Graphics Reference
In-Depth Information
evolve. Presumably, each update will give a more precise result. The rendering
is completed when all the pixels are updated. If the resolution is low enough,
interactive rates can be achieved, which make is possible for a user to interactively
move lights or change the scene geometry. In practice it takes time and a lot of
passes to complete the final image. If it is too slow, the image can be
tiled
into
smaller images that can be rendered independently. Fast tiles finish sooner, and
may provide the user enough information to modify the scene without having to
wait for the slowest tiles to finish.
The implementation of photonmapping in hardware is useful in itself, as is the
ability to view the photon simulation as it evolves. In addition, the approach used
by Purcell and his colleagues opened the door to a different way of looking at the
problem, and also GPU programming in general. The techniques they developed
may be useful in other areas of GPU computing.
2.5.5 The Progress of GPU Computing
The use of the GPU in photorealistic rendering developed in multiple directions,
broadly classified into two categories: using hardware for specialized rendering
situations, and using hardware to improve current techniques. A representative
example of the former is the
ray processing unit
(RPU) described in a 2005 paper
by Sven Woop, Jorg Schmittler, and Philipp Slusallek [Woop et al. 05] The work
is notable in its development of hardware dedicated to ray tracing. Not only does
it implement some acceleration techniques used in software rendering, such as
k
d-trees, but it also leverages the strengths of GPU computing to accelerate ray-
tracing calculations in general. It clearly shows the potential power of the GPU.
An example of using hardware to improve existing techniques is the Light-
speed Automatic Preview System developed in 2007 at the Massachusetts Insti-
tute of Technology (MIT) and Industrial Light & Magic (ILM) [Ragan-Kelley
et al. 07]. This system is capable of re-rendering RenderMan
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scenes with differ-
ent lighting at interactive rates. Although it is not the first such system, Lightspeed
has some significant new abilities. For example, it performs an automatic trans-
lation from the RenderMan shading language to a hardware shading language.
Also it employs two separate framebuffers: one for shading and another custom
“indirect” framebuffer for the final pixels. Lightspeed became the first interac-
tive relighting system able to handle complex effects such as transparency, mo-
tion blur, high quality antialiasing, subsurface scattering, ambient occlusion, and
color bleeding. Furthermore, existing systems were aimed at game development
while Lightspeed was intended for use in feature film production; its creation was
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RenderMan is the name a photorealistic specification and rendering system developed by Pixar.
