There are a number of distance-related computations, measures, and indices that can prove useful in your game programming.
Distance Between Two Points
The easiest way to compute the distance between two points is to use the Euclidean distance metric. Given P and Q in a 3D world, the distance is computed by:
Computing square roots can be costly. Moreover, in some cases (such as spatial sorting by distance), you will not need the distance itself, but just a way to measure objects according to their distance. In this case, you can override the square root and use the expression:
distance_squared = (p.x–q.x)2+(p.y–q.y)2+(p.z–q.z)2
A third method is to use Manhattan distance metrics, which are defined as:
distance_manhattan = abs(p.x–q.x)+abs(p.y-q.y)+abs(p.z-q.z)
Manhattan distances are the 0-level Euclidean norm, defined by the general expression:
Manhattan distances are used as very fast pseudodistances, especially to cull away distant objects. It is a very cheap test that can help us discard irrelevant primitives (both in terms of logic and presentation).
Distance Between Two Lines
Given two lines R and S in (origin, direction vector) form, we can compute the minimum distance between them easily. The lines have the following expression:
These are the parametric equations of the lines, and they are very easy to construct from game data structures. The distance between them is computed with the following:
This formula can be somehow sped up. For crossing lines, the numerator will always be zero, and thus we can save the division and modulo operations in the denominator.
Distance from a Point to a Line
The distance from a point P to a line in parametric form (defined by Org and Dir) can easily be computed with the following:
By storing a normalized direction vector, we can save the divide, thus speeding up the computation.
Distance from a Point to a Plane
Distance from a point P to a plane in 3D space can be computed easily using the normal distance to origin plane representation. A plane can be represented as:
where N is the normal to the plane and D is the minimum distance from the plane to the origin. Assuming this representation, the distance is computed as follows:
As usual, keeping the plane's normal as a unit vector speeds the formula, so we can save the modulo and division in the denominator.
Distance Between Two Planes
Finding the distance between two planes is pretty straightforward:
Test if the two normal vectors are the same; if they are, the planes are parallel. If they are not, distance is 0 (they intersect).
Select a point in one of the planes.
Perform point-plane distance as shown earlier using that point and the opposite plane.