XNA 4 3D Game Development by Example:Beginner's Guide
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While we have a rotation value already associated with the Camera class, it does not mean anything to the class currently. An angle only exists in one plane in our 3D coordinate system, so we cannot simply specify an angle at which to point the camera. We need to convert the angle into a point in 3D space that lies in the direction we wish the camera to face relative to its current location.