Chroma subsampling

[[Mike]]

Hello there, I'm looking for some more indepth info about chroma subsampling and the options provided in the save dialog. Mostly because of the unusual notation used.

I know that 1x1 1x1 1x1 is actually 4:4:4, but how can I figure out the other values. Is there some formula or calculation I can use or apply to go from for example 2x2 2x1 2x1 to its more widely accepted counterpart?

[LeviFiction]

https://zpl.fi/chroma-subsampling-and-j ... g-factors/

1x1,1x1,1x1 = 4:4:4
2x1,1x1,1x1 = 4:2:2
1x2,1x1,1x1 = 4:4:0
2x2,1x1,1x1 = 4:2:0
2x2,2x1,2x1 = 4:4:0
4x2,1x1,1x1 = 4:1:0
etc

[[Mike]]

That's an interesting read, however, I still don't understand how to go from 2x2,1x1,1x1 to 4:2:0 and the other way around. There are some formulas mentioned, but I'm missing a step (or two).

Why doesn't PSP just use the more common notation, that's a lot more understandable.

[LeviFiction]

It says at the beginning of the article that JPEG uses the odd formatting specifically "JPEG uses a different format". JPEG's format has more options and it's not a direct comparison.

As for the article, I intended it as a look-up table, not as an equation. And as you'll see In that article that some of the JPEG subsamplings don't have a representation in jab format, others result in the same jab interpretation. Honestly, all of the samplings listed in PSP are converted for you in that article with the exception of the last one. No idea what that could be because you're right he doesn't properly explain the relationship or why you're getting the relative H and V or how they get turned into the fractions or how the fractions then get turned into the jab format.

Here's the full list, in order, from JPEG format to jab format. All taken from that article.

1x1,1x1,1x1 = 4:4:4
2x1,1x1,1x1 = 4:2:2
1x2,1x1,1x1 = 4:4:0
2x2,1x1,1x1 = 4:2:0
2x2,2x1,2x1 = 4:4:0
4x2,1x1,1x1 = 4:1:0
2x4 1x1 1x1 = n/a
4x1 1x1 1x1 = 4:1:1
1x4 1x1 1x1 = n/a
4x1 2x1 2x1 = 4:2:2
1x4 1x2 1x2 = 4:4:0

4x4 2x2 2x2 = can't tell if this would be H1/1 V1/2 or H1/2 V1/2 = could be anything from 4:4:0 to 4:2:0 no idea.

[[Mike]]

I guess I was looking for an easy conversion rule where there wasn't one.

Thanks for the elaboration!

[Jean-Luc]

Another link :
https://matthews.sites.wfu.edu/misc/jpg ... maSub.html

[[Mike]]

Thanks, the concrete examples really help understanding this better.