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Added some commentary about Xcode PNG files and Apple's version of pngcrush.

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This commit is contained in:
Glenn Randers-Pehrson glennrp@comcast.net
2009-05-01 10:54:41 -05:00
parent 1f8b1427e8
commit 4d9664b8b2
1 changed files with 30 additions and 2 deletions
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32
pngcrush.c
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@@ -25,6 +25,32 @@
* Thanks to Stephan Levavej for some helpful suggestions about gcc compiler
* options and for a suggestion to increase the Z_MEM_LEVEL from default.
*
* Caution: there is another version of pngcrush that has been distributed by
* Apple since mid-2008 as a part of the Xcode SDK. Although it claims
* to be pngcrush by Glenn Randers-Pehrson, it has additional options
* "-iPhone" * and "-speed". It writes files that have the PNG 8-byte signature
* but are not valid PNG files, due to at least
*
* o the presence of the CgBI chunk ahead of the IHDR chunk,
* o nonstandard deflate compression in IDAT, iCCP, and perhaps zTXt chunks,
* o the use of premultiplied alpha in color_type 6 files, and
* o the sample order is ARGB instead of RGBA in color_type 6 files.
*
* The original PNG file cannot be losslessly recovered from such files
* because of the use of premultiplied alpha.
*
* Most PNG decoders will recognize the fact that an unknown critical
* chunk "CgBI" is present and will immediately reject the file.
*
* It is said that the Xcode version of pngcrush is automatically applied
* when PNG files are prepared for downloading to the iPhone unless the
* user takes special measures to prevent it.
*
* I have not seen the source for the Xcode version of pngcrush. All I
* know, for now, is from running "strings -a" on a copy of the executable,
* looking at two Xcode-PNG files, and reading Apple's patent application
* <http://www.freepatentsonline.com/y2008/0177769.html>.
*
*/
#define PNGCRUSH_VERSION "1.6.17"
@@ -4016,8 +4042,10 @@ int main(int argc, char *argv[])
png_crush_pause();
if (found_CgBI)
png_warning(read_ptr,
"Cannot read Xcode CgBI PNG. Even if we could,");
png_error(read_ptr,
"Cannot read Xcode CgBI PNG.");
"the original PNG could be recovered.");
P1( "\nWriting info struct\n");
@@ -4912,7 +4940,7 @@ png_uint_32 png_measure_idat(png_structp png_ptr)
{
printf (" Removing the CgBI chunk.\n");
} else {
printf (" Try \"pngcrush -fix ...\" to convert it to PNG.\n");
printf (" Try \"pngcrush -fix ...\" to attempt to read it.\n");
}
found_CgBI++;
nosave++;