The first step toward ray marching of volumes is to find the point of incidence and the point of emergence for each fragment. This is done by first rendering the positions of back-facing polygons to an off-screen buffer and then render the front-facing polygons while using the off-screen buffer as a texture. The following example draws the depth of the volume using this technique to introduce the first step of a GPU ray marching.
The size of the cube is 2 by 2 by 2 and the depth d is visualized as exp(-d).
Mouse control: orbit - left button, zoom - middle button, pan - right button.