What is ray coherence in volume ray casting?

Any free paper/tutorial defining this term would be appreciated (google hasn't been much help on the free par开发者_开发知识库t).

In volume ray casting, "ray coherence" is the degree to which all rays in a volume hit the same geometry.

If a volume of rays is not coherent, then rays in the volume may hit different objects, which usually means that you will need to test individual rays for this. On the other hand, if your volume is fully coherent, you only need to do geometry computations for the volume as a whole, and individual ray computations for that volume can be completely vectorized.

Coherence is important to the performance of any group-based graphics operation. Even conventional rasterization, on modern GPU's, relies on coherence to efficiently evaluate pixel shaders with conditional branches...