A method of processing on screen display data with an image post processor includes receiving a data stream from a video processor at a post processing device having at least one port, the data stream including on screen display data overlaid on a white background and the on screen display data overlaid on a black background, finding a difference between the on screen display data overlaid on white and the on screen display data overlaid on black, using the difference to determine a complement of an alpha blend value, performing image processing on the image data with the post processor by applying the complement of an alpha blend value to the image data to produce processed image data, and transmitting the processed image data and the on screen display data through a display port.