An unbiased* global illumination ray tracer running in your web browser, 
with a max_trace_depth of 8 and support for shiny, diffuse, transparent 
and checkered spheres. What more do you want!

Just run the attached html on a PC with a fast GPU, you can drag around 
the scene with your pointer. Inside the html file you can change the 
size of the image (right at the end of the file you'll see width and 
height tags), using non-power-of-two values may not work on your 
graphics card.

You can get some pretty cool effects by looking through the glass ball 
into the mirrored ball.

Disclaimer: It works on my machine (Chrome, Win7, nVidia GTX970) but 
I've not tested it on any other machine. Particularly if you have a much 
slower graphics card it may run so slowly that it crashes, or it may not 
run at all. Also make sure you have the latest graphics drivers, no I'm 
not just saying this it often makes a difference. The output should look 
similar to the attached image after a few seconds.

...

After creating the standalone html5 page for blobs thread above, I 
realised it would allow me to do something shadertoy doesn't - that is, 
reuse results from a previous calculation in the next frame. I already 
had an attempt at a path-tracer in C# (which was in turn translated from 
some C++ code I found somewhere) but it was pretty slow. The main 
sticking point I had was learning how to render to texture using webGL. 
So it renders one "frame" at a time, which actually consists of 10 
samples per pixel, it then averages this with all the previous results. 
It then repeats until you move the viewpoint. The result is that once 
you stop dragging the scene around the noise should disappear eventually.

Your browser will probably limit the speed of updates to 60 frames per 
second, so that equals 600 samples per second. You can increase the 
number of samples per frame on line 39. But I found if you make it too 
high that a single frame takes longer than a second or so the video 
driver crashes.

* One of the hardest bits is to get a random number generator running on 
the GPU that is random enough to not show any patterns after a while. 
There is still some subtle non-randomness visible, but it's way better 
than it was originally. That's why there's random bits of code like 
rng.x = sin(r1 - FrameNumber) in there. It seems to do the job.

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