I've started with these series to teach myself image processing and I am wondering when the camera's projection matrix (i.e. intrinsic and extrinsic matrices) is calculated?
Is camera calibration a one-time task at the factory? Or, does it need to be performed regularly for some reason?
Theoretically, the intrinsic parameters of the camera, i. e. things like the focal length, the principal point, and the lens distortion parameters should not change, and thus can be estimated once. In reality, however, they may change over time because of heat or mechanical stresses.
Heat can easily warp the plastic parts of the camera changing the relative positions of the lens and the imaging sensor. Or heat can warp the lens itself, if it is plastic, changing its distortion characteristics. If your camera is mounted on the front of the robot, which repeatedly bumps into walls, that can also cause the lens to become misaligned relative to the imaging sensor.
The extrinsics parameters of the camera are its 3D rotation and translation relative to something. So depending on what that something is, the extrinsics will change if the camera moves. For example, if you have a camera mounted on a mobile robot or a robot arm, you need to know the extrinsics of the camera relative to the robot's origin. Again, that should not change, assuming that the camera is rigidly attached to the robot or the arm, but in reality things can always move, and then you need to recalibrate.
So, in practice, you calibrate once, and hope for the best. If over time the performance of your vision system degrades, then recalibrating should be one of the first things to try.
