Do GIMP's Hue/Saturation/Color/Value blend modes work the same way as Photoshop's color blend modes?

Asked 6/22/2011

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Wikipedia says Photoshop’s Hue, Saturation, Color, and Luminosity blend modes are based on a perceptual color model that differs from HSL/HSV. In GIMP, the comparable modes are Hue, Saturation, Color, and Value. Does GIMP use the same underlying approach as Photoshop, or are GIMP’s modes based on HSV/HSL (with Value instead of Luminosity)? For photography, does this difference matter in practice?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

15y ago

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I guess technically speaking I would call HSL and HSV a "color model", same as RGB or CMYK, as they are tools for modeling and describing color. A "color space" is a tool for calculating color adjustments or comparing colors, such as XYV or Lab. Either way, I am not sure either color spaces nor color models really matter for the question at hand...

Based on the references you have linked, and the terminology used, I can offer two possibilities in regards to how Gimp's blending modes work (which are not really color space or color model related...they are simply layer blending operations that work on different channels, and should work the same regardless of what working color space your image is in, or what color model your image is using.

Option 1

The first option is that Gimp and Photoshop behave the same for those four blending modes. This is based on the assumption that the terms "Luminosity" and "Value" refer to the same thing...the luma axis, the facet of color that determines whether a color is light or dark, irrespective of its chromaticity (hue and saturation.) If this is the case, you can assume that applying the "Value" blend mode will preserve the luminosity of the top layer, and keep the hue and saturation/chroma of the bottom layer.

Option 2

The second option is that Gimp assumes the "value" term is akin to the HSV color model's Value component. In HSV, value is a bit different than luminosity/brightness, in that at maximum value, you achieve maximum color purity for a given saturation. If saturation is zero, you get pure white, where as if saturation is 1.0 (100%) you get that pure hue. This is in contrast with the HSL/B color model, where in luminosity/brightness is an agnostic component. At maximum brightness, regardless of hue and saturation, you get white. At 0.5 (50%) brightness, you achieve maximum color purity for a given saturation. If saturation is zero, you still get pure white, where as if saturation is 1.0 (100%) you get that pure hue. Photoshop uses luminosity according to HSL/B rather than value when you look at things this way.

If Gimp uses Value in the same way as the HSV, I can not say for sure exactly what the outcome of the "Value" blend mode would be. The most logical way to think about it would be that it keeps the value numeric value (say 0.5) of the top layer, and applies that to the hue and saturation values of the bottom layer. If the top layer has a red hue and 100% saturation at 50% value, while the bottom layer has a green hue and 50% saturation at 100% value, I would assume the final outcome is green hue at 50% saturation and 50% value. In other words, a soft, semi-desaturated dark (but not blackish) green.

UPDATE:

Here are some sample images from both Photoshop and GIMP that demonstrate each mode. The differences between the Luminosity blend in Photoshop and the Value blend in GIMP are pretty clear...it does appear as though GIMP treats Value blend according to the rules that govern Value in HSV. GIMP also seems to apply some stronger curves during processing, or simply has a slightly different approach to blending the two colors for each pixel, than Photoshop...producing slightly harsher results. Photoshop on the left, GIMP on the right:

Photoshop HueGIMP Hue
Photoshop SatGIMP Sat
Photoshop ColorGIMP Color
Photoshop LumGIMP Value

Originally by user124. Source · Licensed CC BY-SA 4.0

user124

15y ago

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AI Answer

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No—GIMP’s modes are not exactly the same as Photoshop’s.

From the naming and behavior described, GIMP’s Hue/Saturation/Color/Value modes are generally understood to follow an HSV-style approach, where Value is the brightness channel. Photoshop’s Hue/Saturation/Color/Luminosity modes are described as using a different, more perceptually oriented model, so its Luminosity mode is not the same as HSV Value.

In practice, that means similarly named blend modes can produce different results between the two programs, especially when you are trying to change color without affecting perceived lightness, or adjust lightness contrast without shifting color.

So for photography work, yes, the distinction can matter: Photoshop’s approach is intended to better preserve perceived lightness/chroma relationships, while GIMP’s Value-based blending may behave less perceptually cleanly in some cases.

The terms “color space” and “color model” are a bit mixed up in the Wikipedia wording, but the key takeaway is simple: don’t assume these blend modes are mathematically identical across editors just because the names are similar.

UniqueBot

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15y ago

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