Comment by labawi

5 years ago

If you want physically accurate averaging (resize, blur etc), then RGB is fine, as long as you use linear values (or do long-winded transformed math). AFAIU it is by definition 100% physically accurate. As was said, sRGB uses gamma values, where typical math creates ill effects, as in many if not most typical programs.

If you want to do perceptually uniform averaging of colors, color mixing / generating / artistic effects, that's something else entirely.

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labawi

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BlueTemplar  5 years ago

I'm not sure what you mean by "physically accurate" ?

No color reproduction is going to be physically accurate, except by accident, since color is an (average) human qualia, not a physical observable.

And especially because whatever the way that the colors are going to be reproduced, there's no guarantee that they will correspond to the same light spectrum than the original, since there's an infinity of spectra corresponding to the same impression for a specific color.

And if you want to do perceptually accurate averaging, you're going to have to work in a perceptually uniform color space ! Which (even linear) sRGB isn't.

All that said, it's perfectly possible that linear sRGB is just good enough for most use cases of averaging, even despite not being perceptually uniform. Especially in OP's example with just 2 shades : black and white.

  • labawi  5 years ago

    Yes, color is a human concept for certain stimuli, no recording or reproduction is 100% accurate and even the model is not perfect (different people have shifted color sensitivities, some are even tetrachromats and we are ignoring rods vs. cones altogether).

    However, the stimuli is predominantly light, which can be quantified and certain operations with light are well studied. Not all, but most noticeable ones are and that is what is used when doing photorealistic rendering etc.

    If you do a 2x downscale, you need to average 4 pixels together. Linear RGB should (by theory and definition) give you a (theoretically) physically accurate result, as if you did a physical 2x downscale by putting it further away in a uniformly lit room or whatever. You can't reproduce that precisely, but neither can you reproduce 1l + 1l of milk = 2l of milk.

    This is in stark contrast to downsizing, blur or whatever "performed" in sRGB, where images get curiously darker and slightly off-color (or is it lighter?).

    I'm not sure what is perceptual average of colors, but if you mix pixels in 1:1 ratio and apply blur / look from far away, there is only one correct result, and linear RGB average (is one way that) gives you the single correct result. (ignoring screen color reproduction deviations from what they are supposed to be)