Question

Can someone explain what is happening in the example below:

glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glRotatef(45,0,0,1);
DrawCube();
glTranslatef(4,0,0);
glRotatef(-45,0,0,1);
DrawCube();
glPopMatrix();

I am assuming that it will just rotate the square, shift it right 4 units, and then rotate it back to its original rotation. It seems too easy though, is there something I am missing?

Answer 1

Roughly-

• Make sure OpenGL applies matrix operations to the modelview stack
• Save the current value of the stack (to be reverted by the pop later)
• Load the identity, clearing any and all rotates/transforms/whatever
• Rotate the current matrix transformation 45 degrees around the X axis
• Draw a cube (using the current matrix transformation, which has been rotated)
• Translate the current matrix transformation 4 units in the X axis
• Rotate the current matrix transformation -45 degrees around the x axis
• Draw another cube that has been translated and rotate from the position of the first cube
• Restore the original matrix transform that was saved by the Push

This results in two cubes being drawn. The first is in the center, and rotated 45 degrees, and the second 4 units away on the rotated x-axis, and itself rotated -45 degrees.