A common problem in computational-geometry is to find the intersection of two lines.
In the common case in two dimensions, the result is a point, which can be expressed as a parameter along one or both of the lines. For rays and line segments, the intersection of the containing lines is computed, then its parameters are checked to see whether the intersection falls within the subset of the line.
Common approaches produce poor answers for lines that are (nearly) parallel, so often an angle test is performed first. Parallel rays and line segments can intersect in a ray or line segment rather than a point.
Special search structures exist for finding intersections among many line segments.
In more than two dimensions, almost all pairs of lines have no intersection, but skew lines can be projected into the unique plane (through the origin) parallel to each. The intersection in that plane identifies the point of closest approach between the lines.
