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Weird Photoshop Math

Today I was put into a deep shock with how Photoshop calculates final color of the dot which has several semi-transparent objects on its way. Assume for a second that you have a black background and two 50% white squares on top of each other and the background. What do you think, will the resulting picture show grey square or will it be entirely white?

Yes, I also thought that it should be completely white, but I appeared to be wrong. It looked a bit contrieved to me that instead of white I got some grade of grey. “What a hell!”, I was shouting, staring at the screen. How could it be that 50%+50% gave something, but 100%? Later as my shock had worn off I realized that there’s a kind of logic behind this odd math. And here’s what I ended with…

When you paint 50% white over the 100% black you get grey (something in the middle between black and white). It’s pretty reasonable, I expect. Now that you paint another 50% of white it adds some white to pass only 50% of grey color we got after the first painting. This way, we get only 75% of white as result of the second painting. Well, it makes some sense, but is there any way to get what I was expecting right from the start? This is the question…

By the way, this kind of magic happens not only in Adobe products, but Macromedia got infected as well. I don’t know whether it’s good or not, but simple multi-curtains effects became really complext to make with this. I’ve heard from someone the saying that engineers sometimes are absolutely crazy about math, always trying to mimic the real world in the most precise way, which makes their products completely unusable for ordinary people. Ah, yes, that was about digital cameras.