hi there! your multiply layer math guide is a life-saver!! would you ever consider doing something similar for overlay or 'light-adding' layers?
I'm glad you found it useful! I still want to rewrite it to be clearer for more people.
(here's the original post for other people reading: https://pharanbrush.tumblr.com/post/704602434836791296/in-2019-i-wrote-a-practical-but-boring-blog-post)
As for light-adding layers, what did you have in mind?
Honestly, a lot of the other blend modes are "simple" but doing the reverse to undo their effects can get messy and unreliable. The rest are just not simple.
Divide just happens to be the exact opposite of Multiply.
Likewise, you would use Subtract as the opposite of Add (also known as Linear Dodge).
Overlay literally splits the darker half of the colors as Multiply and the lighter half as Screen, and does a weird parkour backflip with the layers (metaphorically).
Wikipedia actually lists the layer mode math clearly:
But it takes some sitting down and working out the steps to do stuff more complicated than reversing multiply or add. And after the work, there's no guarantee that there's a simple way to do it in your art program with layers. I tried it with the Normal blend mode and you end up with a lot of guesswork for colors and a bunch of folders and layers.
Krita has a crazy number of layer blend modes but I haven't looked into their usefulness for undoing/reversing colors.
