black and white mode

[totaam]

We already have "png/L" support, but we could add a global black-and-white mode to try to save bandwidth:

• jpeg supports TJPF_GRAY
• all the video encoders would need a csc step
• webp would need csc too

I'm not sure this will save enough bandwidth: the self-tuning video encode heuristics normally switch to YUV420P for video, and that already reduces the space taken by the colours to 33% or less of the bitstream.
For non-video, it may allow us to use a lossless encoding more often than we would otherwise.

[totaam]

I don't see the point:

• many things don't degrade well
• none of the video encoders take grayscale as input
• the cost of chroma is a small overhead (just a 33% with YUV420P - worst case scenario - often less if the video encoder interpolates chroma and luma)
• the cost of a single lossless refresh dwarfs any savings we might have made at that point - best to make efficient use of the encodings we have than to invent new ones

[totaam]

Let's give it a go: libyuv does have a ARGBGrayTo function we can use for video and webp codecs.

[totaam]

Updates:

• r26698 adds grayscale as a valid encoding option, ie:

xpra attach --encoding=grayscale

(also works from the tray menu)

• r26699 supports grayscale mode for 'jpeg' and 'webp' via pillow
• r26700 supports grayscale with video modes (via ARGBGrayTo)

[totaam]

Fixups in r27138.