Direct Light 2: integrating shadow rays into a path tracer

Part 1 of 3

Part 3 of 3

 

In my last post, I talked about using shadow rays to sample a light source directly.  Here is our old lovely but noisy path tracer:

vec3 ray_color(ray r)

  if (r hits an object)

       r = ray(hitpoint, generate a scattered  direction from the surface)

       return emitted(hitpoint) + reflectance*ray_color(r)

   else

        return background_color(r)

This works great provided the light emitting objects are big, but otherwise we get a lot of noise.  We also are crossing our fingers we don't get an infinite recursion.

Let's assume we have a magic function direct_light(vec3 p, reflectance_info ref).   Can we just do this to the return line above:

      return emitted(hitpoint) + direct(hitpont, ref_info) +reflectance*ray_color(r)

NOPE!  That would double count direct because that sacttered ray r might hit the light and get the emitted part.  So this is better:

     return direct(hitpont, ref_info) +reflectance*ray_color(r)

 BUT, if you see the light source in the picture, it will be black!  

And before we fix that, another problem is that if the surface is a perfect mirror, the direct lighting itself will be too noisy because only one point on the light matters.

So we need something like a "sees the light" flag on a ray.

vec3 ray_color(ray r)

   vec3 color = (0, 0, 0)

  if (r hits an object)

       if (r.should_see_lights)

            color +=  emitted(hitpoint)

            r = ray(hitpoint, generate a scattered  direction from the surface, should_r_see_lights_flag)

            if (r.should_see_light)

                    color += reflectance*ray_color(r)

            else

                    color += direct(hitpoint, ref_info) +reflectance*ray_color(r)

   else

        color += background_color(r)

  return color

Man that is pretty ugly!  But I don't know a much better way.  Be sure to set the viewing ray flags to "should_see_light = true".

 Are we done?  No.  For perfect mirrors, should_see_lights scattered rays should be set true.  For diffuse reflectors, false.  For glossy objects, it depends on the size of the light source.  Here, a lovely technique from Eric Veach is often used.   I would go with the "balance heuristic"-- it is easiest.  There is a wonderful figure from the paper that shows why this technique is needed:

 

So are we done?   No, there is one more issue.  Does the background "emit" light?  Isn't it a light source.  The answer is you can do it either way.  But if it is a light source, and you can hit it, we can get rid of the   if (r hits an object)  branch-- the background is always hit!  But if it has infinite radius where is the hitpoint?  I would say these design decisions are not obvious and I am on the fence even after trying all of them.

Next time: what goes in here: direct(hitpoint, ref_info)