Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> > dynamic shadowing using shadow volumes 

> I pretty much follow *how* that works, but I'm not sure why you would do 
> that over doing it some other way. It would seem that calculating the shadow 
> volumes is something relatively compute-intensive for something moving.

  There are two competing algorithms for rendering real-time dynamic shadows
with current graphics hardware: Shadow volumes and shadow mapping. They use
completely different approaches to achieve the same (well, similar) result.

  As you might guess, there's no "better" algorithm. It's always about
compromises. One algorithm is better at some things and worse at others.

  Shadow volumes allow rendering pixel-perfect shadows (in fact, even
sub-pixel-perfect if you are using antialiasing). This accuracy is
completely independent of the distance between the shadowed area to
the shadowing object or to the light source causing the shadow. It also
requires less RAM (the shadow polygons are numerous, but polygons are
quite memory-light compared to bitmaps; the only other memory needed is
the stencil buffer.)

  The disadvantage of shadow volumes is that they are considerably more
complicated to implement, and their speed is heavily dependent on the
geometry of the scene. (Also soft shadows are not possible with the
basic algorithm, although I have heard of an expanded algorithm which
would support soft shadows, probably at the cost of rendering speed.)

  Shadow mapping is easier to implement and often (although not always)
faster. Its problem is that it you always have to make a compromise between
accuracy and memory usage (the larger the shadow maps, the more accurate
the result, but the more RAM is required; more accuracy might also affect
rendering speed by some amount). Also, the accuracy is heavily dependent
on the distance between the surface being shadowed and the shadowing surface
as well as the light source (the larger the distance, the blockier and
less realistic the shadow becomes).

  Shadow mapping has also another advantage over shadow volumes: Certain
shadowing effects are extremely light to do with shadow maps because they
require no rendering at all (to get the shadows), while with shadow volumes
you would need to run the full shadow-polygon-creation-and-rendering process
to get the same effect. For example, if you want a moth flying near a lamp,
you can "cheat" and add the moth's shadow to the correspondent shadow maps
algorithmically (instead of rendering the moth with the GPU as usual), and
thus even complex such shadows take virtually no rendering time.

-- 
                                                          - Warp

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