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Zouflain

Creating noise in a 2D array by swapping values, with special conditions

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I have an interesting issue. Technically this is a graphics problem, but the algorithm I'm looking for is not unique to this situation and applies to any array with these constraints. Ultimately I just want to swap 2 pixels in a fragment shader in a reproducable, psuedorandom fashion to simulate GIMP's "spread" noise filter.

As a more general problem, I have a 2 dimensional array of values. I need to be able to iterate every member of the array and swap it with a psuedo-random neighbor (within an nxn neighborhood). This would ordinarily be trivial, but unfortunately at each step of the iteration, a value can only modify [i]itself[/i] and is only aware of the values that exist before iterations and the seed to the whole operation. I cannot cache swaps because each iteration is unaware of them, nor can the iteration be stopped or reversed. Therefore, and more formally, I need a function that takes an x,y pair and produces some psuedorandom x',y' such that f(x,y) = x',y' and f(x',y') = x,y where 0<|x-x'| <= n and 0<|y-y'|<= n. I may be overthinking this (or not thinking at all!) but I'm not sure how to approach this, to be honest.

I suspect that any periocidy would be relatively unnoticeable as the input array is already fairly random. Also, the array is only 4196 x 4196 units large, so a relatively short period would not have a chance of recurring. With these two things in mind, a very bad randomizer is perfectly fine. I also dont care if the algorithm can generate collisions - it's just a noise function, the collisions will be more noise as long as they are not very regular.

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Why not just select a random point in an appropriately size circle (or square) around each pixel and sample the original image at that point? Does it [i]really[/i] need to be swapped with that point? Probably not. (I think this is how GIMP's spread noise works as well)

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One simple way would be to create a texture that encodes which values to swap, e.g. use r and g as indices for the position.Then use the screen position as an index into that texture, and use the r and g as relative indices into the original screen image.You could get as fancy as you want creating your index texture because it wouldn't need to be calculated at runtime.

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@l0calh05t I actually did that while searching for a "good enough" solution, and it worked within reason. The reason I wanted to avoid it was because the effect influences the total concentration of values at each level. For now, this is "good enough" and I'll go with it, but I was wondering if there was a solution.

@Jefferytitan that's a sensible solution that I hadn't considered. I wonder, though, if I would need to use a texture of equal size (4196x4196) or if a smaller, repeating texture would be "random enough?"

Still, two very nice answers. Thank you both.

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I think you could get away with a smaller texture. Remember that you could mix it up somewhat, e.g. it could be rotated and/or reflected, and you could also pack another random texture into the b and a. Combine those together... rotate x 4, horizontal reflect x 2, vertical reflect x 2, other channels x 2, which gives you 32 variations.

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