GluLookAt code

GLU - the OpenGL Utility library is an additional library that contains a handful of functions for additional tasks.
It is traditional and can be found in a lot of tutorials and examples.
gluLookAt's source code is very simple and can be replaced easily.
The code here comes from glh library (OpenGL Helper Library), LGPL license https://sourceforge.net/projects/glhlib


This function computes a matrix and multiplies it with whatever is in *matrix.
eyePosition3D is a XYZ position. This is where you are (your eye is).
center3D is the XYZ position where you want to look at.
upVector3D is a XYZ normalized vector. Quite often 0.0, 1.0, 0.0

The code presented here is slightly modified (but functional and correct) from the one in glhlib.

 void glhLookAtf2(float *matrix, float *eyePosition3D, float *center3D, float *upVector3D)
 {
    float forward[3], side[3], up[3];
    float matrix2[16], resultMatrix[16];
    //------------------
    forward[0] = center3D[0] - eyePosition3D[0];
    forward[1] = center3D[1] - eyePosition3D[1];
    forward[2] = center3D[2] - eyePosition3D[2];
    NormalizeVector(forward);
    //------------------
    //Side = forward x up
    ComputeNormalOfPlane(side, forward, upVector3D);
    NormalizeVector(side);
    //------------------
    //Recompute up as: up = side x forward
    ComputeNormalOfPlane(up, side, forward);
    //------------------
    matrix2[0] = side[0];
    matrix2[4] = side[1];
    matrix2[8] = side[2];
    matrix2[12] = 0.0;
    //------------------
    matrix2[1] = up[0];
    matrix2[5] = up[1];
    matrix2[9] = up[2];
    matrix2[13] = 0.0;
    //------------------
    matrix2[2] = -forward[0];
    matrix2[6] = -forward[1];
    matrix2[10] = -forward[2];
    matrix2[14] = 0.0;
    //------------------
    matrix2[3] = matrix2[7] = matrix2[11] = 0.0;
    matrix2[15] = 1.0;
    //------------------
    MultiplyMatrices4by4OpenGL_FLOAT(resultMatrix, matrix, matrix2);
    glhTranslatef2(resultMatrix, -eyePosition3D[0], -eyePosition3D[1], -eyePosition3D[2]);
    //------------------
    memcpy(matrix, resultMatrix, 16*sizeof(float));
 }

The matrix you get can be uploaded with a call to glLoadMatrixf(matrix) or if you are using shaders, use glUniformMatrix4fv(location, 1, GL_FALSE, matrix).