SketchUp Arc Tool: Arc entities
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Arc entities are a combination of multiple line segments connected together to approximate the curvature of the arc. These entities act as a single line in that they can define the edge of a face and also divide a face. Additionally, selecting one segment of the arc selects the entire Arc entity. However, all inference techniques will operate on the Arc as though it were comprised of segments. For example, every point on the arc is inferred as an endpoint of the segment. Draw Arcs with the Arc tool.
Arc entities are comprised of a length (also called the base chord), bulge, radius, and number of segments. The following image shows an Arc entity.
Arcs with more line segments appear to have smoother curvature than arcs with fewer line segments. However, more line segments increases the size of your model and degrades performance. You can often achieve acceptable results by indicating small segmentation and using smoothing and edge softening to create the impression of smoothness.
If an Arc is deformed in a way that destroys its radial definition, such as with a non-uniform scale operation, it will become a non-parametric Curve entity. Polyline Curves can no longer be edited as arcs.