Apparatus, comprising: a display, comprising a screen; and a processor, configured: to define a bounding region on the screen, and to render a quadric, which is defined in a parameter space, that overlays 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, 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, wherein the processor is further configured to mark a silhouette of the quadric on the screen, by, for each pixel in the bounding region for which the point of intersection does not exist: estimating a shortest distance between the transformed virtual ray and the quadric, and provided that the estimated shortest distance is less than a threshold, marking the pixel as belonging to the silhouette. a display, comprising a screen; and a processor, configured: to define a bounding region on the screen, and to render a quadric, which is defined in a parameter space, that overlays 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, 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, wherein the processor is further configured to mark a silhouette of the quadric on the screen, by, for each pixel in the bounding region for which the point of intersection does not exist: estimating a shortest distance between the transformed virtual ray and the quadric, and provided that the estimated shortest distance is less than a threshold, marking the pixel as belonging to the silhouette. to define a bounding region on the screen, and to render a quadric, which is defined in a parameter space, that overlays 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, 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, wherein the processor is further configured to mark a silhouette of the quadric on the screen, by, for each pixel in the bounding region for which the point of intersection does not exist: estimating a shortest distance between the transformed virtual ray and the quadric, and provided that the estimated shortest distance is less than a threshold, marking the pixel as belonging to the silhouette. 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, 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, wherein the processor is further configured to mark a silhouette of the quadric on the screen, by, for each pixel in the bounding region for which the point of intersection does not exist: estimating a shortest distance between the transformed virtual ray and the quadric, and provided that the estimated shortest distance is less than a threshold, marking the pixel as belonging to the silhouette. estimating a shortest distance between the transformed virtual ray and the quadric, and provided that the estimated shortest distance is less than a threshold, marking the pixel as belonging to the silhouette.