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Skimming the JPEG spec I think the commit author was waaaay off base. The actual spec is very clear that 4 components is the max and are expected to be sequential in nature (4.11 Summary of coding processes) unless I'm reading it wrong. Ultimately the original implementation was wrong fro not including a fourth channel per spec... but not wrong in assuming sequential.

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Even if there are more than 4 components, they are still required to be sequentially ordered. Their decoder also errors out if there are more than 3 components (https://news.ycombinator.com/item?id=27375596), so all you need to do is keep the IDs in an array of 3 entries and check that they match later. That's what the fragment of code I posted from my decoder does.

That's what I'd assumed when I started skimming the spec, but the section I referenced seems to indicate a max of four regardless. I've never heard of a JPEG that's not RGB(A) or CMYK. So it would be highly unusual, not saying it couldn't happen but given how unusual it would probably be replaced with a custom format for that specific application.

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