A scan converter, used in an imaging system, for inverting the R-.theta. format of stored data into a format compatible with video display. As the video display is scanned horizontally, the .theta. addresses are sequentially addressed by means responsive to the scan position. The switch points thus defined are updated within succeeding video lines in accordance with trigonometric relationships between the video lines and the radial lines. Means responsive to the scanning of a reference video line initializes the switch point values in accordance with respective intersection points of the displayed radial lines and the reference video line.The R coordinate is similarly initialized and updated in accordance with trigonometric relationships between the radial lines and the video lines.