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The mapping from 3D to 2D coordinates is (x′, y′) = ( x / w , y / w ). We can convert 2D points to homogeneous coordinates by defining them as (x, y, 1). Assume that we want to find intersection of two infinite lines in 2-dimensional space, defined as a 1 x + b 1 y + c 1 = 0 and a 2 x + b 2 y + c 2 = 0.
The intersection points are: (−0.8587, 0.7374, −0.6332), (0.8587, 0.7374, 0.6332). A line–sphere intersection is a simple special case. Like the case of a line and a plane, the intersection of a curve and a surface in general position consists of discrete points, but a curve may be partly or totally contained in a surface.
(x 0, y 0, z 0) is any point on the line. a , b , and c are related to the slope of the line, such that the direction vector ( a , b , c ) is parallel to the line. Parametric equations for lines in higher dimensions are similar in that they are based on the specification of one point on the line and a direction vector.
The simplest case in Euclidean geometry is the line–line intersection between two distinct lines, which either is one point (sometimes called a vertex) or does not exist (if the lines are parallel). Other types of geometric intersection include: Line–plane intersection; Line–sphere intersection; Intersection of a polyhedron with a line
For example, suppose L, L′ are distinct lines in determined by points x, y and x′, y′, respectively. Linear combinations of their determining points give linear combinations of their Plücker coordinates, generating a one-parameter family of lines containing L and L ′ .
Cyan line has a single point of intersection. Green line has two intersections. Yellow line lies tangent to the cylinder, so has infinitely many points of intersection. Line-cylinder intersection is the calculation of any points of intersection, given an analytic geometry description of a line and a cylinder in 3d space.
The projective lines ū and ȳ intersect at (0, 1, 0). In fact, all lines in K 2 of slope 0, when projectivized in this manner, intersect at (0, 1, 0) in KP 2. The embedding of K 2 into KP 2 given above is not unique. Each embedding produces its own notion of points at infinity. For example, the embedding
If = + is the distance from c 1 to c 2 we can normalize by =, =, = to simplify equation (1), resulting in the following system of equations: + =, + =; solve these to get two solutions (k = ±1) for the two external tangent lines: = = + = (+) Geometrically this corresponds to computing the angle formed by the tangent lines and the line of ...