Warp wrote:

>   I think PNG has options to filter with the previous row, the previous
> column, or both.

Yes.

Each row can be filtered in one of these ways (or not filtered at all). 
The choice is independent per row.

> I assume that if you choose to filter with the previous
> column only, you don't need to store the previous row of pixels in memory
> at all.

Yes.

> (I don't know if libpng optimizes in this way, but at least in
> theory I think it could be possible to implement png compression like that.)

Indeed - I have no idea if libpng has an option to control this...

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