CSC418 Computer Graphics

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 gt 0 is a back face?

8
Backface culling

• N.(P-E) gt0

P
N
V
E
9
Backface culling

• Where in the graphics pipeline can we do backface
  culling?

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

• 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(nresolution)
• 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