Gain that falls as saturation rises
A vibrance adjustment is defined by one rule: the amount of color boost shrinks as a pixel's existing saturation grows. Muted pixels receive a large relative gain, nearly saturated pixels almost none, and fully saturated pixels none at all. Because the gain is relative to each pixel's own state, the same slider position changes a flat image far more than a vivid one, and it never pushes a color beyond its maximum.