i'm writing a small landscape engine and i use bezier patches for rendering the terrain. Basically i generate the patches with an editor starting from an heightmap. Everything works fine (also ppl), i have a very detailed scene without spiked edges and all the other artifacts i would get if i use heightmaps. The problem is with collisions. I have thought to approximate the collisions with the patch computing them with the heightmap i used before. in theory this should work, but the problem is that when i render, the heightmap's vertexes don't lay on the patch (they are in different positions). how can i do to make them coincide?

First of all: don't try to make a collision test between the (mathematically) Bezier-Patch and other objects. You have to subdivide the Bezier Patch in triangles and then test these triangles with other objects. Because this take quite a lot of performance you you do a bounding volumn test before the subdivision and triangle-object test.

but if i start subdividing the patch in triangles i'll waste too rendering time... instead, if i translate the heightmap to the coords of the patch (but without rendering it) i can manage collisions with ~3 lines of code :\

yes,it's a solution but... as you said, it's not very efficient :
well, if i wont find any way to do as i wanted, i'll think about it, thanks ^_^