vertices and data


I have 3-dimensional data that is read from file into a GLSL 3D texture and I have some vertices that are generated (using EmitVertex()) in another part of the code. I would like to use the vertices to get the values of the vertices in the 3-dimensonal data. Generally speaking, one can do that via samplers. But I need to get unclamped values. This is because I need to do fairly minute calculations on the returned vertex values.

I have seen computation done on the data and then sampling done on the data for display purposes such as colour blending etc. But I have not seen computation being performed after sampling. How can I do post-sampling computation?