Almost literal translation from the Haskell version.

edit: And in amazing technicolor: http://zokier.net/stuff/webgl-quasicrystal/color.html

Modifying your main routine to do 3x3 super-sampling reduces flashing/aliasing quite a bit. Check it out.

    void main(void)
    {
        float s = 0.0;
        for( float xx = 0.0; xx < 1.0; xx += 1.0/3.0) {
            for( float yy = 0.0; yy < 1.0; yy += 1.0/3.0) {
                s += combine(vec2(uv.x*xpixels+xx, uv.y*ypixels+yy));
            }
        }
        gl_FragColor = vec4(vec3(s/9.0), 1.0);
    }

[1] https://gist.github.com/f448ba84e94c61ab5924

to put money where my (gut's?) mouth is, the dumb transliteration of my webgl shader to C++, compiled by MSVC in release mode on my win32 machine, takes 100ms to compute a frame at 800x600, on a single core, with precisely no tuning or effort.

with #pragma omp magic, equivalent in pain to the OP's point about almost-free-parallelisation in ghc, I imagine that would drop to around 20ms on 8 cores. and if I used an SSE vector class, probably another 2x, but that could legitimately be disallowed as overly complex.

my point being, you're right, GPUs stomp over CPUs for this kind of work! but my gut told me that this image should not take long for 'even' a CPU to produce; 10 or 20ms without effort, sub millisecond with effort (bytes rather than floats, asm, etc)

maybe I'm just lamenting the abuse of our modern CPUs, which are fantastically fast machines, even for stuff that they are not designed to excel at, like this. </ramble>