A mixed-reality display device comprises an input system, a display, and a graphics processor. The input system is configured to receive a parameter value, the parameter value being one of a plurality of values of a predetermined range receivable by the input system. The display is configured to display virtual image content that adds an augmentation to a real-world environment viewed by a user of the mixed-reality display device. The graphics processor is coupled operatively to the input system and to the display; it is configured to render the virtual image content so as to variably change the augmentation, to variably change a perceived realism of the real world environment in correlation to the parameter value.