Apparatus comprises an image processor configured to provide output video images to a head mountable display, HMD, having one or more display elements to display video images to a wearer of the HMD, for display by the one or more display elements in response to input video images, in which each output video image corresponds to a respective input video image; in which: the image processor comprises a detector configured to detect whether an input image brightness at image locations in the input video images exceeds a threshold image brightness; and the image processor is configured to vary a relationship between the display properties at a given image location in a given output video image and the input image display properties at the given image location in the corresponding input video image in response to a detection that image brightness at the given image location in one or more input video images preceding that corresponding input video image exceeded the threshold image brightness.