A really cool way to do trees can be found on the nVidia site. They have an old demo somewhere in their archive that generated a fractal tree. You could generate a few of these fractal trees at runtime, and use them as imposters during run-time... Could work, who knows.

Trent Polack
trent.codershq.com
trent@codershq.com
Author of Focus on 3D Terrain Programming

Heh, a couple of days ago, I implemented a new tree (and general vegetation) renderer for our engine. I''ve tried a *lot* of different approaches, but wasn''t really happy with any of them. I finally settled with a hybrid 3D/2.5D/2D system, much like a grass renderer, but suited for large scale instanciated objects.

The idea is to render a full 3D tree near the viewer, with trunk, branches and leaves all 3D. Then, when the camera moves away, you can gradually switch to other, less expensive solutions. And that''s where the trouble begins. The trunk and branches are easy, a standard geometric LOD system performs very well here (QSlim for example). But that doesn''t work on the leaves. So I first tried a volumetric approach: rendering the leaves as a sliced set of aligned textures. That looks more or less OK, but takes a huge amount of memory, and can lead to weird visual artifacts, when the trees shape is very irregular (branches fading in and out).

So, I finally used a different method: The tree foliage first gets planar patched, ie. transformed into larger face sets, containing several leaves on a alpha blended planar layer. That looks nice, but takes quite a lot of fillrate. Still not optimal. I was thinking of dynamic parallax layers instead, those should give similar visual results but lower fillrate costs. I haven''t yet tried them.

When the camera gets even further away, I render the tree into a texture, and use that texture as a billboard. The z-information is preserved as an 8bit bumpmap over the billboard, allowing 3D lighting of the tree. That part works very well, is fast, and pretty realistic.

And finally, if the tree is really far away, I drop the bumpmap. Simple per vertex lighting of the quad is enough at those distances.

All in all, I''m happy with the quality and performance. I can post some shots, if people are interested. The only part that makes me still somewhat uncomfortable, is the second LOD, between the full 3D and the bumpmapped billboard. I don''t know, I''ll probably try out some alternatives.

If anyone has weird ideas about organic vegetation rendering, esp. trees, it would be nice if you''d share. Even if it''s totally bizarre, or you think it wouldn''t work, just post it. Sometimes the most weird ideas can get the best results

In my crap ass renderer, I procedurally generate trees, trunk/branches in full geometry, leaves as sets of textured quads..(but i''m unhappy with that atm), anyhow, as the trees get further away, i start dropping branches below a certain size, and at the lowest detail level, all I draw are the leaves...looking into a valley full of trees from 700 - 1000 meters away, only seeing the leaves is good enough at that distance.