In general, a volumetric dataset consists of samples arranged on a regular grid. These samples are also referred to as voxels. While most volume rendering techniques are based on the theoretical framework presented in Section 2.1, several different techniques implementing this optical model have emerged. In the following, we use a taxonomy based on the processing order of the data. We distinguish between image-order, object-order, and hybrid-order. Image-order methods start from the pixels on the image plane and compute the contribution of the appropriate voxels to these pixels. Object-order techniques traverse the voxels and compute what their contribution to the image is. Hybrid-order methods try to combine both approaches. Techniques based on the texture mapping capabilities of the graphics hardware as well as dedicated volume rendering hardware solutions are each discussed in a separate section.