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Hue Blend Mode also Affects Saturation and Luminosity?


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I'm doing a deep dive on all the blend modes and am currently looking at the Component modes (Hue, Saturation, Lightness, and Color).

I (vaguely) knew that Saturation Blend Mode would also slightly change Luminosity. And Luminosity blend mode would also slightly change Saturation. However, I'm surprised that the Hue blend mode ALSO seems to affect saturation and luminosity. Is this expected behavior? Below is one of my examples. I'm aware of the perceived lightness problems and correcting for it (ex: .3R + .6G + .1B), but I didn't expect it to also show up in Hue. 

And while I'm asking: is there any place that actually gives a definitive list of how the blend mode math actually works in Affinity products?  Mostly I've been looking through Photoshop documentation and articles, but even that is kind of vague at times. 

In the image below, top blend layer is HSL = 108, 55, 50.

The bottom layer is HSL = 297, 75, 70.

I thought Hue blend result would be 108, 75, 70.....but it is actually 108, 54, 58.

image.png.ac326972314740b442ab5af6d6298826.png

 

 

Hue_02.afdesign

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The names of the Saturation and Luminosity blend modes are misleading since they do not correspond to the Hue-Saturation-Luminosity colour model.

The Hue, Saturation, Luminosity and Colour blend modes are actually performed in a Hue-Chroma-Luminance colour model.

  • Hue blend mode produces a result with the Hue of the top layer, and Chroma and Luminance of the lower layer.
  • Saturation blend mode produces a result with the Chroma of the top layer, and Hue and Luminance of the lower layer.
  • Luminosity blend mode produces a result with the Luminance of the top layer, and Hue and Chroma of the lower layer.
  • Colour blend mode produces a result with the Hue and Chroma of the top layer, and Luminance of the lower layer.

 

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Wikipedia covers this topic well (and others about HSL/RGB conversions etc)

https://en.wikipedia.org/wiki/Blend_modes

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