They could have created their own chunk type and set the safe-to-copy but to 1, permitting apps to round-trip the chunk without knowing exactly what was in it.

They could have created their own chunk type and set the safe-to-copy but to 1, permitting apps to round-trip the chunk without knowing exactly what was in it. From the spec: > If a chunk's safe-to-copy bit is 1, the chunk may be copied to a modified PNG file whether or not the software recognizes the chunk type, and regardless of the extent of the file modifications.