1. A method of detecting drowsy facial expressions of vehicle drivers under changing illumination conditions comprising: capturing an image of a person's face using an image sensor; detecting a face region of the image using a pattern classification algorithm; performing, using an active appearance model algorithm, local pattern matching to identify a plurality of landmark points on the face region of the image; generating a 3D face model with facial muscles of the face region; determining photometric flows from the 3D face model using an extract photometric flow module; determining geometric flows from the 3D face model using a compute geometric flow module; determining a noise component generated by varying illuminations by comparing the geometric flows to the photometric flows; and removing the noise component by subtracting two photometric flows.