Using OpenGL to calculate transformations

March 28, 2010

For my final art project, I’m working on programmatically generating scenes to be rendered in Sunflow. Lot of the work is placing objects where I want them. The following is just a way to use OpenGL to do your matrix multiplications and transformations.

glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();

// transformations go here

GLfloat modelMatrix[16];
glGetFloatv (GL_MODELVIEW_MATRIX, modelMatrix);
glPopMatrix();

Transformation functions: glTranslate, glRotate, glScale, glMultMatrix

The returned array is a column-major representation of a 4×4 matrix. This section of the OpenGL guide is a good reference for remembering in which order to apply transformations.

For example, if I want to translate an object by (3,4,5) and then rotate it 45 degrees around the z-axis, I would use the following code to retrieve the transformation matrix. Notice the order of the transformation functions.

glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();

glRotatef(45, 0, 0, 1);
glTranslatef(3, 4, 5);

GLfloat modelMatrix[16];
glGetFloatv (GL_MODELVIEW_MATRIX, modelMatrix);
glPopMatrix();

for (int i = 0; i < 4; i++) {
  for (int j = 0; j < 4; j++) {
    printf("%f ", modelMatrix[j * 4 + i]);
  }
  printf("\n");
}

The code to the right outputs the following:

0.707107 -0.707107 0.000000 -0.707107
0.707107  0.707107 0.000000  4.949748
0.000000  0.000000 1.000000  5.000000
0.000000  0.000000 0.000000  1.000000
Comments