A color-coded highlighting process is used to preview node and edge classifications, and indicate if they are candidates for deletion given the current tolerances. A sample image showing this display convention is in Figure 3. A key to the color codes is in Figure 4. Feature and boundary edges are also highlighted, using the appropriate color.
The feature edges, drawn in blue, are determined based on the current value of the feature angle. Feature edges determine the fundamental shape of the resulting geometry, so it is important for them to be clearly visible in the preview. The user can modify the feature angle until only the desired internal contours remain. In this way, task-specific details can be preserved while decimating all other areas.
The two tolerances used as decimation criteria (plane tolerance and edge tolerance) are set interactively through the use of sliders. Before the sliders are presented to the user, each node in the segment is classified, and the appropriate decimation criterion is calculated and cached. During the adjustment of the tolerances, the highlighting described above is drawn, with the following addition: if a node's decimation metric falls within the current tolerance, it is drawn as a grey crosshair at 50 percent of the normal size. This indicates that this node would be deleted if it was the first node examined.
It is important to remember that the algorithm is iterative, and that the local geometry of the object is constantly being altered. Thus, the preview highlighting does not show exactly which nodes will be removed; rather, it is intended to indicate the regions where simplification will occur. Nonetheless, it is an extremely useful addition to the system. It provides useful, real time response to a procedure that is inherently slow, and eliminates the need for numerous, incremental adjustments to the parameters in between long processing delays (minutes, perhaps even hours).