Cheap global illumination

Listening to: Gustavo Santaolalla "Motorcycle Diaries", and MYLO "Destroy Rock N Roll"

So I had this problem, how do you do efficient diffuse lightspill (sometimes referred to as global illumination) when the light source is an area light that's playing an image sequence that updates every frame.

Baking illuminance or using 'indirectdiffuse' in a lightshader might seem like the way to go, except they are both really slow. What I need is a solution that will be quick to render when it comes to the beauty pass. It doesn't matter so much if the setup is slow (in the same ballpark as occlusion is slow anyway).

So I settled on a shader that shoots rays from geometry in the scene towards the image sequence light source, and averages the texture coordinates that get hit, then bakes this to a pointcloud (along with a scalar ratio of hits to misses and an average direction vector). This way, so long as the scene is static, we can bake the spatial relationship of geometry wrt the light source and then use these baked texture coordinates as a texture lookup that's cheap to do. We can change our image sequence each frame, and the texture coordinate lookup is still valid.

They are a bit like shadowmaps though, you need one per light source, and if the scene is non-static, then you need to bake once per frame that's got geometry in a new position. But for a static scene, they are a winner because they are view independent.

So why didn't they work at first when I used the bake3d and texture3d calls?

Well, if you have a cloud of points, then you need to know what space they are stored in and what space is used in the retrieval.

So the answer, and the tip for today, is that bake3d and texture3d by default will transform P and N that you give it into world space before doing the lookup. That means you don't usually have to care about what space the P and N are in that you pass to it. (Not counting Pref etc, of course. But my scene doesn't have any deformations, so no problem).

First post!

Listening to : CD of Mighty Boosh radio show.

Wow, first post! I guess I wanted to set this blog up ages ago. In fact, back in 2003 when I first started using renderman in film vfx for my living, I wanted to keep a log of all the things I had to learn to re-train from being a 3D Programmer in the games industry. That opportunity to chronicle the initial learning curve involved has probably passed, rats, but hey-ho I can still post things as I learn them....