Maximum Intensity Casting ? How

I have a 3D grid of vertices which are assigned varying alpha values. I want to render the volume (perspective and real time) so that for a given pixel in the framebuffer, the final colour is that of vertex with highest alpha under that pixel.

Essentially pass a ray from eye perpendicular through all parts of the volume and get highest alpha.

I was hoping this could be acheived by some glblend combination rather than ave to do the sorting maths myself, but i have so far failed. I know this technique is used in medical imaging but haven’t found an opengl solution for it. Any help greatfully recieved.

What about

Hope that helped.