Question

I’m trying to rotate a cube around the axis and what I’m doing is:

    glTranslatef(0.0f, 0.0f, -60.0f);
    glRotatef(angle, 0.0f, 1.0f, 0.0f);

I’m expecting it to move to -60 and rotate around the y axis in circle, but instead it’s just spinning around it self at -60 coordinate. When I write it like this:

    glRotatef(angle, 0.0f, 1.0f, 0.0f);
    glTranslatef(0.0f, 0.0f, -60.0f);

I get what I need but I don’t understand why?
Why are they doing to opposite?
Can someone please explain.

Answer 1

When you apply a transform it is applied locally. Think of it as a coordinate system that you are moving around. You start with the coordinate system representing your view, and then you transform that coordinate system relative to itself. So in the first case, you are translating the coordinate system -60 along the Z axis of the coordinate system, and then you are rotating the coordinate system around the new Y axis at the new origin. Anything you draw is then drawn in that new coordinate system.

This actually provides a simpler way to think about transformations once you are used to it. You don’t have to keep two separate coordinate systems in mind: one for the coordinate system that the transforms are applied in and one for the coordinate system that the geometry is drawn in.