Reverting Apple’s “PNG’s”
Essentially, the authors of this tool figured out the optimizations that Apple were doing, and reversed the steps to convert them back into standard PNG images. The tool relied upon a compiled-in version of the libpng library.
Recently when I went to use this tool again, I noticed that it did not work reliably. It dumped lots of memory errors and complained of “extra compressed data” in the input files. I then proceeded to spend too many hours trying to fix it, bashing my head against libpng, before my friend Tom Harrington alerted me to an Xcode-bundled tool that can do the job reliably:
iPhoneOS.platform/Developer/usr/bin/pngcrush
…a more accurate description of the issue is that they are not PNGs despite using a PNG header and similar file structure and purloining the .png extension.
The PNG specification has very specific conformance requirements specifically to avoid incompatible variants popping up, and disguising the iPhone-optimized pictures as PNGs is a downright Microsoftishly destructive thing to do.
