#### Re: How does Luminance slider in Camera Raw split colors by their hue?

Dan Margulis

On Jan 20, 2021, at 8:39 AM, John <johnny.php@...> wrote:

the HSL panel in Camera Raw allows adjusting Luminance for 8 color ranges I assume.

Does anybody know how those ranges are calculated?

For ex. what does "Reds" actually mean, how does it determine what is red and not orange for ex.

it's clear that RGB or other color space values are in play here, but how does the math actually work.

John,

The flip, or wisecracking, response to the question is,

1) Probably nobody remembers what methodology was used. The Photoshop team often has picked up pieces of existing code to create new commands. Also there are several complicating factors in a command like this that might require some less than desirable shortcuts.

2) It is hard to see what good knowing this information would do. There are good reasons to have an exact knowledge of what curves or white balance do, or what constitutes “red” in sRGB as opposed to Adobe RGB as opposed to ProPhoto. But the nature of this particular command is that it can only be used in a by-guess-and-by-gosh way where we twiddle the sliders and try certain settings, and if we don’t like the particular way in which the affected area is computed we make a better mask ourselves.

The realistic response is:
If we are designing this command from scratch there are two basic approaches:

1) Do it in a logical mathematical way that will appeal to color scientists.

2) Do it in an illogical way with the idea that a slider labeled “Reds” will affect only areas that a non-color scientist might consider red.

These two approaches cannot be reconciled. Assuming that you have decided to select areas that the average person might consider red or blue or yellow, the size of the selection covered by each couldn’t possibly be the same. Yellow, for example, is only perceived as yellow in very light tones, whereas blues are recognized as blues in a range three times as large.

Assuming that you could somehow pinpoint the exact color that the average person considers red or blue or yellow, the selection areas would also not be symmetrical. The irregularities have been known for some time. In his 1858 On Colour John Gardner Wilkinson complained,

I have stated that the names of colours are uncertain and indefinite, and in proof of this it is only necessary to ask what idea is conveyed to the mind by the mere mention of a red, or a blue, color? A scarlet coat is called red; and the term red is applied to a rose, a brick, port wine, mulberries, cherries, and other things of very different hues…

If you force people to pick one color and call it red, they’re probably going to choose something in the neighborhood of scarlet. So a command targeting “Reds” has to be strongest in that zone. In a command designed for color scientists, the selection would tail off equally on both sides. However, in real life we can’t go much in the yellow direction before declaring that the resulting color is orange and not red. All the objects above cited by Wilkinson are on the other side—we still consider things to be red when they have moved sharply toward magenta.

With blues it’s the same idea. If we have a basic agreement on what blue means we will accept quite a large hue shift before we call it aqua. But if the shift is in the other direction it won’t take much for us to call the color purple.

That ad hoc approach is the one I would take if I were designing the command.

Dan Margulis

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