To animate a scene is nothing else than applying the appropriate local transformations to objects in it. For instance, if we have a cone and a sphere and we want to move them, each one of them will have a corresponding local transformation that will describe its location, orientation, and scale. In the previous section, we saw that matrix stacks allow recovering the original Model-View transform so we can apply the correct local transform for the next object to be rendered.
Knowing how to move objects with local transforms and matrix stacks, the question that needs to be addressed is: When?
If we calculated the position that we want to give to the cone and the sphere of our example every time we called the draw function, this would imply that the animation rate would be dependent on how fast our rendering cycle goes. A slower rendering cycle would produce choppy animations and a too fast rendering cycle would create the illusion of objects jumping from one side to the...