Traditional Volume Rendering vs. 3D Texture Mapping Based Method

Traditional Volume Rendering(=Ray Casting) (a)

1. Rays are fired from a projection plane.
   
2. Volume data are resampled along the rays at evenly spaced sampling points.
   
3. Volume data are shaded at the sampling points using normal vectors, view vector and light vectors.
   
4. Shaded colors are composited to the projection plane.
   

Texture Based Volume Rendering(=Volume Slicing) (b) & (c)

1. Volume data is converted to 3D texture data.
   
2. Texture mapped planes are generated and composited to a view plane using accelerated texture mapping hardware and compositing hardware
   

Ray Casting vs. Volume Slicing (from volumizer manual)

1. Ray casting under orthographic projection ((a) & (b) in the above figure) is equivalent to taking a series of slices along planes parallel to the viewport and compositing.
   
2. Ray casting under perspective projection ((c) in the above figure) is equivalent to samling along a series of concentric spherical shells centered atthe eye.