This implements a very basic path tracer in VOPs inside houdini, taking a silly abusive experiment ( vimeo.com/20700092 ) another step. It's actually a bit more conceptually simple than previously, though slower to process (this video is sped up 3x).
Its using a vop network to trace rays from the points of a grid 'camera', path trace through the scene, and store the resultant colour in the camera's point color attribute.
This supports multi bounce illumination from geometry and sky, and diffuse (lambertian) surfaces only. Geometry lights are created by giving primitives an 'emit' float attribute. It's using a unconventional method of accumulating light along the paths and isn't bothering much with pdfs, russian roulette, anything like that - just a bit of fun!
file here: mke3.net/projects/houdini/pathtrace4.hipnc.zip