gfx/thebes/gfxLineSegment.h

   1 /* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   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), mEnd(aEnd) {}
  15
  16   bool PointsOnSameSide(const gfxPoint& aOne, const gfxPoint& aTwo) {
  17     // Solve the equation
  18     // y - mStart.y - ((mEnd.y - mStart.y)/(mEnd.x - mStart.x))(x - mStart.x)
  19     // for both points
  20
  21     gfxFloat deltaY = (mEnd.y - mStart.y);
  22     gfxFloat deltaX = (mEnd.x - mStart.x);
  23
  24     gfxFloat one = deltaX * (aOne.y - mStart.y) - deltaY * (aOne.x - mStart.x);
  25     gfxFloat two = deltaX * (aTwo.y - mStart.y) - deltaY * (aTwo.x - mStart.x);
  26
  27     // If both results have the same sign, then we're on the correct side of the
  28     // line. 0 (on the line) is always considered in.
  29
  30     if ((one >= 0 && two >= 0) || (one <= 0 && two <= 0)) return true;
  31     return false;
  32   }
  33
  34   /**
  35    * Determines if two line segments intersect, and returns the intersection
  36    * point in aIntersection if they do.
  37    *
  38    * Coincident lines are considered not intersecting as they don't have an
  39    * intersection point.
  40    */
  41   bool Intersects(const gfxLineSegment& aOther, gfxPoint& aIntersection) {
  42     gfxFloat denominator =
  43         (aOther.mEnd.y - aOther.mStart.y) * (mEnd.x - mStart.x) -
  44         (aOther.mEnd.x - aOther.mStart.x) * (mEnd.y - mStart.y);
  45
  46     // Parallel or coincident. We treat coincident as not intersecting since
  47     // these lines are guaranteed to have corners that intersect instead.
  48     if (!denominator) {
  49       return false;
  50     }
  51
  52     gfxFloat anumerator =
  53         (aOther.mEnd.x - aOther.mStart.x) * (mStart.y - aOther.mStart.y) -
  54         (aOther.mEnd.y - aOther.mStart.y) * (mStart.x - aOther.mStart.x);
  55
  56     gfxFloat bnumerator = (mEnd.x - mStart.x) * (mStart.y - aOther.mStart.y) -
  57                           (mEnd.y - mStart.y) * (mStart.x - aOther.mStart.x);
  58
  59     gfxFloat ua = anumerator / denominator;
  60     gfxFloat ub = bnumerator / denominator;
  61
  62     if (ua <= 0.0 || ua >= 1.0 || ub <= 0.0 || ub >= 1.0) {
  63       // Intersection is outside of the segment
  64       return false;
  65     }
  66
  67     aIntersection = mStart + (mEnd - mStart) * ua;
  68     return true;
  69   }
  70
  71   gfxPoint mStart;
  72   gfxPoint mEnd;
  73 };
  74
  75 #endif /* GFX_LINESEGMENT_H */