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sampleImage: using alpha to affect simple choker
Hi everyone,
I have an effect in mind and after some research I found out that sampleImage would be the best way to do it, but I don’t really do expressions much at all so I need help to write down the expression.
Basically I’ll have a source layer with varying alpha value, and I want to use this alpha value to affect the effect of Simple Choker on my target layer. So basically where in the source layer is transparent, the target layer won’t be choked, and then the “Choke Matte” value will go up where the source layer is more opaque.
I hope I’ve explained it well. So can anyone help write the code for that?
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4 Replies
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I don’t think you can do that with an expression. You could set the Choke Matte value to be based on the alpha of a particular pixel or region, but you couldn’t generate different values for different regions in a single expression.
Dan
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That is unfortunate, so is there no way to connect sampleImage to the parameter of an effect in any way?
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No, you can, it’s just that whatever you sample sets the effect parameter for the entire layer. sampleImage() gives you pixel-level access, but it’s read-only, so expressions don’t have any way to control anything at the pixel level.
Dan
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I see, I’ll figure out other ways to do the effect then. Thanks for the information.
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