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I can kind of make sense of why humans(animals?) evolved to see brightness in relative terms. The same object will have different amount of light reflecting off it based on the time of day so you don't want your brain to register those as different scenes.

Hue also makes sense. It's the distribution of wavelengths being reflected.

But what does saturation correspond to? When do things become saturated/desaturated in the natural world?

Humans eyes don't see color as HSV; we (probably) use the https://en.wikipedia.org/wiki/Opponent_process.

This means that the signal getting to our brains is actually a report of "how much {(red minus green), (yellow minus blue)} relative intensity" there is in view.

Saturation is our name for the feeling of having strong visual qualia, brought about by the magnitude of one or both of the relative-intensity signals being large.

• This is why oversaturated bright colours can trigger migraines in people—they're the strongest encoded signals, causing the most synaptic firing.

• This is also why some oversaturated colours don't seem as "bright" as others—you can make a "Brown 2.0" that's ultra-mega-brown, but it can't be as intense in {R-G, Y-B} terms as the pinkest pink (https://culturehustle.com/products/pink-50g-powdered-paint-b...), since pink/magenta is a full relative intensity pin {1-0, 0-1}.

• This is also why staring at oversaturated colours for long periods leaves more of a visual afterimage than staring at equally-bright white does. It's the {R-G, Y-B} intensity, not the brightness, that's using up the electrochemical messenger molecules in your retina.

Take an image and force all colors to the same saturation, and you may get an idea. If nothing else, there is advantage to extracting as much detail from the signal as rapidly as possible. Not everything has to have a direct, straight line relationship; second-order and third-order effects are perfectly selectable too. In fact straight-line relationships are the exception; even the ones we think we see are the confluence of a lot of individual second- and nth-order outcomes.

If you take the absolute value of differences between each pair of the three "color" signals, and find how varied they are, that is saturation.


I suspect a lot of natural stripe patterns require saturation to distinguish them.

The (high) opposite of saturation, the "grayness" is a characteristic of glare, specular reflections, metallic materials (and salts).


Sunbleached? Roughened surface?

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