Conditions for error-free polygon table

When the geometric tables are produced, they should be checked for consistency. The usual conditions are:

1. Every vertex is listed as an end point of at least two edges.
2. Every edge is part of at least one polygon (for a closed solid, exactly two polygons).
3. Every polygon is closed: its edges form a closed loop, so the last vertex joins back to the first.
4. Every polygon has at least one shared edge, so polygons are connected.
5. If the edge table contains pointers to polygons, every edge referenced by a polygon must point back to that polygon (consistent back-pointers).
6. Each polygon is planar, with its vertices lying in one plane; triangles are always planar.
7. No duplicate vertices or edges, and no zero-length edges.
8. Vertices are listed in a consistent order (all counter-clockwise when viewed from outside) so that normals point outwards.

Checking these rules helps to find missing or repeated data before the model is rendered, which would otherwise cause holes or wrong shading.