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root/radiance/ray/doc/notes/rendering.note
Revision: 1.1
Committed: Sat Mar 15 17:32:55 2003 UTC (21 years, 4 months ago) by greg
Branch: MAIN
CVS Tags: rad5R4, rad5R2, rad4R2P2, rad5R0, rad5R1, rad3R7P2, rad3R7P1, rad4R2, rad4R1, rad4R0, rad3R5, rad3R6, rad3R6P1, rad3R8, rad3R9, rad4R2P1, rad5R3, HEAD
Log Message:
Added and updated documentation for 3.5 release

File Contents

# User Rev Content
1 greg 1.1 Photo-realistic vs. Physically-based Rendering
2    
3     Photo-realistic rendering places emphasis on the appearance of its
4     output rather than the techniques used to derive it. Anything goes,
5     basically, as long as the final image looks nice. There is no
6     attempt to use physically realistic values for the light sources
7     or the surface reflectances. In fact, the light sources themselves
8     often have physically impossible characteristics like 1/r falloff (as
9     opposed to 1/r^2) or there is a lot of ambient lighting that comes from
10     nowhere but somehow manages to illuminate the room. (You are probably
11     saying, "Hey! Doesn't Radiance use an ambient term?" The answer is
12     yes, but only as a final approximation to the interreflected component.
13     The renderers I'm talking about use the ambient level as a main source
14     of illumination!) Also, surfaces typically have color but there is no
15     reflectance given, so all the surfaces appear to have roughly the same
16     brightness.
17    
18     Such numerical shortcuts are often just conveniences provided so the
19     user can get results easily and quickly without having to worry about
20     fussy details, like where to put the light sources and what to use
21     for reflectances. As you might expect, there is a penalty paid besides
22     meaningless values, and that is fake-looking images. Have you noticed
23     how these renderings always look pastel and glowing? You're seeing
24     the visual equivalent of AM radio.
25    
26     Physically-based rendering, on the other hand, follows the physical
27     behavior of light as closely as possible in an effort to *predict*
28     what the final appearance of a design will be. This is not an
29     artist's conception anymore, it is a numerical simulation. The
30     light sources start in the calculation by emitting with a
31     specific distribution, and the simulation computes the reflections
32     between surfaces until the solution converges. The most popular
33     technique for this computation is usually referred to as "radiosity",
34     or flux transfer, and it does this by dividing all the surfaces into
35     patches that exchange light energy within a closed system. This type
36     of calculation is limited for the most part to simple scenes with
37     diffuse surfaces where the visibility calculation and the solution
38     matrix are manageable.
39    
40     Radiance, in contrast to most flux transfer methods, uses ray tracing
41     to follow light in the reverse direction and does not require the same
42     discretization as radiosity techniques. This has significant
43     advantages when the scene geometry is complex, and permits the modeling
44     of some specular interactions between surfaces. In general, Radiance
45     is faster than radiosity if the scene contains more than a few thousand
46     surfaces or has significant specularity.