Thread: Area of irregular shape

  1. 06-22-2004 #1
    Brian
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    Area of irregular shape

    I'm writing a space game with customisable spaceship shapes, but I'm a bit overwhelmed by a problem, I want to figure out the mass of an irregular 2d shape (the spaceship) based on its area.

    The shape itself is stored as a sort of linked list

    Point x1, y1 -----> Point x2, y2 ----> Point x3, y3 ----> point x4, y4

    and the last point joins back onto the first point. Every 1 unit (unit being 1 integer distance between points) cubed has a mass of 1.

    so a triangle of, say:
    0, 0
    0, 17
    12, 0

    would have a mass of 17/2 * 12 = 102.
    I know I will have to split the main polygon down into triangles, but using every 3 points will not work because sometimes a line back between points could overlap.

    I hope I make sense, could I please have any advice on how to go about splitting my shape into triangles.
  2. 06-22-2004 #2
    Salem
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    http://homepages.borland.com/efg2lab...olygonArea.htm
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  3. 06-22-2004 #3
    MrWizard
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    Is your polygon limited to the convex case? If so...

    Do you have them in either counter-clockwise or clockwise order? If so just pick the first point as your starting point. So you have 6 points represented like so:

    Code:
       *
  *  *
  *  *
   *
    Let point 1 be the bottom and start moving in a clockwise motion.

    Triangles:

    123
    134
    145
    156

    It will be a bit more work for concave polygons. Let me know if that's your case.

    Edit:

    Yeah...or Salem's way for the general case.
    "...the results are undefined, and we all know what "undefined" means: it means it works during development, it works during testing, and it blows up in your most important customers' faces." --Scott Meyers
  4. 06-22-2004 #4
    vNvNation
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    also keep in mind that you don't have to have exact area. You can approximate the shape by enclosing it with a box, or a circle, or some other basic geometry shape for which you know how to calculate the area.
  5. 06-23-2004 #5
    Brian
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    cool, I have it now, thanks for the help

    btw this site gives polygon area source code for any of you searchers:

    http://astronomy.swin.edu.au/~pbourk...etry/polyarea/
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