Internal IDs for fragment references

[Crissov]

https://github.com/Crissov/PNG-extensions/blob/master/fmAP.md

Make parts or sub-images addressable with fragment identifiers (#foo) in URLs.

• individual fdAT/fcTL frames of APNGs APNG is widely supported, add to spec, register image/apng #26
• rectangular sections of PNGs, e.g. in sprite sheets

[Crissov]

Crissov/PNG-extensions#1

[ProgramMax]

That first link for the proposed new fmAP chunk allows up to 79 bytes for the fragment name.
Is there a reason to limit it's size?

[ProgramMax]

I realized that several chunks have Latin-1 strings up to 79 bytes.
For example, the iCCP profile name or tEXt chunk's keyword field.

So the 79 byte fragment name might be because of these precedents?

But you'll also notice the tEXt chunk also have a 0-or-more-byte string for the contents of the chunk. Since Media Fragments can be an arbitrary length, I suggest we not limit to 79 bytes.

Additionally, we should probably put a null separator between the language and the fragment_name parts.

[Crissov]

Is there any chance that the named addressing of APNG frames could still be added to the upcoming Third Edition?

Named spatial regions can wait for the Fourth.

[svgeesus]

@Crissov If we have, say, an APNG with 8 frames, and there is a way to use a fragment ID to link to the fourth frame, is the result:

• the fourth frame is displayed, as a static image
• an animation is displayed, starting with the fourth frame

[svgeesus]

There is no need to limit the length of the language tag. For example iTXT does not limit it; instead it is delimited by a null separator.

An empty or zero sequence number refers to IDAT

However, the first sequence number in an APNG where IDAT is not the first frame is also zero:

The fcTL and fdAT chunks have a zero-based, 4 byte sequence number. Both chunk types share the sequence.

[Crissov]

To be honest, I only ever considered addressing a single named frame, not a sub-sequence. If one could target a range of indexed frames, I guess some people would then also want to freely reorder them, e.g. animation.apng#3,5-8,3,9,3,0-2,10-. If users could to that, suddenly named alternate sequences would be another usage scenario, so the above could be predefined inside the file and used as animation.apng#altseq.