PGON defintion – help

Home Forums Problems and solutions in GDL 3D modelling PGON defintion – help

Tagged: , ,

This topic contains 2 replies, has 2 voices, and was last updated by  Pavel Čapek 1 year, 5 months ago.

  • Author
    Posts
  • #3553

    Pavel Čapek
    Participant

    Hello,
    I have a question for PGON definition. I follow gdl documentation http://gdl.graphisoft.com/reference-guide/primitive-elements. But I don’t understand on which base is set a MINUS or NONE sign for edge indexes.

    A negative index changes the direction of the stored normal vector or edge to the opposite in the polygon.
    (The stored vector or edge does not change; other polygons can refer to it using the original orientation with a positive index.)

    Thank you for an answer.

    Pavel Čapek
    BIM Project

  • #3643

    Péter Baksa
    Participant

    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
    Software Engineer, Library Team
    GRAPHISOFT SE

  • #3644

    Pavel Čapek
    Participant

    Hi Péter,
    Thank you for your answer. After a while I discovered this by trial and error method. But I definitely make a revision of my code by your advice.

    Pavel Čapek
    BIM Project

You must be logged in to reply to this topic.