- Ray tracing is slow
- Ray intersect object is often expensive
- Improve speed two ways
- Reduce the cost of ray intersect object
- Intersect ray with fewer objects

**Bounding Boxes**- :
- Idea: Place a bounding box around each object
- Only compute ray intersect object if the ray intersects
the bounding box
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- If box aligned with coordinate axes, ray intersect box is very cheap
- Be careful with CSG/Hierarchical
- Can also use bounding spheres
- Most useful when ray intersect object is VERY expensive
Example: polygonal object with lots of facets

- Construction of good bounding box can be difficult

**Spatial Subdivision**- :
- Idea: Divide space in to subregions
- Place objects from scene in to appropriate subregions
- When tracing ray, only intersect with objects in
sub-regions through which the ray passes
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- Useful when lots of small objects in scene
- Determining subdivision is difficult

CS488/688: Introduction to Interactive Computer Graphics

University of Waterloo

Computer Graphics Lab