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Jittered Sampling
(Term of computer graphics)
A stochastic process in which values are sampled
uniformly over a rectilinear subspace. The exact position
of the respective sample in each subrectangle is thereby
varied randomly.
In the Radiance software, this method can be
applied in several situations of the simulation process:
On the image plane, view rays may be sent through
random locations within the area of a pixel instead
of through its center. This feature should be
applied together with
image filtering, and helps to eliminate aliasing
effects ("staircases") in diagonal edges of the image.
Without filtering, pixel jittering may result in a slightly
rough appearance of such edges.
Specular hightlights can be sampled with rays that
deviate from the exact mirror direction of a surface to a degree
controlled by the roughness attribute of the material.
This feature results in correctly displayed highlights
on materials that exhibit a high specular reflection component,
but not such a smooth surface as to visually mirror
their environment.
If no filtering is applied
to the final image, the boundary areas of specular
highlights may show some speckle.
In shadow testing for a point in the scene towards
a light source, rays can be aimed at random points on the
source surface. This will
result in soft penumbras, given that the image is
filtered to a lower resolution after rendering. If the
image is not reduced, then the stochastic sampling
of light sources may result in speckle in the penumbra.
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