On Wednesday 12 May 2004 20:04, Andy Ross wrote: > Lee Elliott wrote: > > Was the stencil shadow stuff for generating object shadows? How far > > off usable was it, and did it only work with your terrain engine? > > It was decidedly "demo" quality. But it was part of the model code, > not the terrain engine. Doing shadows on terrain is sort of a 2D > problem, and actually a little simpler (computationally faster, if not > algorithmically easier) than doing a full-on general shadow > implementation. > > Basically, there are two general techniques for doing shadows with 3D > hardware: > > The first is to draw the object casting the shadow into a > 1-bit-plus-depth "shadow buffer" from the point of view of the light > source. You then use this buffer as a modulating texture for the > light source when drawing the objects on which the shadow falls. This > is a relatively straightforward process (although it requires some > form of rendering to a texture, which wasn't standardized in OpenGL > until recently) and works fast. The problems are that the resolution > is limited to what you pick for the texture, so you can see pixelation > effects if the viewer is close to an object which is "far" from the > shadow caster. More seriously, you cannot use this technique for > objects which cast shadows on *themselves* since the depth information > in the shadow buffer isn't precise enough. > > Stenciling is the other trick. This is a geometric technique where > you draw the "shadow volume" of an object into the stencil buffer. > For each triangle, for example, you draw a tetrahedron containing its > vertices and a vertex projected "infinitely" far away from the light > source. You then use some nifty tricks involving the stencil buffer > to tell which screen pixels are lit by the light source. This is a > great technique, and works correctly in a very nice general way for > every surface on the screen. > > It's also abysmally slow when implemented naively. Every (!) polygon > ends up beign drawn as a big swath from its real position to one edge > of the screen. This eats fill rate like there's no tomorrow. > Production implementation need to do lots of bookeeping work to > optimize the shadow volume such that only polygons on the silouette of > the object are drawn (others are essentially useless). This is the > part I didn't finish. :) > > Andy
Thanks for the info. It's something I've wondered about, in a 'how do they do that?' sort of way, but it's a bit beyond my programming ability to do anything practical about it. Something to look forwards to, perhaps;) LeeE _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
