Method for selecting silhouette and visible edges in wire frame images

Abstract

A first method for producing wire-frame images of tessellated objects in real time utilizing existing hardware capabilities removes hidden lines by displaying the frame slightly away from the actual edges and toward the viewing direction to ensure the portion of the frame intended to be visible is not obscured by the visible faces and that the hidden portion of the frame is obscured by the visible faces. A second method allows for visible and hidden lines to be shown in unique line styles. This is done by first displaying all lines in the hidden line style while suppressing the faces. Then, the first method is performed and the lines that are visible lines are overwritten in another line style. A third method shows only visible silhouette and intersection edges. The visible silhouette edges are obtained by displaying the frame in thick lines behind the visible faces. Then, intersection edges are displayed in front of the faces so that only visible ones can be seen. A fourth method builds on the third by also showing hidden silhouette and intersection edges in a unique line style. Each object's silhouette edges are first obtained in hidden line style as if alone. Then, on all objects taken as one, all intersection edges are displayed and the third technique is performed in visible line style. The methods provide real time performance and can be implemented using standard graphics hardware. In addition, no edge-face adjacency information of objects is needed.