Apple screenshot PNGs contain non-standard chunk

I did some idle poking around in the PNG file format, and found something weird.

Here’s the gist.

I read up just enough on the .PNG standard to get a sense of how the data is structured. The first few bytes in the file are the same on every PNG, followed by a series of “chunks” that are structured the same way.

Each chunk has a chunk type, which is one of set of predefined chunks, such as IHDR for the image header, and IDAT for the image data. I wrote a simple loop to run through each chunk and print out the chunk type and its length in bytes.

Oddly enough, the .PNG I was using to test out my code had a chunk type that wasn’t documented: iDOT. Here’s the out (note that because there are so many IDAT chunks in a given file, I just print the number and combined size at the end, or else the terminal output would just be way too long.):

     Valid PNG signature
     ====================
     chunk_length: 13
     chunk_type:  IHDR
     width:  2818
     height:  2328
     bit_depth:  8
     colour_type:  6c
     compression_method:  0
     filter_method:  0
     interlace_method:  0
     ====================
     chunk_length: 2728
     chunk_type:  iCCP
     ====================
     chunk_length: 9
     chunk_type:  pHYs
     ====================
     chunk_length: 415
     chunk_type:  iTXt
     ====================
     chunk_length: 28
     chunk_type:  iDOT
     ====================
     chunk_length: 0
     chunk_type:  IEND
     ====================
     Found a total of 642 IDAT chunks consuming 10512756 bytes.