src/fl_arc.cxx

   1 //
   2 // "$Id: fl_arc.cxx 7903 2010-11-28 21:06:39Z matt $"
   3 //
   4 // Arc functions for the Fast Light Tool Kit (FLTK).
   5 //
   6 // Copyright 1998-2010 by Bill Spitzak and others.
   7 //
   8 // This library is free software; you can redistribute it and/or
   9 // modify it under the terms of the GNU Library General Public
  10 // License as published by the Free Software Foundation; either
  11 // version 2 of the License, or (at your option) any later version.
  12 //
  13 // This library is distributed in the hope that it will be useful,
  14 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  15 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16 // Library General Public License for more details.
  17 //
  18 // You should have received a copy of the GNU Library General Public
  19 // License along with this library; if not, write to the Free Software
  20 // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
  21 // USA.
  22 //
  23 // Please report all bugs and problems on the following page:
  24 //
  25 //     http://www.fltk.org/str.php
  26 //
  27
  28 /**
  29   \file fl_arc.cxx
  30   \brief Utility functions for drawing arcs and circles.
  31 */
  32
  33 // Utility for drawing arcs and circles.  They are added to
  34 // the current fl_begin/fl_vertex/fl_end path.
  35 // Incremental math implementation:
  36
  37 #include <FL/fl_draw.H>
  38 #include <FL/math.h>
  39
  40 // avoid problems with some platforms that don't
  41 // implement hypot.
  42 static double _fl_hypot(double x, double y) {
  43   return sqrt(x*x + y*y);
  44 }
  45
  46 void Fl_Graphics_Driver::arc(double x, double y, double r, double start, double end) {
  47
  48   // draw start point accurately:
  49
  50   double A = start*(M_PI/180);          // Initial angle (radians)
  51   double X =  r*cos(A);                 // Initial displacement, (X,Y)
  52   double Y = -r*sin(A);                 //   from center to initial point
  53   fl_vertex(x+X,y+Y);                   // Insert initial point
  54
  55   // Maximum arc length to approximate with chord with error <= 0.125
  56
  57   double epsilon; {
  58     double r1 = _fl_hypot(fl_transform_dx(r,0), // Horizontal "radius"
  59                           fl_transform_dy(r,0));
  60     double r2 = _fl_hypot(fl_transform_dx(0,r), // Vertical "radius"
  61                           fl_transform_dy(0,r));
  62
  63     if (r1 > r2) r1 = r2;               // r1 = minimum "radius"
  64     if (r1 < 2.) r1 = 2.;               // radius for circa 9 chords/circle
  65
  66     epsilon = 2*acos(1.0 - 0.125/r1);   // Maximum arc angle
  67   }
  68   A = end*(M_PI/180) - A;               // Displacement angle (radians)
  69   int i = int(ceil(fabs(A)/epsilon));   // Segments in approximation
  70
  71   if (i) {
  72     epsilon = A/i;                      // Arc length for equal-size steps
  73     double cos_e = cos(epsilon);        // Rotation coefficients
  74     double sin_e = sin(epsilon);
  75     do {
  76       double Xnew =  cos_e*X + sin_e*Y;
  77                 Y = -sin_e*X + cos_e*Y;
  78       fl_vertex(x + (X=Xnew), y + Y);
  79     } while (--i);
  80   }
  81 }
  82
  83 #if 0 // portable version.  X-specific one in fl_vertex.cxx
  84 void fl_circle(double x,double y,double r) {
  85   _fl_arc(x, y, r, r, 0, 360);
  86 }
  87 #endif
  88
  89 //
  90 // End of "$Id: fl_arc.cxx 7903 2010-11-28 21:06:39Z matt $".
  91 //