I was thinking about color, and am curious about what happens to gamut vs brightness in jpgs. With 256 levels per channel they can give a total gamut of 16M colors, but that's across all brightness levels. What dynamic range does a single color have?
Let's take 100,100,100 as an example. A nice pure grey. It's easy to see that this color will scale across the entire 8-bit dynamic range without changing in hue. Reduce it to 50,50,50, and it is still grey but at reduced brightness. As long as RGB are equal, the hue is the same.
But what about 30,40,50? The highest this color can go without changing hue is 153,205,255, and the lowest it can go is 3,4,5. This color has less than 5-bit dynamic range without changing hue.
Of course there are unique colors in those 16M. Any which have primes >128 in any channel, or have no common denominator. For instance 2^x, 3^y, 5^z where any x, y, or z causes that channel to be >128. And a host of colors with high brightness are unique, such as 255,254,253. If you wanted to scale this color to 10% brightness, I guess you'd say it was 26, 25, 25? Close enough?
On the other end, the number of colors seems to reduce dramatically as brightness drops. The number of colors up to a certain grey scale is equal to the product of the RGB up to that level? So for instance, up to 1,1,1 there are only 8 colors. Up to 2,2,2 there are 27. I guess this makes little difference since at the lowest levels we are less sensitive to hue changes?
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