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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. |
