Accelerating Meshes using Bounding Boxes

scene rendered with mesh acceleration. This scene contains two meshes - a goblet and that of a table.
Novel meshes courtesy of turbo squid

xml file used to create images


The above scene when rendered with the bounding box implemented took approximately 450 seconds. This time included super-sampling and soft shadows. When we removed the bounding box and rendered the scene with no acceleration, the rendering took approximately 1400 seconds. The speed increased then by a factor of 3.
In addition, and as expected, The progress bar that indicates how much of the scene has been rendered goes very quickly at first, as we test the intersection with the box and this test fails, yet the rendering slows down dramatically as we near the middle of the scene, which is where the mesh is located.

The bounding box algorithm works by intersecting the ray with each pair of planes associated with the box, for the x, y, and z axes.
Rather than get the intersection and check whether that point is on the plane, instead I keep a track of the closest intersection parameter gotten, and the farthest. If the closest is larger than the farthest, we know that there is no intersection. However if we've checked all the planes and the closest intersection is never larger than the largest far intersection we know that there was an intersection and we have to check the geometry.

While this is vastly more efficient than trawling the entire hierarchy at every point, there are still many optimisations we can make. For example adaptively deciding whether we want to continue sampling a pixel.

Adaptive Super-sampling