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The case of the fourth letter indicates whether the chunk is safe to copy by editors that do not recognize it.If lowercase, the chunk may be safely copied regardless of the extent of modifications to the file.The case of the first letter indicates whether the chunk is critical or not.If the first letter is uppercase, the chunk is critical; if not, the chunk is ancillary.
The original PNG specification was authored by an ad-hoc group of computer graphics experts and enthusiasts.
A chunk consists of four parts: length (4 bytes, big-endian), chunk type/name (4 bytes), chunk data (length bytes) and CRC (cyclic redundancy code/checksum; 4 bytes).
The CRC is a network-byte-order CRC-32 computed over the chunk type and chunk data, but not the length.
Chunks declare themselves as critical or ancillary, and a program encountering an ancillary chunk that it does not understand can safely ignore it.
This chunk-based storage layer structure, similar in concept to a container format or to Amigas IFF, is designed to allow the PNG format to be extended while maintaining compatibility with older versions—it provides forward compatibility, and this same file structure (with different signature and chunks) is used in the associated MNG, JNG, and APNG formats.