A method of image processing includes capturing two frames of image data using a camera having a phase detection auto focus (PDAF) sensor, subtracting phase difference pixel data associated with the two frames of image data to produce residual phase difference values, detecting a region-of-interest change based on the residual phase differences values, and determining a focus for the PDAF camera sensor based on the detected region-of-interest change. The method may further include compressing the residual phase difference values, sending the compressed residual phase difference values to a processor, and reconstructing, with the processor, the phase difference pixel data from the compressed residual phase difference values.