GridsData structure: a 3-D array of cells (voxels) that tile
spaceEach cell points to list of all surfaces intersecting that
cell
Mais sobre GridsBe Careful! The fact that a ray passes through a
cell and hits na object doesnt mean the ray hit that object in that
cellOptimization: cache intersection point and ray id in mailbox
associated with each objectGrids are a poor choice when the world
is nonhomogeneous (clumpy)e.g. a teapot in a stadium: many polygons
clustered in a small spaceHow many cells to use?too few many
objects per cell slowtoo many many empty cells to step through
slowGrids work well when you can arrange that each cell lists a few
(ten, say) objectsBetter strategy for some scenes: nested grids
Octrees
Which Structure is Best for Ray Tracing?
K-d Trees and BSP Trees
Building a BSP Treethe subdivisionof space it impliesa BSP
treeusing 2 as root231123abcdabcdviewpoint
Building the Tree 212a2b332a2b1viewpointUsing line 3 for the
root requires a split
Building a Good Tree - the tricky part
Uses for Binary Space Partitioning (BSP) Trees
Painters Algorithm with BSP trees
Drawing a BSP Tree front_to_back(tree, viewpt) { if (tree ==
null) return; if (positive_side_of(root(tree), viewpt)) {
front_to_back(positive_branch(tree, viewpt);
display_polygon(root(tree)); front_to_back(negative_branch(tree,
viewpt); } else { draw negative branch first} }
Drawing Back to Front12a2b332a2b1viewpointHidden surface
removal
