next up previous
Next: Bi-directional Path Tracing Up: A Review of Monte Previous: Related Work

Monte Carlo Light Tracing

Dutre proposed an algorithm that does not require mesh generation and can handle all types of light-surface interactions [DLW93]. This algorithm is based on particle model and direct computation of pixel intensities. Particles perform random walk through the scene and whenever a surface is hit, a ray is traced from the intersection point to the eye and direct light source contribution is added to the relevant pixel.

The random walks are denoted by a sequence of pairs tex2html_wrap_inline891 , where tex2html_wrap_inline893 and tex2html_wrap_inline895 is the last point generated. For the ith particle the pair tex2html_wrap_inline899 is the pair tex2html_wrap_inline901 representing the starting point and starting direction of the ith particle.

for each particle ( tex2html_wrap_inline905 ):
	determine the starting position  tex2html_wrap_inline907 
	trace a ray towards the eye and add the direct light source
	contribution to the relevant pixel
	determine the new direction  tex2html_wrap_inline909 
	absorbed = False;  tex2html_wrap_inline911 
	repeat until absorbed:
	  	compute  tex2html_wrap_inline913 
		trace a ray from  tex2html_wrap_inline915  to the eye and add the reflection
		contribution to the relevant pixel
		use the subcritical PDF to generate  tex2html_wrap_inline917  or
		to absorb the particle
determine  tex2html_wrap_inline921  for each pixel
display image

Figure: Particle tracing

The new direction tex2html_wrap_inline917 is chosen according to the subcritical pdf (spdf) on the basis of importance sampling:


where tex2html_wrap_inline925 is the bi-directional reflection distribution function (BRDF). The pdf is subcritical because it does not integrate to 1 over all possible angles, at least for physically valid BRDFs. The actual value of the integration gives the chance that random walk is continued, which ensures that the random walk terminates. This technique is commonly called Russian roulette.

Csébfalvi Balázs
Tue Apr 15 18:39:13 METDST 1997