Realtime Ray Tracing and Interactive Global Illumination - Scientific ...

30 Chapter 3: A Brief Survey of Ray Tracing Acceleration Methods
primitives that are “close” to a given ray, and to skip primitives that are far
away. During “traversal” of this acceleration structure, only those potential
candidates thus have to be intersected, and the total number of intersection
tests can be significantly reduced.
Over the last 20 years, many different kinds of acceleration structures
have been developed, like e.g. uniform, non-uniform, recursive and hierarchical
grids [Amanatides87, Klimaszewski97, Cohen94, Gigante88, Fujimoto86,
Hsiung92, Jevans89], Octrees [Glassner84, Cohen94, Whang95, Samet89],
Bounding Volume Hierarchies [Rubin80, Kay86, Haines91a, Smits98], BSP
trees (or kd-trees) [Sung92, Subramanian90a, Havran01, Bittner99], and even
higher-dimensional, directional techniques such as ray classification [Arvo87,
Simiakakis95].
As already noted by Kay [Kay86] in 1986, in principle these techniques
mainly differ in whether they hierarchically organize the scene primitives (as
done by Bounding Volume Hierarchies), or whether they subdivide object
space (or ray space) into a set of unique voxels (as done by BSPs, kd-trees,
or Grids).
3.3.2.1 Bounding Volume Hierarchies
In the first class (Bounding Volume Hierarchies) each primitive is stored
only once in the hierarchy. This usually leads to a predictable memory consumption,
and guarantees that each primitive is intersected only exactly once
during traversal of the hierarchy (which is usually not the case for spatial
subdivision techniques, see below). On the other hand, different parts of the
hierarchy may overlap the same regions in space. This often leads to inefficient
traversal such as intersecting the primitives in the wrong order 10 or
traversing the same space multiple times).
3.3.2.2 Spatial Subdivision (Grids, Octrees, BSPs, etc.)
In contrast to BVHs, spatial subdivision techniques subdivide three-dimensional
space into a finite, non-overlapping set of volume elements (voxels), in
which each voxel keeps a list of references to all the primitives overlapping
that respective voxel.
Traversing a ray through the spatial acceleration structure then sequentially
iterates through all the voxels encountered by a ray, thereby intersecting
10 The correct order can be guaranteed by organizing the not-yet-traversed parts of the
hierarchy in a priority queue and always traversing the closest one. This however incurs
additional cost in each traversal step. Also, please keep in mind that the correct order is
not important at all for shadow rays.