The color-contrast module in darktable is one that I use on very many occasions but appears to confuse a number of people. The module has 2 very simple sliders; magenta/green and blue/yellow both of which are set at the slider default value of 1.00.
When I initially used thus function I wrongly assumed that moving the magenta/green slider to the right would increase the magenta saturation and moving the slider to the left would increase the green saturation. I was totally wrong! Moving the slider to the right (values >1.00) will increase both the magenta and green saturation together. The same is true as to how the blue/yellow slider works.
My main use of this module is to ‘break apart’ vegetation and allowing a more varied, and I believe natural, look to the landscape. Since pushing both of the sliders to greater values also changes the sky saturation I generally use a simple parametric cutoff to eliminate any blue shift.
I use the Tone Curve in LAB independent channel mode to do this type of work. It permits me to adjust green/magenta and blue/yellow independent of one another. The curve works the same way as the L curve, but adds saturation and color contrast.
I started this thread on account of having seen a video that implied that the sliders worked in an entirely different way; changing (adding) contrast green-to-the left and magenta-to-the-right. I hoped that my note would clarify the operation for others.
Ya they are just opposing colors…add magenta is like lowering green and same for blue/yellow…same as a and b channel in LAB tone curve…more control in the tone curve …you can select with the picker a particular part of the vegetation and tweak that…and the curves are more visual…I prefer this as you can also tweak the Luma channel at the same time…personally never use the color contrast module…
Hi Todd … just what I intended to debunk … moving the (magenta/green) slider to the right boosts both the magenta and green.
If you want to pull-up the green only one must use the parametric control to chop off that part of the ‘a’ channel.
Also the Color Zone module excels here. Quite easy to raise the saturation of a specific color.
The subject is/was the misuse of one module and not other modules.
OK should I remove all my comments?
both are right: citing the doc:
the effect of this module’s sliders are similar to applying a steepened or flattened a- or b-curve in module tone curve
→ a good example for darktable way to provide a simple solution in addition to a more customizable expert tool