replacement malloc_fn().
+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
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
PNG_FLAG_MNG_EMPTY_PLTE
PNG_FLAG_MNG_FILTER_64
PNG_ALL_MNG_FEATURES
- feature_set is a png_32_uint that is the logical AND of
+ 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.
IX. Y2K Compliance in libpng
-July 8, 2002
+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 0.71 and
-upward through 1.2.4 are Y2K compliant. It is my belief that earlier
+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
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