A system is provided for simultaneously displaying video data of multiple video sources (040, 042) on a display. The system comprises a video input (120) for receiving the video data (122) of the multiple video sources; a display processor (140) for generating display data (142) to display the video data of the multiple video sources in respective viewports (1A-2F) on a display. The display processor is further arranged for, if needed to fit respective ones of the viewports, spatially scaling the video data of one or more of the multiple video sources to obtain said fit; and generating a visual indicator (300-320) for visually indicating whether the video data in one of the viewports has been spatially scaled from its native spatial resolution. By providing the visual indicator as part of the display data, the system may warn the user that the video data in one of the viewports may comprise undesirable artifacts which may hinder interpretation of the video data.