gfx/thebes/gfxLineSegment.h

   1 /* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 4 -*-
   2  * This Source Code Form is subject to the terms of the Mozilla Public
   3  * License, v. 2.0. If a copy of the MPL was not distributed with this
   4  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   5
   6 #ifndef GFX_LINESEGMENT_H
   7 #define GFX_LINESEGMENT_H
   8
   9 #include "gfxTypes.h"
  10 #include "gfxPoint.h"
  11
  12 struct gfxLineSegment {
  13   gfxLineSegment(const gfxPoint &aStart, const gfxPoint &aEnd)
  14     : mStart(aStart)
  15     , mEnd(aEnd)
  16   {}
  17
  18   bool PointsOnSameSide(const gfxPoint& aOne, const gfxPoint& aTwo)
  19   {
  20     // Solve the equation y - mStart.y - ((mEnd.y - mStart.y)/(mEnd.x - mStart.x))(x - mStart.x) for both points
  21
  22     gfxFloat deltaY = (mEnd.y - mStart.y);
  23     gfxFloat deltaX = (mEnd.x - mStart.x);
  24
  25     gfxFloat one = deltaX * (aOne.y - mStart.y) - deltaY * (aOne.x - mStart.x);
  26     gfxFloat two = deltaX * (aTwo.y - mStart.y) - deltaY * (aTwo.x - mStart.x);
  27
  28     // If both results have the same sign, then we're on the correct side of the line.
  29     // 0 (on the line) is always considered in.
  30
  31     if ((one >= 0 && two >= 0) || (one <= 0 && two <= 0))
  32       return true;
  33     return false;
  34   }
  35
  36   /**
  37    * Determines if two line segments intersect, and returns the intersection
  38    * point in aIntersection if they do.
  39    *
  40    * Coincident lines are considered not intersecting as they don't have an
  41    * intersection point.
  42    */
  43   bool Intersects(const gfxLineSegment& aOther, gfxPoint& aIntersection)
  44   {
  45     gfxFloat denominator = (aOther.mEnd.y - aOther.mStart.y) * (mEnd.x - mStart.x ) -
  46                            (aOther.mEnd.x - aOther.mStart.x ) * (mEnd.y - mStart.y);
  47
  48     // Parallel or coincident. We treat coincident as not intersecting since
  49     // these lines are guaranteed to have corners that intersect instead.
  50     if (!denominator) {
  51       return false;
  52     }
  53
  54     gfxFloat anumerator = (aOther.mEnd.x - aOther.mStart.x) * (mStart.y - aOther.mStart.y) -
  55                          (aOther.mEnd.y - aOther.mStart.y) * (mStart.x - aOther.mStart.x);
  56
  57     gfxFloat bnumerator = (mEnd.x - mStart.x) * (mStart.y - aOther.mStart.y) -
  58                          (mEnd.y - mStart.y) * (mStart.x - aOther.mStart.x);
  59
  60     gfxFloat ua = anumerator / denominator;
  61     gfxFloat ub = bnumerator / denominator;
  62
  63     if (ua <= 0.0 || ua >= 1.0 ||
  64         ub <= 0.0 || ub >= 1.0) {
  65       //Intersection is outside of the segment
  66       return false;
  67     }
  68
  69     aIntersection = mStart + (mEnd - mStart) * ua;
  70     return true;
  71   }
  72
  73   gfxPoint mStart;
  74   gfxPoint mEnd;
  75 };
  76
  77 #endif /* GFX_LINESEGMENT_H */