BEFORE MOVING ON WITH OPENGL, we look at the mathematical background in a little more depth. The mathematics of computer graphics is primarily linear algebra, which is the study of vectors and linear transformations. A vector, as we have seen, is a quantity that has a length and a direction. A vector can be visualized as an arrow, as long as you remember that it is the length and direction of the arrow that are relevant, and that its specific location is irrelevant. If we visualize a vector V as starting at the origin and ending at a point P, then we can to a certain extent identify V with P — at least to the extent that both V and P have coordinates, and their coordinates are the same. For example, the 3D point (x,y,z) = (3,4,5) has the same coordinates as the vector (dx,dy,dz) = (3,4,5). For the point, the coordinates (3,4,5) specify a position in space in the xyz coordinate system. For the vector, the coordinates (3,4,5) specify the change in the x, y, and z coordinates along the vector. If we represent the vector with an arrow that starts at the origin (0,0,0), then the head of the arrow will be at (3,4,5).

The distinction between a point and a vector is subtle. For some purposes, the distinction can be ignored; for other purposes, it is important. Often, all that we have is a sequence of numbers, which we can treat as the coordinates of either a vector or a point at will.

Matrices are rectangular arrays of numbers. A matrix can be used to apply a transformation to a vector (or to a point). The geometric transformations that are so important in computer graphics are represented as matrices.

In this section, we will look at vectors and matrices and at some of the ways that they can be used. The treatment is not very mathematical. The goal is to familiarize you with the properties of vectors and matrices that are most relevant to OpenGL.


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