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- Projective in homogeneous coordinates: bothandare defined
- In Projective Space, we single out a particular line and call it the line at infinity.
- With this line at infinity, we are able to define parallelism, and equal length of two intervals.
- In Affine Geometry, we further single out two circular points in the line at infinity.
- Two circular points:which satisfy a pair of real equations:.
- With two circular points, we are able to define angle and length ratios.