Question

There is something I have never understood. How can a great big PC game like GTA IV use 50% of my CPU and run at 60fps while a DX demo of a rotating Teapot @ 60fps uses a whopping 30% ?

Answer 1

In general, it’s because

1. The games are being optimal about what they need to render, and
2. They take special advantage of your hardware.

For instance, one easy optimization you can make involves not actually trying to draw things that can’t be seen. Consider a complex scene like a cityscape from Grand Theft Auto IV. The renderer isn’t actually rendering all of the buildings and structures. Instead, it’s rendering only what the camera can see. If you could fly around to the back of those same buildings, facing the original camera, you would see a half-built hollowed-out shell structure. Every point that the camera cannot see is not rendered — since you can’t see it, there’s no need to try to show it to you.

Furthermore, optimized instructions and special techniques exist when you’re developing against a particular set of hardware, to enable even better speedups.

The other part of your question is why a demo uses so much CPU:

… while a DX demo of a rotating Teapot @ 60fps uses a whopping 30% ?

It’s common for demos of graphics APIs (like dxdemo) to fall back to what’s called a software renderer when your hardware doesn’t support all of the features needed to show a pretty example. These features might include things like shadows, reflection, ray-tracing, physics, et cetera.

This mimics the function of a completely full-featured hardware device which is unlikely to exist, in order to show off all the features of the API. But since the hardware doesn’t actually exist, it runs on your CPU instead. That’s much more inefficient than delegating to a graphics card — hence your high CPU usage.