On the Mac, raster files are stored in a different file format, suffixed .png.binltl. These files are neither PNG format nor are they encrypted.

The format of the file is as follows. Note that all data types are little-endian.

Offset Datatype Description
0x00 unsigned 16-bit int   width of the final image
0x02 unsigned 16-bit int height of the final image
0x04 unsigned 32-bit int size of compressed data
0x08 unsigned 32-bit int size of uncompressed data
0x12 ... compressed image data

The image data is compressed using zlib deflate. When uncompressed it is a stream of RGBA bytes in that order, so four bytes per pixel.

The actual dimensions of the uncompressed image are always square, with each side being a power of two. The dimensions are the smallest power-of-two square that completely encloses the actual source image (whose size appears in the header).

Thus, a 18x12 image would be stored in a 32x32 square, but a 512x512 image would be in a 512x512 square.

The square should be cropped to the image size specified in the header. Pixels outside these bounds have undefined values.