Oriented projective geometry

Oriented projective geometry is an oriented version of real projective geometry.

Whereas the real projective plane describes the set of all unoriented lines through the origin in R³, the oriented projective plane describes lines with a given orientation. There are applications in computer graphics and computer vision where it is necessary to distinguish between rays light being emitted or absorbed by a point.

Elements in an oriented projective space are defined using signed homogeneous coordinates. Let $\mathbf {R} _{*}^{n}$ be the set of elements of $\mathbf {R} ^{n}$ excluding the origin.

Oriented projective line, $\mathbf {T} ^{1}$ : $(x,w)\in \mathbf {R} _{*}^{2}$ , with the equivalence relation $(x,w)\sim (ax,aw)\,$ for all $a>0$ .
Oriented projective plane, $\mathbf {T} ^{2}$ : $(x,y,w)\in \mathbf {R} _{*}^{3}$ , with $(x,y,w)\sim (ax,ay,aw)\,$ for all $a>0$ .

These spaces can be viewed as extensions of euclidean space. $\mathbf {T} ^{1}$ can be viewed as the union of two copies of $\mathbf {R}$ , the sets (x,1) and (x,-1), plus two additional points at infinity, (1,0) and (-1,0). Likewise $\mathbf {T} ^{2}$ can be view two copies of $\mathbf{R}^2$ , (x,y,1) and (x,y,-1), plus one copy of $\mathbf {T}$ (x,y,0).

An alternative way to view the spaces is as points on the circle or sphere, given by the points (x,y,w) with

x²+y²+z²=1.

Distances between two points

p=(p_{x},p_{y},p_{w})

and

q=(q_{x},q_{y},q_{w})

\mathbf {T} ^{2}

can be defined as elements in $\mathbf {T} ^{1}$

((p_{x}q_{w}-q_{x}p_{w})^{2}+(p_{y}q_{w}-q_{y}p_{w})^{2},\mathrm {sign} (p_{w}q_{w})(p_{w}q_{w})^{2})\,.

References

Stolfi, Jorge (1991). Oriented Projective Geometry. Academic Press. ISBN 978-0-12-672025-9.
From original Stanford Ph.D. dissertation, Primitives for Computational Geometry, available as .
A. G. Oliveira, P. J. de Rezende, F. P. SelmiDei An Extension of CGAL to the Oriented Projective Plane T2 and its Dynamic Visualization System, 21st Annual ACM Symp. on Computational Geometry, Pisa, Italy, 2005.
Ghali, Sherif (2008). Introduction to Geometric Computing. Springer. ISBN 978-1-84800-114-5.
Nice introduction to oriented projective geometry in chapters 14 and 15. More at authors web-site. Sherif Ghali.

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