Documentation on grAb data?

[DavidN]

I'm writing a tool for myself to do a couple of things that setpng can't, such as being able to copy grAb data from one sprite to another - is there any documentation on the grAb chunk's format? The source for setpng doesn't seem to be available any more, and the wiki page https://zdoom.org/wiki/GrAb describes it as 8 bytes long, representing the X and Y coordinates of the offset - but there are four more mystery bytes that I'm not sure about.



This is a file with offsets [1, 1] compared to the same image with offsets [35, 148] - 00 00 00 01 00 00 00 01 and 00 00 00 23 00 00 00 94. But what are the next four bytes before the chunk ends?

[Graf Zahl]

The final 4 bytes are not part of the chunk itself but a CRC checksum that must be prepended to each PNG chunk, it's part of the PNG format.

[DavidN]

Thank you! I'll go and look that up.