Volume Rendering Tex-Coords - Graphics

I think the most standard approach is to compute the proxy geometry in
model-space. For a 3D texture-based approach, your proxy geometry
should be orthogonal to the vector:

modelviewInverse * (0,0,1)

Use that vector as the normal to a plane that you move through volume
geometry, computing the intersection (a polygon) of the plane and the
volume geometry at each step. The tricky part is ordering the vertices
from the intersection so you get a polygon -- refer to a computational
geometry text.

There is a good SIGGRAPH tutorial out there by Engel et al. (which has
recently been published as a book with the same title):

Engel, K., Hadwiger, M., Kniss, J. M., Lefohn, A. E., Salama, C. R.
and Weiskopf, D. (2004). Real-time volume graphics. SIGGRAPH '04
Course Notes: Proceedings of the conference on Computer graphics and
interactive techniques (p. 29). New York, NY, USA: ACM Press.

-Kenny