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HSL-Blend Ranges luminosity conflict

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Start with a colour wheel image, add HSL adjustment, then turn saturation half-way down and luminosity half-way up.

Now add Blend Ranges and pull down left node of right-hand chart. Colours like yellow are strongly affected but blue and red are hardly affected at all.

Why is this happening? My guess is that lightness conversion in Blend Ranges uses luminosity perception, where yellow is seen as a brighter colour and blue a darker colour. However HSL luminosity is value-based and so shows the same amount of greyness.

Hence we have a conflict in the treatment of luminosity. I don't know if it is a bug, but it is certainly bugging me as it clogs an effect I was trying to create. At the very least it is a usability defect as it easily causes confusion.

Maybe one way to address it is with a checkbox on Blend Ranges that switches between perceptual (as current) and value-based luminosity variation.

Thank you for your kind consideration and have a nice day.

Dave Straker

Cameras: Sony A7R2, RX100V

Computers: Win10: Chillblast i9 Custom + Philips 40in 4K & Benq 23in; Surface Pro 4 i5; iPad Pro 11"

Favourite word: Aha. For me and for others.

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I had a similar question watching your video (How the Graphs in Affinity Photo's Blend Ranges Really Work!) at 9:17 when you set Saturation to -100%. Why were the gray levels all the same? I think part of the answer lies in the HSL model. https://en.wikipedia.org/wiki/HSL_and_HSV#Saturation Saturation is chroma scaled to always fit between 0-1, looking at fig 14. The problem is discussed in the Saturation section, see purple boxes.

BTW your videos are a great help to understanding the fundamentals!

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