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Photo-realistic vs. Physically-based Rendering
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Photo-realistic rendering places emphasis on the appearance of its
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output rather than the techniques used to derive it. Anything goes,
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basically, as long as the final image looks nice. There is no
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attempt to use physically realistic values for the light sources
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or the surface reflectances. In fact, the light sources themselves
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often have physically impossible characteristics like 1/r falloff (as
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opposed to 1/r^2) or there is a lot of ambient lighting that comes from
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nowhere but somehow manages to illuminate the room. (You are probably
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saying, "Hey! Doesn't Radiance use an ambient term?" The answer is
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yes, but only as a final approximation to the interreflected component.
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The renderers I'm talking about use the ambient level as a main source
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of illumination!) Also, surfaces typically have color but there is no
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reflectance given, so all the surfaces appear to have roughly the same
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brightness.
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Such numerical shortcuts are often just conveniences provided so the
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user can get results easily and quickly without having to worry about
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fussy details, like where to put the light sources and what to use
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for reflectances. As you might expect, there is a penalty paid besides
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meaningless values, and that is fake-looking images. Have you noticed
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how these renderings always look pastel and glowing? You're seeing
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the visual equivalent of AM radio.
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Physically-based rendering, on the other hand, follows the physical
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behavior of light as closely as possible in an effort to *predict*
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what the final appearance of a design will be. This is not an
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artist's conception anymore, it is a numerical simulation. The
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light sources start in the calculation by emitting with a
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specific distribution, and the simulation computes the reflections
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between surfaces until the solution converges. The most popular
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technique for this computation is usually referred to as "radiosity",
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or flux transfer, and it does this by dividing all the surfaces into
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patches that exchange light energy within a closed system. This type
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of calculation is limited for the most part to simple scenes with
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diffuse surfaces where the visibility calculation and the solution
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matrix are manageable.
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Radiance, in contrast to most flux transfer methods, uses ray tracing
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to follow light in the reverse direction and does not require the same
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discretization as radiosity techniques. This has significant
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advantages when the scene geometry is complex, and permits the modeling
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of some specular interactions between surfaces. In general, Radiance
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is faster than radiosity if the scene contains more than a few thousand
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surfaces or has significant specularity.
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