Hi, 

I'm wondering what the "subtract" blend mode is supposed to do. 

Looking at the source code, when opacity is 1 and ignoring clamping, the 
formula seems to be roughly "b + a - 1". That looks very much like a "add" 
operator more than a "subtract". 

Actually, looking at the Git history, commit 
https://github.com/darktable-org/darktable/commit/52b057dec32ea62c62f4c968a216bebf8ae89c92
 turned the formula from "a-b" to essentially "b+a-max". 

This looks like a bug introduced a long time ago, unnoticed because no one 
really use this operator. Is it the case? If not, what did I miss? 

Thanks, 

-- 
Matthieu Moy 
https://matthieu-moy.fr/ 


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