Bounding Boxes, Spatial Subdivision

• 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

  \

• 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

  \

• Useful when lots of small objects in scene
• Determining subdivision is difficult