Wireframe Methods

When only the outline of an object is to be displayed, visibility tests are applied to surface edges. Visible edge sections are displayed, and hidden edge sections can either be eliminated or displayed differently from the visible edges. For example, hidden edges could be drawn as dashed lines, or we could use depth cueing to decrease the intensity of the lines as a linear function of distance from the view plane. Procedures for determining visibility of object edges are referred to as wireframe-visibility methods. They are also called visible-line detection methods or hidden-line detection methods. Special wireframe-visibility procedures have been developed, but some of the visible-surface methods discussed in preceding sections can also be used to test for edge visibility.

A direct approach to identifying the visible lines in a scene is to compare each line to each surface. The process involved here is similar to clipping lines against arbitrary window shapes, except that we now want to determine which sections of the lines are hidden by surfaces. For each line, depth values are compared to the surfaces to determine which line sections are not visible. We can use coherence methods to identify hidden line segments without actually testing each coordinate position. If both line intersections with the projection of a surface boundary have greater depth than the surface at those points, the line segment between the intersections is completely hidden, as in Fig. 6.1(a). This is the usual situation in a scene, but it is also possible to have lines and surfaces intersecting each other. When a line has greater depth at one boundary intersection and less depth than the surface at the other boundary intersection, the line must penetrate the surface interior, as in Fig. 6.1(b). In this case, we calculate the intersection point of the line with the surface using the plane equation and display only the visible sections.

Fig.6.1.Hidden-line sections (dashed) for a line that (a) passes behind a surface and (b) penetrates a surface.

Some visible-surface methods are readily adapted to wireframe visibility testing. Using a back-face method, we could identify all the back surfaces of an object and display only the boundaries for the visible surfaces. With depth sorting, surfaces can be painted into the refresh buffer so that surface interiors are in the background color, while boundaries are in the foreground color. By processing the surfaces from back to front, hidden lines are erased by the nearer surfaces. An area-subdivision method can be adapted to hidden-line removal by displaying only the boundaries of visible surfaces. Scan-line methods can be used to display visible lines by setting points along the scan line that coincide with boundaries of visible surfaces. Any visible-surface method that uses scan conversion can be modified to an edge-visibility detection method in a similar way [6].

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