Using GLM for mathematics

Since GLM is a header-only library, installation is simple. Download the latest GLM distribution from http://glm.g-truc.net. Then, unzip the archive file, and copy the glm directory contained inside to anywhere in your compiler's include path.

To use the GLM libraries, it is simply a matter of including the core header file, and headers for any extensions. For this example, we'll include the matrix transform extension as follows:

#include <glm/glm.hpp>
#include <glm/gtc/matrix_transform.hpp>

Then the GLM classes are available in the glm namespace. The following is an example of how you might go about making use of some of them:

glm::vec4 position = glm::vec4( 1.0f, 0.0f, 0.0f, 1.0f );
glm::mat4 view = glm::lookAt( glm::vec3(0.0,0.0,5.0),glm::vec3(0.0,0.0,0.0),glm::vec3(0.0,1.0,0.0) );
glm::mat4 model(1.0f);   // The identity matrix
model = glm::rotate( model, 90.0f, glm::vec3(0.0f,1.0f,0.0) );
glm::mat4 mv = view * model;
glm::vec4 transformed = mv * position;

The GLM library is a header-only library. All of the implementation is included within the header files. It doesn't require separate compilation and you don't need to link your program to it. Just placing the header files in your include path is all that's required!

The previous example first creates a vec4 (four coordinate vector) representing a position. Then it creates a 4 x 4 view matrix by using the glm::lookAt function. This works in a similar fashion to the old gluLookAt function. Here, we set the camera's location at (0, 0, 5), looking towards the origin, with the "up" direction in the direction of the y-axis. We then go on to create the model matrix by first storing the identity matrix in the variable model (via the single argument constructor), and multiplying by a rotation matrix using the glm::rotate function. The multiplication here is implicitly done by the glm::rotate function. It multiplies its first parameter by the rotation matrix (on the right) that is generated by the function. The second parameter is the angle of rotation (in degrees), and the third parameter is the axis of rotation. Since before this statement, model is the identity matrix, the net result is that model becomes a rotation matrix of 90 degrees around the y-axis.

Finally, we create our modelview matrix (mv) by multiplying the view and model variables, and then using the combined matrix to transform the position. Note that the multiplication operator has been overloaded to behave in the expected way.

It is not recommended to import all of the GLM namespace by using the following command:

using namespace glm;

This will most likely cause a number of namespace clashes. Instead, it is preferable to import symbols one at a time, as needed. For example:

#include <glm/glm.hpp>
using glm::vec3;
using glm::mat4;

glm::mat4 proj = glm::perspective( viewAngle, aspect, nearDist, farDist );
glUniformMatrix4fv(location, 1, GL_FALSE, &proj[0][0]);