Definitions of different types of coherence relevant for volume rendering

• Pixel Space Coherence: Coherence between pixels in image space.

  Acceleration of image processing taking advantage of the asumption that between two similar shaded pixels ly only pixels with similar color. Thus, coherent pixels can be computed with interpolation.

• Object Space Coherence: Coherence between voxels in Object space (e.g. empty areas)

  Avoid sampling of in 3D regions having uniform or similar values: Detection of this areas by building up octrees. Samples located in a coherent area are calculated only once. "It is obvious that empty space does not have to be sapmled - its has only to be crossed as fast as possible" (Space leaping) []

• Inter Ray Coherence: In parallel projection all ray have the same slope, thus only a template of a discretisation of such a ray is generated by a 3D line voxelization algorithm.
• Frame-to-Frame Coherence: The coherence between successive images in animations.

  Exploiting the coherence of empty space in conscutive images, the ray path can be shorten.