Described embodiments include an apparatus that includes a display, including a screen, and a processor. The processor is configured to define a bounding region on the screen. The processor is further configured to render a quadric, which is defined in a parameter space, over a three-dimensional electroanatomical map of a surface of a heart that is displayed on the screen, by, for each pixel in the bounding region, transforming, to the parameter space, a virtual ray that passes through the pixel, and ascertaining whether a point of intersection between the transformed virtual ray and the quadric exists in the parameter space, and, for each pixel in the bounding region for which the point of intersection exists, rendering the pixel on the screen, based on properties of the point of intersection. Other embodiments are also described.
