During the rendering process, the optical model describes how points in the volume interact with light. Optical
parameters are usually taken as the data values directly, or they are derived from applying one or more transfer functions to the data values. The choice of a transfer function emphasizes desired features within the data and can be implemented as lookup tables. Output images are generated by sampling the volume along all viewing rays and accumulating the resulting optical properties to compute a pixel for each ray. Texturebased techniques perform the sampling step by rendering a set of 2D geometric primitives inside the volume.