Takes about 1-2 seconds to do the DE for a typical 2048x1024 image.
Much faster than my previous attempts which were O(dim^2) per pixel, or O(dim^4) for the whole image (totally impractical for non-tiny images).
I've written a blog post about it which will be published on Saturday.
I ran some further benchmarks. At higher resolutions (8k^2) it takes 2mins, while fractorium takes 5 seconds. The improvement in appearance of my method at low iteration counts can be outweighed by instead iterating points for another 1m55s - I got 666M iterations per second in one test, 1m55s at 8k^2 gives another 1k samples per pixel.
My way became slower because I needed to subdivide into finer bands and oversample in the filters to avoid posterization artifacts from the quantisation...
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