PNGDecrush

PNGDecrush is a C# library for reversing the optimization process that is applied to PNG files in an iOS project.

Background

When building an iOS app using Xcode, PNG files are run through the pngcrush tool with some proprietry Apple modifications (accessed through the -iphone option).

xcrun -sdk iphoneos pngcrush -iphone ...

Specifically, the following steps are performed:

• The proprietry and non-standard CgBI chunk is added to the start of the PNG.
• Zlib headers and checksums are removed from all IDAT chunks (leaving just raw deflate data).
• Pixel data is converted to BGR/BGRA byte order.
• Pixel color values are pre-multiplied with the alpha.

Existing implementations are broken

There are some existing 'decrush' implementations out there if you google around, all of which are broken for a number of different reasons, including:

• They perform a very naive byte swap on the raw PNG image bytes without reversing the precompression line filters beforehand. This will cause sometimes subtle, sometimes not-so-subtle artifacts in the resulting image.
• They do not handle multiple IDAT chunks correctly.
• Their code quality is lacking and they do not have tests.

This implementation

This implemention performs the following steps:

• The CgBI chunk is removed
• Chunk data is inflated and deflated again with zlib headers intact
• The resulting PNG is read into .NET's standard image manipulation classes where:
  • The byte swap is reversed
  • The premultiplied alpha is reversed

We have unit tests, and everything is stream based.

A simple example of using the library:

using (FileStream input = File.OpenRead('/path/to/input.png'))
using (FileStream output = File.Create('/path/to/output.png'))
{
	try
	{
	    PNGDecrusher.Decrush(input, output);
	}
	catch (InvalidDataException)
    {
        // decrushing failed, either an invalid PNG or it wasn't crushed
    }
}