Path: utzoo!attcan!uunet!ogicse!ucsd!nosc!crash!jcs
From: jcs@crash.cts.com (John Schultz)
Newsgroups: comp.graphics
Subject: Re: Polygon orientation test ?
Message-ID: <5201@crash.cts.com>
Date: 22 Oct 90 22:31:47 GMT
References: <4158.272311a1@cc.curtin.edu.au>
Organization: Crash TimeSharing, El Cajon, CA
Lines: 56





  Yes, there are at least two easy ways:

    1. Use the cross product of two of the points, the sign determines
       the orientation (Subtract point 0 from points 1 and 2, compute
       the cross product between the new points 1 and 2).

    2. Use the Newell method (in many graphics texts).

  Here's a snippet of code to get you started:

  if (count > 3) { /* Newell method */

    orient = 0;
    for (i=0; i < count; i++) {

      if (i == (count-1)) {
        j = 0;
      } else {
        j = i+1;
      }

      orient += (point[i].x - point[j].x)*(point[i].y + point[j].y);

    } 

  } else { /* Move to origin, and do cross product */

    d1.x = point[1].x - point[0].x; 
    d1.y = point[1].y - point[0].y; 
    d2.x = point[2].x - point[1].x; 
    d2.y = point[2].y - point[1].y;

    orient = d1.x*d2.y - d1.y*d2.x;
  } /* if count > 3 */

  /* Right handed coordinate system with +z into screen */

  sgnorient = sgn(orient);

  if (sgnorient < 0) { /* do counter-clockwise case */

  } else if (sgnorient > 0) { /* do clockwise case */

  } else { /* Polygon is a line segment or point (coplaner points) */

  } /* if */
  
  Hope this helps,



  John