dashed-canvas.js

   1 /**
   2  * @license
   3  * Copyright 2012 Dan Vanderkam (danvdk@gmail.com)
   4  * MIT-licensed (http://opensource.org/licenses/MIT)
   5  */
   6
   7 /**
   8  * @fileoverview Adds support for dashed lines to the HTML5 canvas.
   9  *
  10  * Usage:
  11  *   var ctx = canvas.getContext("2d");
  12  *   ctx.installPattern([10, 5])  // draw 10 pixels, skip 5 pixels, repeat.
  13  *   ctx.beginPath();
  14  *   ctx.moveTo(100, 100);  // start the first line segment.
  15  *   ctx.lineTo(150, 200);
  16  *   ctx.lineTo(200, 100);
  17  *   ctx.moveTo(300, 150);  // start a second, unconnected line
  18  *   ctx.lineTo(400, 250);
  19  *   ...
  20  *   ctx.stroke();          // draw the dashed line.
  21  *   ctx.uninstallPattern();
  22  *
  23  * This is designed to leave the canvas untouched when it's not used.
  24  * If you never install a pattern, or call uninstallPattern(), then the canvas
  25  * will be exactly as it would have if you'd never used this library. The only
  26  * difference from the standard canvas will be the "installPattern" method of
  27  * the drawing context.
  28  */
  29
  30 /**
  31  * Change the stroking style of the canvas drawing context from a solid line to
  32  * a pattern (e.g. dashes, dash-dot-dash, etc.)
  33  *
  34  * Once you've installed the pattern, you can draw with it by using the
  35  * beginPath(), moveTo(), lineTo() and stroke() method calls. Note that some
  36  * more advanced methods (e.g. quadraticCurveTo() and bezierCurveTo()) are not
  37  * supported. See file overview for a working example.
  38  *
  39  * Side effects of calling this method include adding an "isPatternInstalled"
  40  * property and "uninstallPattern" method to this particular canvas context.
  41  * You must call uninstallPattern() before calling installPattern() again.
  42  *
  43  * @param {pattern | Array<Number>} A description of the stroke pattern. Even
  44  * indices indicate a draw and odd indices indicate a gap (in pixels). The
  45  * array should have a even length as any odd lengthed array could be expressed
  46  * as a smaller even length array.
  47  */
  48 CanvasRenderingContext2D.prototype.installPattern = function(pattern) {
  49   if (typeof(this.isPatternInstalled) !== 'undefined') {
  50     throw "Must un-install old line pattern before installing a new one.";
  51   }
  52   this.isPatternInstalled = true;
  53
  54   var dashedLineToHistory = [0, 0];
  55
  56   // list of connected line segements:
  57   // [ [x1, y1], ..., [xn, yn] ], [ [x1, y1], ..., [xn, yn] ]
  58   var segments = [];
  59
  60   // Stash away copies of the unmodified line-drawing functions.
  61   var realBeginPath = this.beginPath;
  62   var realLineTo = this.lineTo;
  63   var realMoveTo = this.moveTo;
  64   var realStroke = this.stroke;
  65
  66   this.uninstallPattern = function() {
  67     this.beginPath = realBeginPath;
  68     this.lineTo = realLineTo;
  69     this.moveTo = realMoveTo;
  70     this.stroke = realStroke;
  71     this.uninstallPattern = undefined;
  72     this.isPatternInstalled = undefined;
  73   };
  74
  75   // Keep our own copies of the line segments as they're drawn.
  76   this.beginPath = function() {
  77     segments = [];
  78     realBeginPath.call(this);
  79   };
  80   this.moveTo = function(x, y) {
  81     segments.push([[x, y]]);
  82     realMoveTo.call(this, x, y);
  83   };
  84   this.lineTo = function(x, y) {
  85     var last = segments[segments.length - 1];
  86     last.push([x, y]);
  87   };
  88
  89   this.stroke = function() {
  90     if (segments.length === 0) {
  91       // Maybe the user is drawing something other than a line.
  92       // TODO(danvk): test this case.
  93       realStroke.call(this);
  94       return;
  95     }
  96
  97     for (var i = 0; i < segments.length; i++) {
  98       var seg = segments[i];
  99       var x1 = seg[0][0], y1 = seg[0][1];
 100       for (var j = 1; j < seg.length; j++) {
 101         // Draw a dashed line from (x1, y1) - (x2, y2)
 102         var x2 = seg[j][0], y2 = seg[j][1];
 103         this.save();
 104
 105         // Calculate transformation parameters
 106         var dx = (x2-x1);
 107         var dy = (y2-y1);
 108         var len = Math.sqrt(dx*dx + dy*dy);
 109         var rot = Math.atan2(dy, dx);
 110
 111         // Set transformation
 112         this.translate(x1, y1);
 113         realMoveTo.call(this, 0, 0);
 114         this.rotate(rot);
 115
 116         // Set last pattern index we used for this pattern.
 117         var patternIndex = dashedLineToHistory[0];
 118         x = 0;
 119         while (len > x) {
 120           // Get the length of the pattern segment we are dealing with.
 121           segment = pattern[patternIndex];
 122           // If our last draw didn't complete the pattern segment all the way
 123           // we will try to finish it. Otherwise we will try to do the whole
 124           // segment.
 125           if (dashedLineToHistory[1]) {
 126             x += dashedLineToHistory[1];
 127           } else {
 128             x += segment;
 129           }
 130
 131           if (x > len) {
 132             // We were unable to complete this pattern index all the way, keep
 133             // where we are the history so our next draw continues where we
 134             // left off in the pattern.
 135             dashedLineToHistory = [patternIndex, x-len];
 136             x = len;
 137           } else {
 138             // We completed this patternIndex, we put in the history that we
 139             // are on the beginning of the next segment.
 140             dashedLineToHistory = [(patternIndex+1)%pattern.length, 0];
 141           }
 142
 143           // We do a line on a even pattern index and just move on a odd
 144           // pattern index.  The move is the empty space in the dash.
 145           if (patternIndex % 2 === 0) {
 146             realLineTo.call(this, x, 0);
 147           } else {
 148             realMoveTo.call(this, x, 0);
 149           }
 150
 151           // If we are not done, next loop process the next pattern segment, or
 152           // the first segment again if we are at the end of the pattern.
 153           patternIndex = (patternIndex+1) % pattern.length;
 154         }
 155
 156         this.restore();
 157         x1 = x2, y1 = y2;
 158       }
 159     }
 160     realStroke.call(this);
 161     segments = [];
 162   };
 163 };