1. CSC418 Computer Graphics
Back Faces
Visibility Algorithms

2. Visibility Problem
What is NOT visible?

3. Visibility Problem What is NOT visible?
primitives outside of the field of view
back-facing primitives
primitives occluded by other objects closer to the camera

4. Backface culling

5. Backface culling

6. Backface culling N V E

7. Backface culling
N.V > 0 is a back face?

8. Backface culling
N.(P-E) >0 P N V E

9. Backface culling canonical projection
Where in the graphics pipeline can we do backface culling?
canonical
projection

10. Occluded faces
Does backface culling always determine visibility completely for a single object?

11. Occluded faces
In typical scenes some polygons will overlap, we must determine which portion of each polygon is visible to eye!

12. Painters Algorithm B C A Sort primitives in Z.
Draw primitives back to front (CBA). B C A

13. Painters Algorithm
Problems
Large faces
Intersecting faces
Cycles

14. Visibility Problem Image space algorithms
Operate in display terms pixels, scanlines
Visibility resolved to display resolution
Examples: Z-buffer, ray-tracing
O(n*resolution)
Object Space algorithms
Analytically compute visible fragments
Examples: painters algorithm, BSP
O(n2 )

15. CSC418 Computer Graphics Next Lecture BSP trees Depth sorting
Z-buffer A-buffer