Creating a computer graphic model
of a uniform material like woven cloth or finished wood can be done by modeling
a small volume, like one yarn crossing, and repeating it over and over, perhaps
with minor modifications for color or brightness. But the final rendering step,
where the computer creates an image of the model, can require far too much
calculating for practical use. Cornell graphics researchers have extended the
idea of repetition to make the calculation much simpler and faster. Rendering
an image of a patterned silk tablecloth the old way took 404 hours of
calculation. The new method cut the time to about one-seventh of that, and with
thicker fabrics, computing was speeded up 10 or 12 times. A computer graphic
image begins with a 3D model of the object’s surface. To render an image, the
computer must calculate the path of light rays as they are reflected from the
surface. Cloth is particularly complicated because light penetrates into the
surface and scatters a bit before emerging and traveling to the eye. It’s the
pattern of this scattering that creates different highlights on silk, wool or
felt. They previously used high-resolution CT scans of real fabric to guide
them in building micron-resolution models.
Brute-force rendering computes
the path of light through every block individually, adjusting at each step for
the fact that blocks of different color and brightness will have different
scattering patterns. The new method pre-computes the patterns of a set of
example blocks – anywhere from two dozen to more than 100 – representing the
various possibilities. These become a database the computer can consult as it
processes each block of the full image. For each type of block, the pre-computation
shows how light will travel inside the block and pass through the sides to
adjacent blocks. In tests, the researchers first rendered images of
plain-colored fabrics, showing that the results compared favorably in
appearance with the old brute-force method. Then they produced images of
patterned tablecloths and pillows. Patterned fabrics require larger databases
of example blocks, but the researchers noted that once the database is
computed, it can be re-used for numerous different patterns. The method could
be employed on other materials besides cloth, the researchers noted, as long as
the surface can be represented by a small number of example blocks. They
demonstrated with images of finished wood and a coral-like structure.
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