Comment by twic

I tried the first couple of ideas:

https://gist.github.com/tomwhoiscontrary/337cb8aaef013327a89...

I only went as far as generating threshold maps, not actually using them. Couldn't see how to do that using ImageMagick, and didn't want to write it manually!

The high-pass filter maps "look okay", but i haven't looked at their spectrum. How important is it that they have a specific frequency response? What is the failure mode if they don't?

The shuffling maps don't "look" so hot. There aren't border artifacts or repeating blocks (and you wouldn't expect these a priori - not sure why you think that), but indeed, it's not very different to white noise.