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The TRACE mode is used to remove lines that are obscured by opaque bodies. Line drawings made with TRACE mode ON make use of zone (or cell) descriptions, including the transparency of the zones. When a segment of a line or curve is ready to be drawn, the path from the ends of the segment to the viewpoint is traced to see if opaque bodies are encountered. If either trace makes such an encounter, the segment is considered to be hidden and is not drawn. The number of such segments is proportional to the resolution settings. Low resolution, and hence long segments, can result in gaps in the drawings.
If TRACE is OFF (Trace in the Line Drawing Dialog unchecked), then all bodies are assumed to be transparent (even if some non-transparent zones have been defined) and all of the geometry is visible. The TRACE OFF mode results in shorter rendering speeds; the time saving may be significant for complicated models.
Several other options only work in line drawing mode when TRACE mode is ON. Geometry testing (Test) requires TRACE ON (geometry testing when ray tracing (Color or Etch) does not depend on the setting of TRACE). When testing the geometry, all transparency information is ignored. If TRACE is OFF, no check is performed on the validity of the viewpoint; the viewpoint may be inside a body or on a surface. In surface geometry mode, TRACE is always forced to be ON.