Reply To: PGON defintion – help

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#3643
Péter Baksa
Keymaster

Hi Pavel,

To define polygons, you have to list the indices of it’s edges. These indices are generated internally, incremented after each EDGE command, and reset to 1 with the BASE command. The edges have a direction. If you include an edge with -ID, the GDL interpreter will include the edge in opposite direction. The edge directions of a polygon should form a continuous counter-clockwise loop. When you have two neighboring polygons with a common edge, going counterclockwise around both polygons means that the common edge will be positive direction in one polygon, and negative in the other. The negative ID is a shortcut for reusing the same edge in opposite direction, you don’t have to define them twice.
If you include an edge with ID = 0, the following edges will define a hole. The edge directions of holes should form a continuous clockwise loop.

I hope this gets you closer to understanding.

Péter Baksa
Library Platform, Software Engineer
GRAPHISOFT SE