> I'm rather new to this news group, and I'm interested in implementing
> real-time shadows in my 3D engine (the follow-up of ALPHA2). Is there
> any info on this subject available on the internet?
> 
> I already gave this subject some thought (together with a colleague):
> BSP-rendering (or portals) both have excellent characteristics to limit
> the time needed to calculate the shadow planes. It's clear that shadows
> need to be calculated by intersecting potentially lit/shadowed polygons
> with light-volumes, or shadow-volumes (We thought the first is better).
> So, we start with n polygons that need to be intersected with n volumes,
> since every polygon potentially casts a volume. That's n*n
> intersections, when there's only ONE light source. This huge number can
> be decreased, but are there people reading this that have good ideas?
> 
> I believe shadows can dramatically improve graphics quality and
> immersion, so I hope this can be discussed thoroughly.

This can be decreased to n*n/2 because you can sort (non-intersecting)
polygons by distance from the light source, then you only need to check
every polygon for shadowing by the ones nearer to the light source.

Another method is shadow buffering, where you render the scene from the
point of view of the light source to see which polygons are exposed. It
works something like this:

Assign each polygon in the scene with a different number (using, say a
16-bit shadow buffer would allow 65535 polygons)
:
Render the scene in the normal way from the point of view of the light
source (ie position the camera at the light source and point it towards
the scene), but instead of any texturing or shading use a single value -
the number assigned to this polygon. Also store the shadow buffer
co-ordinates of all the vertices.
:
Now render the scene normally, but for each triangle or polygon as well as
doing your normal shading use the co-ordinates of the corners of this
polygon to interpolate in the shadow buffer, and every 'pixel' in the
shadow buffer which has a value different to the value assigned to this
polygon means that a point is in shadow (because from the point of view of
the light source this point of the polygon isn't showing - it's obscured
by a nearer polygon). If this point is in shadow plot a darker version of
that pixel.

phew. Hope you got all that.. it's quite hard explaining something like
that without diagrams (I'll have to put a tutorial of this on my web pages
sometime..)

This method works quite well for discrete objects, where the whole object
can easily be rendered into the shadow buffer. For full 360 degree scenes
this is not really possible.

-lks
 www.chu.cam.ac.uk/home/lk213/