Help using the color grading filter

I am using 2 video sources and I would like to tweak the sky color.

I tried using the highlights setting and the following setting seems quite good to my eye.
R: 0% G: -3% B: -1.5%

I then thought, hey why not calculate the perfect settings.
So switched off the filter, took a screen shot and pasted it into inkscape.
I sampled the more correct looking bit of sky: R:185 G: 205 B: 231
The less correct bit was: R:175 G:225 B:255

I calculated the correction as roughly R: +5.7% G: -8.9% B:-9.5%
I then used those numbers in the filter… way off.
Resulting color is: R:193 G:186 B:209

So I tried R: +2.8 G:-4.5 B:-4.8
Bingo! perfect match. Why are my calcs wrong?

Your calculations would have been correct if processing was done in gamma space. But the color grading filter processes midtones and highlights in linear space (which means light intensity increases like an exponential curve rather than a constant angle on a graph). Since these color values are sitting at the halfway intensity point on a graph, the exponential curve is getting steeper and it takes less percentage movement (the X axis) to get bigger changes in color (the Y axis). Some notes are included in the filter’s documentation:

https://www.mltframework.org/plugins/FilterLift_gamma_gain/

2 Likes

Thank you Austin for the prompt and clear answer.