In this recipe, we will implement volume rendering using single-pass GPU ray casting. There are two basic approaches for doing GPU ray casting: the multi-pass approach and the single-pass approach. Both of these approaches differ in how they estimate the ray marching direction vectors. The single-pass approach uses a single fragment shader. The steps described here can be understood easily from the following diagram:

First, the camera ray direction is calculated by subtracting the vertex positions from the camera position. This gives the ray marching direction. The initial ray position (that is, the ray entry position) is the vertex position. Then based on the ray step size, the initial ray position is advanced in the ray direction using a loop. The volume dataset is then sampled at this position to obtain the density value. This process is continued forward advancing the current ray position until either the ray exits the volume...