Thursday, September 10, 2009

Assignment #2

CS513, Assignment #2

Source is here

Images can be clicked on for a larger view.

Scene viewed normally from the camera:

Scene viewed from the light source:

How to build and run:

tar jcf jbowles-assign2.tar.bz2
cd jbowles-assign2/shadowmap

Press 'c' to toggle light/eye camera
Press space to pause/unpause teapot rotation
Press 'r' to reload shaders

Problems and fixes:

The shadow is not in the correct position. I suspect that this is due to the problem that Joe described with the light projection and view matrices not being set up right. When the view is from the light position, everything looks correct to me, but there seems to be some subtle issue. You can see this in the light view screen cap. The shadow should not be visible.

I'm not sure what the fix is in this case. I'll keep looking into this.

Transforming the model coordinate into light texture space was problematic. Initially I was just multiplying the untransformed coordinate in the vertex shader by the light model view projection matrix.

Solution: perspective divide and pre multiply the MVP with a clip to texture matrix. The clip to texture matrix moves the vertex into the light's canonical view volume.

Row major vs column major storage of matrices was also a problem. I incorrectly assumed that the way OpenGL wants it matrices would be the same as the GPU. There is no reason why this assumption should be true and it's clear from the Cg examples that this is not the case. While I was getting a lot of the math correct on the CPU side,
I was sending the GPU matrices that needed to be transposed.

I plan on changing my matrix class to store everything row major. If I need to load one of my matrices into the OpenGL state, then I'll transpose it. Right now I transpose all matrices before sending them to the GPU because I store them in column major order.

No comments:

Post a Comment