A method includes the steps of obtaining a frame from an image sensor, the frame comprising a number of pixel values, detecting a change in a first subset of the pixel values, detecting a change in the second subset of the pixel values near the first subset of the pixel values, and determining an occupancy state based on a relationship between the change in the first subset of the pixel values and the second subset of the pixel values. The occupancy state may be determined to be occupied when the change in the first subset of the pixel values is in a first direction and the change in the second subset of the pixel values is in a second direction opposite the first direction.