Chris Cason <del### [at] deletethistoopovrayorg> wrote:
> therefore it is entirely possible that, as counterintuitive as it may seem,
> there is potential for a scene with global illumination to be *faster* in
> interactive rendering than one without.

  It's easier to understand when one knows how the global illumination
algorithm works in POV-Ray, and how radiosity works in scanline-rendering.
In the latter, global illumination is precalculated into lightmaps, and
these lightmaps are then just used as texture modifiers on the object
surfaces (this is very fast because of hardware support). Rendering using
lightmaps is much faster than using dozens of light sources.

  The data structure used by POV-Ray to store global illumination samples
is not very far from a lightmap. It's a kind-of "sparse 3D lightmap".
Retrieving values from that data structure is not slow, and thus once it
has been sufficiently calculated (ie. no more illumination samples are
necessary), rendering becomes quite fast, especially if there are no
light sources.

  (Of course the main problem with this technique has to do with the
"sparse" part. When samples are too far apart, illumination will be
quite inaccurate.)

-- 
                                                          - Warp

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