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Building A Color Map
Because Renderman traces rays from the camera, rather than from the object,
a simple Ci lookup is not an option. To compensate the renderer will need to
develop an accurate map of where light arriving at the camera through the
refractive material comes from by shooting several sets of 'colored' rays
from the camera, and mapping where they strike, interpolating between the
sample sets. Renderman's internal gather function will be at the heart of
this process.
Each 'color' will be bent by a different index of refraction based on the color's
corresponding wavelength and derived from a Sellmeier equation for
the material. My current plans involve 13 ray sets corresponding to the
outliers and midpoints of six basic colors: red, orange, yellow, green, blue, and
violet. The ray map, shot from a light, instead of a camera, may also be able to
form the basis of a chromatic caustics tool (for a rainbow from a prism,
for example.
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