Quote - so this is my result with using (from left to right) GC, sRGB, and the new power method.  same properties, infinite lights, my very bright setup.   i have no clue what i've done wrong to produce the color shift, but i thought i'd post.

Did you have a blue IBL in the lighting?

GC and sRGB tend to take fractional values and bring them closer to 1. The ratio of the new value versus the old value is smallest when the value is already close to 1 and largest when the old value is near 0.

Knowing that, consider the color RGB .75, .5, .25. These color components are in the ratio of 3:2:1. What happens when you apply GC(2.2) to that color?

You get .88, .73, .53. The ratios are now roughly 1.65:1.37:1.

Roughly speaking, the sensation we call hue is due to which components are strongest and second strongest. There are six bands of hue possible; Rg, Rb, Gr, Gb, Br, and Bg. This color is in the Rg (Red, green) band. Now the particular hue within the band is due to the particular ratio of the brightest to middle value. So the new color has a somewhat different hue than the old color.

Roughly speaking, the sensation we call saturation is due to the relationship between the strongest and the weakest component, not ratio exactly, but we can still understand how saturation is being influenced by comparing ratios. We started with 3:1 and ended with 1.65 to 1. Clearly this is a reduction of saturation.

So, GC shifts hue, decreases saturation, and increases luminance. We compensate for this by applying anti-GC to the incoming colors first, which does the opposite in a way that attempts to balance the hue and saturation effect, while not balancing the luminance effect. The net effect is to increase luminance with a minimal shift in hue and saturation.

The problem with GC is if you do not anti-GC the incoming material to balance the HS effects, you end up with a washed out appearance. Everything moves closer to white. Pastel (weak saturation, high luminance) colors become white, and colors with strong saturation and medium luminance become pastel. So it is really not useful to apply GC alone as a final processing step.

The new technique I showed attempts to preserve hue and saturation, only altering luminance. And since it does, if there is any slight amount of blue in your lighting, and you're testing with a white prop, the blue will tend to disappear after GC, while not so in the "exponential" technique.

By the way, the technique is similar to (or maybe exactly the same as) the tone mapping technique called HSV Exponential Tone Mapping or something like that. I can't find any clear CG-community approved exact mathematical definitions. I can only find verbal descriptions of these techniques in other products, like VRay and Kerkythea. In all cases they talk about adjusting luminance while preserving hue and saturation. It is from those little clues and the words HSV and Exponential that I guessed at how to do it.

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