Change Chroma Subsample for Image files

Is there a setting in vips jpegsave that allows to change chroma subsampling of input images. For web use I think the recommended setting is 4:2:0 or 2x2.

thanks.

[jcupitt]

For jpeg write it's just on or off. I'll tag this as an ehancement.

Edit: default (4:2:0) or off, I should say.

Thanks @jcupitt for the prompt response. Is it possible to support it the way GraphicsMagick does:

http://www.graphicsmagick.org/GraphicsMagick.html#details-sampling-factor

Typically if you take screenshot or framegrab from the video sources they may come with subsampling as 4:4:4 and I think it may make sense to provide at least following options:

1. 4:4:4 (or no subsampling)
2. downsample to 4:2:2
3. downsample to most popular 4:2:0

thanks.

[jcupitt]

Looking at the definition of the subsampling notation here:

http://www.poynton.com/PDFs/Chroma_subsampling_notation.pdf

Current libvips behaviour is:

• vips copy x.png x.jpg[Q=80] below Q90, libvips will write 4:2:0, ie. 2:1 horizontal and vertical subsampling of Cr and Cb, the jpeg standard.
• vips copy x.png x.jpg[Q=95] above Q90, libvips will write 4:4:4, ie. no subsampling of Cr and Cb, following ImageMagick's practice.
• vips copy x.png x.jpg[Q=80,no-subsample] overrides the automatic enabling of subsample and will write 4:4:4.

You can't write Q > 90 and subsample, but that must be rather rare, I think.

It doesn't seem that easy to add all these options, especially the offset chroma you need for 4:2:2. Could you explain the use case?

[jcupitt]

From 8.9 there is a subsample-mode option to jpegsave. You can set it on, off or auto. In auto mode, chroma subsample is disabled if Q is 90 or more. The default mode is auto.