> Indeed, my plan is to write a custom renderer. I was more asking about 
> which kinds of algorithms might be appropriate to use.

AFAIK they are just using a normal backward raytracer like POV, but with 
some optimisations to find the fractal surface quickly rather than just 
binary search along the ray.  You know how when you render a 2D mandelbrot 
you can use that double-log formula to get a smooth iteration count?  Well I 
think they're using a similar formula to get an indicator of how far away 
they are from the surface.  If you calculate that at 2 or 3 points close 
together on your ray, you should be able to extrapolate to get a pretty good 
estimate of where the surface actually is.  Repeat until you're at 
sufficient accuracy.

To estimate the surface normal I think you could re-calculate the fractal 
(using the smooth iteration count formula) a short distance away on each 
axis.  This can then be used for the lighting algorithm or your choice (it 
seems a lot of them then shoot a few hundred rays out for some AO lighting 
in addition to diffuse and shadows).

It certainly looks exciting, after my df3 isosurface attempts I'm very 
tempted to try this myself too.  I'm going to think about if a GPU version 
is possible.

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