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74 Chapter 17: Rendering
Pixel radius of 10
Left: 10 rays per sample
Middle: 50 rays per sample
Right: 150 rays per sample
Increasing the number of rays per sample can
greatly increase rendering time. The images on the
right can take nearly six times as long to render as
the images on the left. Increasing the filter rad ius
also increases render time, but not as dramatically.
Clamp Values (cd/m^2)—This control is expressed
as a luminance value. Luminance (candelas per
metersquared)representshowbrightlyyou
perceive a material. Clamp Value sets an upper
limit on the luminance that will be considered
intheRegatheringstage. Useittoavoidthe
appearance of brig ht spots.
Bright polygons in the scene can create a “sparkle” effect of
bright spots.
Thesebrightspotsareartifactsnotofthenumber
of samples cast, but rather of the presence of bright
polygons in your s cene . During the In itial Quality
stage, this bright energy gets bounced in random
directions, leading to a “sparkle” effect. Typically
you can detect these polygons before regathering.
During the final Regathering stage, bright spots
can b e avoided by setting Clamp Values somewhat
below the luminance of these br ight surfaces and
spots.
Bright sp ots have been reduced by clamping.
Tip: You can query the luminance of these surfaces
by using the Lighting Analysis tool (page 3–76).
Tip: Use Render Reg ion (page 3–13) to render just
theareaofthebrightspotstofindrapidlytheright
clamp value to use.
Be careful with this control: Clamp Va lues let
you clamp any intensity, and the rendering might
become darker than it should be because you
have clamped indirect illumination that is to be
expected, thus dimming the effect of the radiosity
solution.
Adaptive Sam pling group
These controls can help you speed up rendering
time. They reduce the number of light samples