Getting level of rotation with UIAccleration

Games like FroggyJump for iPhone figu开发者_StackOverflowre out the rotation of the iphone. I'm getting confused with the acceleration values. How do I calculate the level of rotation? I suppose I need to consider when the iphone isn't perfectly upright.

Thank you.

I'm also wanting to use the new Core Motion framework with the "Device Motion" for iPhone 4 for extra precision. I guess I'll have to use that low pass filter for the other devices.

It's the yaw.

Having given Froggy Jump a quick go, I think it's likely directly using the accelerometer's x value as the left/right acceleration on the frog. If it is stationary, you can think of an accelerometer as giving you the vector that points upward into space, relative to the local axes. For something like a ball rolling or anything else accelerating due to tilt, you want to use the values directly.

For anything that involves actually knowing angles, you're probably best picking the axis around which you want to detect rotation then using the C function atan2f on the accelerometer values for the other two axes. With just an accelerometer, there are some scenarios in which you can't detect rotation — for example, if the device is flat on a table then an accelerometer can't detect yaw. The general rule is that rotations around the gravity vector can't be detected with an accelerometer alone.