The present application relates to a display controller and display system and a method of operating thereof. At a filtering stage display image data are generated on the basis of received pixel-mapped image data. The filtering operation accepts a plurality of pixels out of the received image data as input values to generate a pixel of the display image data as output value. It is further determined whether the plurality of pixels being the input values to the filtering operation are marked. If all pixels thereof are marked, the output pixel being the output value is marked. The marked pixels in the display image data are validated on the basis reference data.