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() {}
  14   gfxLineSegment(const gfxPoint& aStart, const gfxPoint& aEnd)
  15       : mStart(aStart), mEnd(aEnd) {}
  16
  17   bool PointsOnSameSide(const gfxPoint& aOne, const gfxPoint& aTwo) {
  18     // Solve the equation
  19     // y - mStart.y - ((mEnd.y - mStart.y)/(mEnd.x - mStart.x))(x - mStart.x)
  20     // 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
  29     // line. 0 (on the line) is always considered in.
  30
  31     if ((one >= 0 && two >= 0) || (one <= 0 && two <= 0)) return true;
  32     return false;
  33   }
  34
  35   /**
  36    * Determines if two line segments intersect, and returns the intersection
  37    * point in aIntersection if they do.
  38    *
  39    * Coincident lines are considered not intersecting as they don't have an
  40    * intersection point.
  41    */
  42   bool Intersects(const gfxLineSegment& aOther, gfxPoint& aIntersection) {
  43     gfxFloat denominator =
  44         (aOther.mEnd.y - aOther.mStart.y).value * (mEnd.x - mStart.x).value -
  45         (aOther.mEnd.x - aOther.mStart.x).value * (mEnd.y - mStart.y).value;
  46
  47     // Parallel or coincident. We treat coincident as not intersecting since
  48     // these lines are guaranteed to have corners that intersect instead.
  49     if (!denominator) {
  50       return false;
  51     }
  52
  53     gfxFloat anumerator = (aOther.mEnd.x - aOther.mStart.x).value *
  54                               (mStart.y - aOther.mStart.y).value -
  55                           (aOther.mEnd.y - aOther.mStart.y).value *
  56                               (mStart.x - aOther.mStart.x).value;
  57
  58     gfxFloat bnumerator =
  59         (mEnd.x - mStart.x).value * (mStart.y - aOther.mStart.y).value -
  60         (mEnd.y - mStart.y).value * (mStart.x - aOther.mStart.x).value;
  61
  62     gfxFloat ua = anumerator / denominator;
  63     gfxFloat ub = bnumerator / denominator;
  64
  65     if (ua <= 0.0 || ua >= 1.0 || ub <= 0.0 || ub >= 1.0) {
  66       // Intersection is outside of the segment
  67       return false;
  68     }
  69
  70     aIntersection = mStart + (mEnd - mStart) * ua;
  71     return true;
  72   }
  73
  74   gfxPoint mStart;
  75   gfxPoint mEnd;
  76 };
  77
  78 #endif /* GFX_LINESEGMENT_H */