I've always had an interest in computer generated plants so I was pleased to read Self-organising tree models for image synthesis from Siggraph this year.
The paper basically pulls together a bunch of techniques that have been around for a while and uses them to generate some really good looking tree models.
Seeing as I've had a bit of time on my hands between Batman and before I start at LucasArts I decided to put together an implementation in OpenGL (being a games programmer I want realtime feedback).
Some screenshots below and a Win32 executable available - Plant.zip
I implemented both the space colonisation and shadow propagation methods. The space colonisation is nice in that you can draw where the plant should grow by placing space samples with the mouse, this allows some pretty funky topiary but I found it difficult to grow convincing real-world plants with this method. The demo only uses the shadow propagation method.
Creating the branch geometry from generalised cylinders requires generating a continuous coordinate frame along a curve without any twists or knots. I used a parallel transport frame for this which worked out really nicely, these two papers describe the technique and the problem:
Getting the lighting and leaf materials to look vaguely realistic took quite a lot of tweaking and I'm not totally happy with it. Until I implemented self-shadowing on the trunk and leaves it looked very weird. Also you need to account for the transmission you get through the leaves when looking toward the light:
There is a nice article in GPU Gems 3 on how SpeedTree do this.
The leaves are normal mapped with a simple Phong specular, I messed about with various modified diffuse models like half-Lambert but eventually just went with standard Lambert. It would be interesting to use a more sophisticated ambient term.
Still a lot of scope for performance optimisation, the leaves are alpha-tested right now so it's doing loads of redundant fragment shader work (something like Emil Persson's particle trimmer would be useful here).
If you want to take a look at the source code drop me an email.
On my NVIDIA card when the vert count is > 10^6 it runs like a dog, I need to break it up into smaller vertex buffers.
Some ATI mobile drivers don't like the variable number of shadow mapping samples. If that's your card then I recommend hacking the shaders to disable it.