- 3D 2D projections map all
*z*to same value. - Need
*z*to determine occlusion, so a 3D to 2D projective transformation doesn't work. - Further, we want 3D lines to map to 3D lines (this is useful in hidden surface removal)
- The mapping maps lines to lines, but loses all depth information.
- We could use
Thus, if we map the endpoints of a line segment, these end points will have the same relative depths after this mapping.

BUT: It fails to map lines to lines

\

- In this figure,
*P*,*Q*,*R*map to*P*',*Q*',*R*' under a pure projective projection. - With the mapping
*P*,*Q*,*R*actually map to , which fail to lie on a straight line. - The map
**does**map lines to lines,**and**it preserves depth information.\

CS488/688: Introduction to Interactive Computer Graphics

University of Waterloo

Computer Graphics Lab