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Chapter 14: Optimizing Rendering
Back patch culling usually reduces the work done by the graphics hardware but it does
increase the workload of the host. Performance is enhanced when the time spent
performing back patch culling is roughly equal to the time spent processing culled
triangles in the graphics hardware.
When to Use Back Patch Culling
Back patch culling is most effective when:
• The primitives are composed of many elements.
• Most primitives are not on the front and back sides of geometric shapes.
If the primitives are short, the processing time of the back patch culling might
approximate the rendering time for no net gain in performance. If most of the primitives
wrap around to both sides of a geometry, few primitives are back patch culled.
Method of Calculation
To determine which side of a geometry an element of a primitive is on, the angle between
the viewing vector and the normal to each element of a primitive is calculated. If the
viewing angle is less than 90 degrees, the element is on the front of the geometry, as
shown in Figure 14-2.
Figure 14-2 Viewing Angle
View vector
Face Normal
Viewing angle
