common.glsl: Introduce linearToRelativeLuminance()

[Mugen87]

see #11986

[Mugen87]

@WestLangley I've just changed LuminosityShader for now. Do you think we need further comments like proposed in #11986 (comment) ?

[WestLangley]

For now, I would just call the function relativeLuminance() and not differentiate between linear or sRGB inputs. This is a subtlety that most users will neither understand, nor care about.

If @bhouston feels strongly about keeping the name linearToRelativeLuminance, then you should modify your examples to only use the function when you know the input space is linear.

[Mugen87]

Okay!

[WestLangley]

Let's make sure @bhouston finds this acceptable.

[bhouston]

Let's make sure @bhouston finds this acceptable.

It is for linear RGB color spaces, it isn't a general function. There is a different function for dealing sRGB or gamma corrected spaces, basically you need to decode sRGB or gamma RGB to linear and then apply this.

So I much prefer the original name because it will remind people to be in linear space before using this conversion.

[Mugen87]

Only two examples make use of LuminosityShader right now. webgl_postprocessing_sobel and webgl_shaders_tonemapping. In both cases LuminosityShader works on a linear input. So linearToRelativeLuminance() should be consistent with regard to the examples.

I think only ToneMapShader needs a fix.

[Mugen87]

Should be okay now!

[mrdoob]

Thanks!