libpng.txt - A description on how to use and modify libpng
- libpng version 1.2.6 - August 15, 2004
+ libpng version 1.2.7 - September 12, 2004
Updated and distributed by Glenn Randers-Pehrson
<glennrp@users.sourceforge.net>
Copyright (c) 1998-2004 Glenn Randers-Pehrson
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
(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
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:
(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
of optimized features, see contrib/gregbook/readpng2.c in the libpng
source-code distribution.
-VI. MNG support
+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.
them. You may wish to consider using libmng (available at
http://www.libmng.com) instead.
-VII. Changes to Libpng from version 0.88
+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
png_uint_32 application_vn = PNG_LIBPNG_VER;
-VII. Y2K Compliance in libpng
+IX. Y2K Compliance in libpng
-August 15, 2004
+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 1.2.6 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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