Features

Listing 2. The X Intercept Calculation.

// Procedure: PointInPoly (EDGE CROSSING VERSION)
// Purpose: Check if a point is inside a polygon
// Returns: TRUE if Point is inside polygon, else FALSE

BOOL CFateView::PointInPoly(tSector *sector, tPoint2D *hitPos)
{
// Local Variables
    short edge, first, next;
    tPoint2D *pnt1,*pnt2;
    BOOL inside = FALSE; // INITIAL TEST CONDITION
    BOOL flag1,flag2;

    edge = first = sector->edge; // SET UP INITIAL CONDITIONS
    pnt1 = &m_edgelist[edge].pos;
    flag1 = ( hitPos->y >= pnt1->y ) ; // IS THE FIRST                                         VERTEX OVER OR                                         UNDER THE LINE

     /* LOOP THROUGH THE VERTICES IN A SECTOR */
    do {
      next = m_edgelist[edge].nextedge; // CHECK THE NEXT VERTEX
      pnt2 = &m_edgelist[next].pos;
      flag2 = ( hitPos->y >= pnt2->y ); // IS IT OVER OR UNDER
      if (flag1 != flag2) // MAKE SURE THE EDGE ACTUALLY                        // CROSSES THE TEST X AXIS
       {
     // CALCULATE WHETHER THE SEGMENT ACTUALLY CROSSES THE X TEST AXIS
    // A TRICK FROM GRAPHIC GEMS IV TO GET RID OF THE X INTERCEPT DIVIDE
    if (((pnt2->y - hitPos->y) * (pnt1->x - pnt2->x) >=
      (pnt2->x - hitPos->x) * (pnt1->y - pnt2->y)) == flag2 )
      inside = !inside; // IF IT CROSSES TOGGLE THE INSIDE                      // FLAG (ODD IS IN, EVEN OUT)
      }
    pnt1 = pnt2; // RESET FOR NEXT STEP
    edge = next;
    flag1 = flag2;
    } while (edge != first);
    return inside;
}

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