Assignment 5 – Bounding Volume Hierarchy
When wanting to draw many polygons it is not realistic to think of producing an image in any short period of time by cycling through every object for every ray. This is especially true when wanting to do anti-aliasing and hence sampling multiple rays per pixel.
A way around this is by bounding groups of polygons into boxes and checking the box first to see if the ray went through it. When the ray misses that box completely then there is no need to check any of the polygons bounded by that box. This save huge amounts of time when you’re speaking of drawing scenes with over a million polygons.
For example the Stanford Buddah contains 543,652 vertices and 1,087,716 triangles. I built bounding boxes around groups of these polygons by dividing them along different axis on my way down to having one polygon to check. Somehow my times haven’t turned out great and running gprof shows that the bounding box hitBox() function is what takes the most time. But it looks like what was discussed in class. So I think I must be taking a hit somewhere else. double tmin, tmax, temp_min, temp_max; Vector3 eye = ray.origin(); Vector3 direction = ray.direction(); // dividing once here gets rid of extra divides. double temp = 1 / direction.x(); // now just check against temp instead of direction.x if( temp > 0 ) { tmin = ( min_.x() - eye.x() ) * temp; tmax = ( max_.x() - eye.x() ) * temp; } else { tmin = ( max_.x() - eye.x() ) * temp; tmax = ( min_.x() - eye.x() ) * temp; } // divide again for y temp = 1 / direction.y(); if( temp > 0 ) { temp_min = ( min_.y() - eye.y() ) * temp; temp_max = ( max_.y() - eye.y() ) * temp; } . . .
Compiler	Num Samples		BVH build	Ray-trace		Total
Visual Studio 6.0	1		118.632 sec	130.858 sec		249.49 sec (4 min)
Visual Studio 6.0	25		115.868 sec	3606.256 sec (1 hr)		3722.124 sec
g++ -O3	1		130.750 sec	122.730 sec		253.480 sec
g++ -O3	25		204.820 sec	3600.00+ sec		3804.82+ sec
1 sample per pixel		25 samples per pixel			1 sample marble buddah