Contrast
Adjust contrast with a power curve anchored at the median — predictable, and hue-preserving.
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Contrast is a power curve pivoted on the image's own median luminance. In the log-luminance domain this is exactly a linear scaling, which is why the amount of contrast added is predictable rather than dependent on how bright the photo happened to be.
The same curve is applied to all three channels, so channel ratios are preserved and hue does not drift as contrast increases.