Can somebody help me with 3d trees?
I am searching for a good algorithm to generate 3d trees for a game.

I have been texture mapping trees onto a quad and using the quad like a sprite (always pointing towards viewer. The area of the image outside of the tree of course is masked for transparency. Search for “transparency” & “sprite” & “billboard” on a search engine.

I may be wrong, I’m still a newbie, but if you use the billboarding technique the front side of the trees will always face the viewer. So the trees will look like s* if you look at them from above. If that’s the case I think you’ll have to use more texture mapped quads per tree but without billboarding.

You might want to look into slicing and blending… i don’t know any websites right off hand but do a search for it and you should find what you need.
Using one polygonal model and 8-12 alphablended quads I got pretty realistic looking, 3-dimensional results that don’t drain too much GPU power.