I'm trying to draw shadows for some texture-mapped polygons with alpha of either 0 or 1 at each pixel, which I'm using for trees, fences, etc. I read several tutorials on shadow mapping, and that sounds like it would work great, except that my graphics card doesn't support two required extensions, ARB_depth_texture and ARB_shadow. I know, I could get a better card - but I'm wondering whether there's any other approach I could try. Shadow volumes won't work, since some polygons are transparent, but perhaps there's something else. I'm willing to make a few compromises. Specifically, - I only need a single, directional light (the Sun). - I'm fine with only drawing shadows onto the terrain, rather than allowing self-shadowing and having different objects cast shadows on each other. My terrain is made of square tiles, all the same size, but not necessarily coplanar (it can have elevation). - My view will be sort of isometric, like an RTS, so I won't have too many objects in it at once. However, I would like the shadows to be re-calculated each frame, so I can have animation. One idea I have is to somehow render the visible part of the scene, other than the terrain, onto some plane perpendicular to the light direction, drawing everything in black; then somehow project this image onto the terrain, so the shadows fall on the terrain. I don't know how to do this though, especially the rendering to a texture. Here's a list of the OpenGL extensions my card supports. The actual card is GeForce 4 440 Go, with the latest drivers.

The technique you describe is called projective shadowing and could be suitable solution.

Frustum has a nice OpenGL demo of this with source:
http://www.frustum.org/3d/index.php?demo=9

Thanks, I'll look into that. I also dug up my copy of Game Programming Gems (1), which came out before the shadow extensions were common, and found a demo there.