| The cross ratio |
|
|
|
|
|
|
| A Projective
Invariant |
|||||
| • | Something that
does not change under projective transformations |
||||
| (including
perspective projection) |
|||||
| The cross-ratio of 4
collinear points |
| P4 |
| P3 |
| P2 |
| P1 |
|
|
|
|
|
|
| Can permute the
point ordering |
|||||
| • | 4! = 24 different
orders (but only 6 distinct values) |
||||
| This is the
fundamental invariant of projective geometry |
|||||