Graphics Reference
In-Depth Information
scattering is working properly. (By the way, you can add a perfectly transmis-
sive and completely nonreflective sphere with refractive index 1.0 to a scene, and
it should make no difference to the scene's appearance. Of course, if your ren-
derer bounds the number of ray-surface interactions, it may have an effect on the
rendering nonetheless.) The transparent sphere reflects some light from the other
sphere, reflects some light onto it, and generates a diffuse pattern of light on the
floor. None of these effects (color bleeding between diffuse surfaces, the caustic,
or reflected light on the solid sphere) would be visible in a ray-traced version of
the scene.
The third scene is the Cornell box, a standard test scene with diffuse surfaces,
and an area light, in which color bleeding and multiple inter-reflections are evident
in an accurate rendering, but are missing from the ray tracing.
The final scene consists of a large area light tilted toward the viewer, and a
large mirror below it, also tilted toward the viewer, with a diffuse rectangle behind
it to form a border for the mirror. The viewer sees not only the light, but also its
reflection in the perfect mirror. Together these give the appearance of a single long
continuous rectangle.
Figure 32.10 shows a path-traced version of the first scene. There are some
obvious differences between this and the ray-traced scene. First, the area light,
which we ignored in the ray tracing, has been included in the path tracing. Sec-
ond, there's
noise
in the path-traced image—everything looks somewhat speckled.
We'll return to this presently. Third, the shadows are softer. Light is reflecting from
the floor onto the sphere, lighting the lower half somewhat, which in turn helps
light the shadowed part of the floor. Fourth, there is color bleeding: The pinkish
color on the floor and back wall is from light that's reflected from the sphere.
The softened shadows and color bleeding are what you should expect when
you consider how path tracing works. The noise, however, seems like a serious
drawback. On the other hand, the ray-traced version exhibits aliasing, especially
on the shadow edges. That's because in the ray tracer, the rays from the eye
through two nearby pixels end up reflecting from (or refracting through) the sphere
in almost parallel directions. In the path tracer, there's a coin toss: About 80% of
Figure 32.10: A path-traced scene.
