As mentioned before, projective geometry preserves neither distances nor
ratios of distances. However, the cross ratio, which is a ratio of
ratios of distances, is preserved and is therefore a useful concept.
Given four collinear points
denote the Euclidean distance between two points
Then, one definition of the cross ratio is the following:
The Euclidean distance between two points
is computed from the 2D Euclidean points
obtained by dividing by the third coordinate, as mentioned in section
Although the cross ratio is invariant once what the order of the
points has been chosen, its value is different depending on that order. Four
points can be chosen 4! = 24 ways, but in fact only six distinct values
are produced, which are related by the set
As we hinted before, there are other measures of the cross ratio, all of which are also invariant under projective transformations. Not surprisingly, duality leads to a cross ratio for four concurrent lines by replacing the Euclidean distance between two points with the sine of the angle between two lines (I have not confirmed whether the cosine also works). Another less obvious way to measure the cross ratio between four concurrent lines is to use a new, arbitrary line that intersects them; the cross ratio of the lines is then defined as the cross ratio of the four points of intersection (The cross ratio will be the same no matter which line is used).
As a final comment on the cross ratio, it is worth noting that it does not require that the original points be collinear. For example, given five points in a star configuration, as shown in figure 6, we can connect the dots as shown in (a) to yield lines containing four collinear points, the points of intersection, whose cross ratio can be used. Another possibility is to draw lines from one of the points to the other four, as shown in (b), thus yielding four concurrent lines whose cross ratio can be used.