Default color space for untagged surfaces

All color managing compositors need to be able to handle surfaces that the client has not indicated any image description (color space) for, commonly known as untagged surfaces.

Since the old and widespread standard that has been implicitly used with monitors and computer content is sRGB, at first hand it would appear that sRGB is the obvious choice for any untagged surface.

However, I suspect that may cause end users to be upset. The standard sRGB is fairly dim in luminance and narrow in color gamut. OTOH, consumer monitors have been long displaying an unidentified RGB signal with a brighter image and wider color gamut than sRGB defines (right?). One could have set their monitor to real sRGB mode, but that looks dull and does not let the monitor to show off its capabilities, so who would use that? People have accustomed to the brighter and wider colors. Enabling HDR support which mandates color management would therefore turn all color unaware applications pretty dull and dim in their colors.

Luminance of legacy/SDR content is something we have already thought might need an end user setting to overcome the above deviation from sRGB.

I hadn't thought about the color gamut until now, though. It likely has the same problem. Maybe the default color space for untagged content should be wider than sRGB? But how wide? Maybe it needs to depend on the monitor at hand?

One option would be to try to guess how a monitor would display an unidentified RGB signal - well, not really, but to guess what signal colorimetry would look nice on that monitor. A compositor could manufacture a different default content description for each output: some dynamic range that looks fine as SDR, and some color gamut that fits end user expectations, maybe derived from monitor capabilities and adjustable in desktop monitor settings with a list of a few presets.

My main point here is that claiming sRGB is the best default is probably not true.