simple gpu computation

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Hello,

I'm processing images like this

Images are very large (8192*8192). I need to do two things with these images.

First: Get all the unique colors into an array.

Second: Get the bounding box dimensions of each unique color.

I've no idea how to do both of these things with gpu. For example, I'm using unordered_set to record unique pixel values in my cpu version. GPU doesn't have something like that I assume, so how do I do that? Any suggestions?