Polygons

• Need an area primitive
• Simple polygon:
  • Planar set of ordered points,

    (sometimes we repeat at end of list)

  • No holes
  • No line crossing

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• Normally define an interior and exterior

  Points ordered in counter-clockwise order

  To the ``left'' as we traverse is inside

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• Try to avoid degeneracies, but sometimes unavoidable

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• We prefer Convex polygons to Concave polygons

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  Polygon is convex if for any two points inside polygon, the line segment joining these two points is also inside.

  Convex polygons ``behave'' better when shading, etc

• Although convex polygons normally ``well behaved'' under affine transformation, affine transformations may introduce degeneracies

  Example: Orthographic projection may project entire polygon to a line segment.