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MODELLING CONCEPTS
Convex vs Concave vs Straight polygons.
A convex polygon is one where all of the internal angles are less than 180 degrees (acute, right or obtuse angles)
A concave polygon is one where one or more of the internal angles is more than 180 degrees (reflex angle)
A Straight polygon is one where one or more of the angles is exactly 180 degrees. (straight angle)
Concave and straight polygons can be misinterpreted by renderers, other software and game engines and will very likely cause flickering. Some modifiers in blender (i.e. the Surface Deform Modifier) will give an error message of you try to apply them to a mesh which has concave faces,
We should always try to make sure that we have no concave polygons in our meshes. They can occur with both quads and N-gons but are much more likely to happen with N-gons. The Catmul-Clark subdivision surface modifier will never create convex polygons.
Examples of convex and concave polygons
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