1. An image-based motion sensor, comprising: a camera system, arranged for generating an image output including a plurality of captured images; and a processing system, arranged for obtaining a motion sensor output by processing the image output, and identifying a user input as one of a plurality of pre-defined user actions according to the motion sensor output, wherein different functions of at least one application performed by one electronic device are controlled by the pre-defined user actions; and the motion sensor output includes information indicative of at least one of a motion status and an orientation status of the image-based motion sensor; wherein each of the captured images comprises more than one color component, and only values of one single color component are involved in obtaining the motion sensor output.