1. A video output checker which is configured to receive incoming video data for rendering an image on a display, the video data comprising pixel data which comprise, for each pixel, a set of colour component values for a given colour model, the video output checker configured to compare each colour component for a pixel in a selected area of the image with a corresponding test range of values and, if a component value falls outside the range, to measure a deviation, the video output checker further configured to sum deviations for the selected area to provide an error value for the selected area, and to compare the error value against a threshold number.