ASC-CDL color adjustments in PhotoFlow

[Elle (Elle Stone)]

@Carmelo_DrRaw

range of sliders:
as already mentioned, the current sliders follow the same logic as those in Darktable, in order to have some common ground. In particular:

• slope = slider + 1
  a value of “0” in the slider corresponds to slope=1, i.e. no change
  a value of “+1” corresponds to slope=2
  a value of “-1” corresponds to slope=0, i.e. pure black

Putting darktable to one side, I’m just trying to understand what these sliders are supposed to do. I can’t emulate a log2 curve in GIMP, but when working on linear RGB, I was assuming that modifying the slope in each channel individually is the same as multiplying by a color:

ASC-CDL compliant adjustments and reference colorspace

@Carmelo_DrRaw - thanks! for posting above - I was wondering about what might be equivalent to what. The Wikipedia article says nine operations, slope/offset/power applied to each channel individually. I’m feeling very mathematically stupid at the moment, worse than usual! So is below correct? Multiply/divide by a color (including gray): offset in log2 encoding <–> slope in linear encoding Add/subtract a color (including gray): offset in linear encoding → ??? in log2 encoding Rai…

@Elle

I though that “slope” on linear RGB would be the equivalent of multiplying by a color. I set the R/G/B values to 0.981, 0.883, 0.677, expecting the image to get slightly darker and more yellow. But instead it got a lot brighter. So what does this “slope” on linear RGB actually do if it doesn’t multiply the channel values by the given inputs?

But multiplying the image by a color in GIMP and moving the sliders as shown in the screenshot in PhotoFlow still doesn’t produce the same result, even after subtracting the multipliers from 1.0:

Why is there a difference? Did I type in the wrong values in PhotoFlow? What is CDL slope on linear RGB if it’s not multiplying by a color?