Ok, so here’s how to emulate a tone curve with the color correction module. We start from neutral:
We use power, pivot and offset to add contrast:
This is certainly “punchier” but the highlights are blown (check the clouds). To understand why, it is useful to have a look at the curve that is generated by the sliders. The formula is:
out = \left(\frac{in - offset}{pivot}\right)^{power}*pivot
which visually is something like this (in green):
It’s clear now that the values close to 1 (i.e. the highlights) get pushed way past 1, so they will be clipped on output.
There are two solutions to this:
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use a parametric mask to create a “rolloff” effect, simlarly to what you would do in a classic S-shaped curve
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use “log tone mapping” to push back the highlights.
Here, I’m going to show you 1, because it’s closer to what you would do with a tone curve. Here’s the mask that I used and its effect:
Also in this case, it’s useful to plot the curves to visualize what is going on. Here are the mask (in red) and the resulting combined curve (in orange):
Now, this looks somewhat familiar, doesn’t it?
Finally, I added a bit of (output) saturation to complete the picture:
Clearly, even though I only used “global” edits, you can apply all this locally, by using all the masking options that are available in the color correction module.
Here’s the final picture and the arp (for the current master version of ART):
tone-curve.arp (10.6 KB)
HTH