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Snapples

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Photoshop normal blend formula

Hi

I'd like to know the blend formula that Photoshop uses in it's Normal blend mode. Any documentation I find it says it replaces the layer's color by the brush color, which is true but nowhere does it say what happens with the alpha value.

For example, when I paint with magenta (255,0,255) with a 50% opacity and then paint cyan (0,255,255) also at 50% opacity on top of it, I get 85,170,255 with an alpha value of 0.75 as a result color. So obviously opacity of the brush color has a stronger influence than the opacity of the layer.

Just like the other blendmodes the Normal one has to have a formula to calculate the final color, does anyone know it?

Thanks in advance
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tomaugerdotcom
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Snapples

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I'm sorry I took so long to respond, I've been busy with work related things and forgot about this thread.

Thank you for the formula. I didn't know whether Photoshop used the formula for regular alpha blending, premultiplied alpha blending or something else entirely.
I'm writing a painting app for a smartphone and I'd like to get the same behavior as one would get in Photoshop, and with your formula I do. So thank you.

Also thank you for that second post, it made me realise I had made a mistake, I was drawing the brushstroke to the layer every frame (about 30 times per second), rather than once every time the mouse is pressed and released. So I fixed it by drawing on a temporary layer from the moment the user taps the screen until he releases it and then flatten it with the layer below it.
Thank you, everything I needed to know.
Right on, Snapples, good luck with the app! Sounds cool.

T