Why does the Hue blend mode change saturation and brightness?

Question

My understanding is that the Hue blend mode is supposed to use the hue from the top layer, and the saturation and brightness from the bottom layer.

• I placed a red in the bottom layer. RGB = (255, 0, 0), HSB = (0°, 100%, 100%)

• I placed a blue in the top layer. RGB = (0, 255, 255), HSB = (180°, 100%, 100%)

• The resulting color was as follows: RGB = (1, 108, 108), HSB = (180°, 99%, 42%)

As you can see, the saturation dropped slightly and the brightness dropped a lot. That goes against everything I've read about this blend mode.

• Why is this happening?
• How can I predict the effect on the resulting color's saturation and brightness?

I've found that the resulting color's saturation and brightness will drop by various amounts depending on the hue of the top layer, or the hue of the bottom layer. Unfortunately, I can't identify a pattern, it seems fairly sporadic.

Also, changing the saturation or brightness of the top layer's color does not affect the resulting color's RGB or HSB values. Changing the saturation or brightness of the bottom layer does alter the resulting color's saturation and/or brightness, but not the hue.

I'm testing this in Photoshop (Version 24.0.0). I'm checking the colors with the color picker and Photoshop's Info panel. I'm not sure if it matters, but under Image > Mode, I have RGB Color and 8 Bits/Channel both checked (I think those are default settings).

Answer 1

The brightness is not taken from the back layer. The rgb-numbers of the shown mixing result are calculated so that the saturation and the luminosity of the mixing result are both same as in the back layer. Hue is taken from the front layer. You have obviously guessed that the brightness is taken from the back layer as is, but that's a wrong guess. Adobe has not revealed exact math behind the color mixes caused by blending modes in Photoshop, but you can find them from PDF file format specifications. Search for PDF blending modes.

Answer 2

It changes the hue while keeping the luminosity constant. Luminosity is defined as something like 0.3R + 0.6G + 0.1B, though I cannot specify the exact coefficients that Photoshop uses. I tested all four of the last blend modes in Photoshop (Hue, Saturation, Color, Luminosity), and they all have the same luminosity definition. They all work this way. Therefore, the brightness in HSV will change because its definition is different, and hence, the saturation will change as well.