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Like all curves, this operation is extremely useful for stylizing the image. You can create custom vibrance operations, where you selectively increase the saturation of low-saturated regions of the image in different ways to give the picture more “pop.” It’s also an excellent tool for taking care
of over-saturated areas of the picture when adhering to conservative QC requirements. You can specifically desaturate only the most over-saturated parts of the picture, without affecting similar hues at lower levels of saturation.
In the following example, you can see that the portion of the image with the highest saturation has been desaturated, while the rest of the image has been left alone.
Desaturating only the highest saturated elements in the picture by lower control points at the right of the Sat vs. Sat curve
Sat vs. Lum Curve
The inverse of the Lum vs. Sat curve, Sat vs. Lum lets you quickly adjust the luminance of pixels that falls within a particular range of saturation. This is useful for cases where regions of the image you want to make lighter or darker happen to coincide with an identifiably consistent range of saturation.
To help guide your adjustment, the histogram drawn for this curve lets you see how many pixels there are at each level of saturation found within the image, with the leftmost part of the curve representing minimum saturation, and the rightmost part of the curve representing maximum saturation. Seeing where the levels of saturation lie in the current image help you to quickly make targeted luminance adjustments when you sample the region of the image you want to adjust to add points to this curve.
The Sat vs. Lum curve
In the following example, the right side of this curve is lowered so that the most saturated parts of the image become darkened. You might want to make an adjustment like this to emulate one of the properties of film, where highly saturated colors block more light from passing through the emulsion than less saturated colors. Or, you may do this in an effort to desaturate bright colors in highlights that might otherwise clip past maximum white. Or, you might do this to quickly darken highly saturated skies.
(Top Left) The original image, (Top Right) The highly saturated blue sky and sea reflections are darkened to a more intense blue by lowering the right part of the Sat vs. Lum curve (Bottom).