Illustration of the 2-pass volume rendering algorithm implemented in PRISM.
In the first pass, the front and back faces of the bounding box of the volume are rendered in two separate textures as illustrated in (a) and (b). The color assigned to the bounding box vertices encodes the normalized coordinate of the vertices in the volume. The box is clipped using near and far clipping planes of the renderer to avoid creating holes in the image. In the second pass, textures (a) and (b) are used to compute ray equation and run the integration. Bounds of the ray are adjusted using depth buffer accumulated by VTK for the rendering of other primitives, in this case, the 3D surface of a tumor (c). Doing so allows the correct interleaving of surfaces and volumes as shown in (d).