To allow new chunk types to be added to PNG, it is necessary to establish rules about the ordering requirements for all chunk types. Otherwise a PNG editing program cannot know what to do when it encounters an unknown chunk.
We define a "PNG editor" as a program that modifies a PNG file and wishes to preserve as much as possible of the ancillary information in the file. Two examples of PNG editors are a program that adds or modifies text chunks, and a program that adds a suggested palette to a truecolor PNG file. Ordinary image editors are not PNG editors in this sense, because they usually discard all unrecognized information while reading in an image. (Note: we strongly encourage programs handling PNG files to preserve ancillary information whenever possible.)
As an example of possible problems, consider a hypothetical new ancillary chunk type that is safe-to-copy and is required to appear after PLTE if PLTE is present. If our program to add a suggested PLTE does not recognize this new chunk, it may insert PLTE in the wrong place, namely after the new chunk. We could prevent such problems by requiring PNG editors to discard all unknown chunks, but that is a very unattractive solution. Instead, PNG requires ancillary chunks not to have ordering restrictions like this.
To prevent this type of problem while allowing for future extension, we put some constraints on both the behavior of PNG editors and the allowed ordering requirements for chunks.
The rules for PNG editors are:
These rules are expressed in terms of copying chunks from an input file to an output file, but they apply in the obvious way if a PNG file is modified in place.
See also Chunk naming conventions.
The ordering rules for an ancillary chunk type cannot be any stricter than this:
The actual ordering rules for any particular ancillary chunk type may be weaker. See for example the ordering rules for the standard ancillary chunk types (Summary of standard chunks).
Decoders must not assume more about the positioning of any ancillary chunk than is specified by the chunk ordering rules. In particular, it is never valid to assume that a specific ancillary chunk type occurs with any particular positioning relative to other ancillary chunks. (For example, it is unsafe to assume that your private ancillary chunk occurs immediately before IEND. Even if your application always writes it there, a PNG editor might have inserted some other ancillary chunk after it. But you can safely assume that your chunk will remain somewhere between IDAT and IEND.)
Critical chunks can have arbitrary ordering requirements, because PNG editors are required to give up if they encounter unknown critical chunks. For example, IHDR has the special ordering rule that it must always appear first. A PNG editor, or indeed any PNG-writing program, must know and follow the ordering rules for any critical chunk type that it can emit.