A data processing method performed by a computer for determining breathing signal data which represents a breathing cycle of a patient, comprising the steps of: acquiring image data representing a sequence of training thermal images of at least a part of the surface of the patient's body over time, the sequence covering at least one half breathing cycle and being captured by a thermographic camera； and tracking at least one tracked point in the image data over the sequence of training thermal images to find a trajectory of the tracked point as the breathing signal data, wherein the tracked point is a point on the surface of the patient's body.
