It's not as trivial as you might think.

We tried that relative-to-an-absolute scheme on our iPhone FPS and it was very confusing. The best scheme was a relative-to-a-relative center where the origin would "follow" the thumb, but then we had a full screen to do that.

The problem with a relative-to-an-absolute scheme (even one with the little raised circle) is you have no idea where the thumb is until you touch the control but then it's too late.

Not saying it can't be done, it's just a little bit tricky.