Drawing graphics with canvas

  • Revision slug: HTML/Canvas/Drawing_Graphics_with_Canvas
  • Revision title: Drawing Graphics with Canvas
  • Revision id: 75328
  • Created:
  • Creator: Nickolay
  • Is current revision? No
  • Comment /* See also */ link to [[HTML:Canvas]]

Revision Content

Introduction

With Firefox 1.5, Firefox includes a new HTML element for programmable graphics. <canvas> is based on the WHATWG canvas specification, which itself is based on Apple's <canvas> implemented in Safari. It can be used for rendering graphs, UI elements, and other custom graphics on the client.

<canvas> creates a fixed size drawing surface that exposes one or more rendering contexts. We'll focus on the 2D rendering context (incidentally, the only currently defined rendering context). In the future, other contexts may provide different types of rendering; for example, it is likely that a 3D context based on OpenGL ES will eventually be added to the <canvas> specification.

The 2D Rendering Context

A Simple Example

To start off, here's a simple example that draws two intersecting rectangles, one of which has alpha transparency:

Example 1.

<html>
 <head>
  <script type="application/x-javascript">
function draw() {
 var canvas = document.getElementById("canvas");
 var ctx = canvas.getContext("2d");

 ctx.fillStyle = "rgb(200,0,0)";
 ctx.fillRect (10, 10, 55, 50);

 ctx.fillStyle = "rgba(0, 0, 200, 0.5)";
 ctx.fillRect (30, 30, 55, 50);
}
  </script>
 </head>
 <body onload="draw()">
   <canvas id="canvas" width="300" height="300"></canvas>
 </body>
</html>

The draw function gets the canvas element, then obtains the 2d context. The ctx object can then be used to actually render to the canvas. The example simply fills two rectangles, by setting fillStyle to two different colors using CSS color specifications and calling fillRect. The second fillStyle uses rgba() to specify an alpha value along with the color.

The fillRect, strokeRect, and clearRect calls render a filled, outlined, or clear rectangle. To render more complex shapes, paths are used.

Using Paths

The beginPath function starts a new path, and moveTo, lineTo, arcTo, arc, and similar methods are used to add segments to the path. The path can be closed using closePath. Once a path is created, you can use fill or stroke to render the path to the canvas.

Example 2.

<html>
 <head>
  <script type="application/x-javascript">
function draw() {
  var canvas = document.getElementById("canvas");
  var ctx = canvas.getContext("2d");

  ctx.fillStyle = "red";

  ctx.beginPath();
  ctx.moveTo(30, 30);
  ctx.lineTo(150, 150);
  ctx.quadraticCurveTo(60, 70, 70, 150);
  ctx.lineTo(30, 30);
  ctx.fill();
}
   </script>
 </head>
 <body onload="draw()">
   <canvas id="canvas" width="300" height="300"></canvas>
 </body>
</html>

Calling fill() or stroke() causes the current path to be used. To be filled or stroked again, the path must be recreated.

Graphics State

Attributes of the context such as fillStyle, strokeStyle, lineWidth, and lineJoin are part of the current graphics state. The context provides two methods, save() and restore(), that can be used to move the current state to and from the state stack.

A More Complicated Example

Here's a little more complicated example, that uses paths, state, and also introduces the current transformation matrix. The context methods translate(), scale(), and rotate() all transform the current matrix. All rendered points are first transformed by this matrix.

Example 3.

 <html>
  <head>
   <script type="application/x-javascript">
 function drawBowtie(ctx, fillStyle) {
 
   ctx.fillStyle = "rgba(200,200,200,0.3)";
   ctx.fillRect(-30, -30, 60, 60);
 
   ctx.fillStyle = fillStyle;
   ctx.globalAlpha = 1.0;
   ctx.beginPath();
   ctx.moveTo(25, 25);
   ctx.lineTo(-25, -25);
   ctx.lineTo(25, -25);
   ctx.lineTo(-25, 25);
   ctx.closePath();
   ctx.fill();
 }
 
 function dot(ctx) {
   ctx.save();
   ctx.fillStyle = "black";
   ctx.fillRect(-2, -2, 4, 4);
   ctx.restore();
 }
 
 function draw() {
   var canvas = document.getElementById("canvas");
   var ctx = canvas.getContext("2d");

   // note that all other translates are relative to this
   // one
   ctx.translate(45, 45);

   ctx.save();
   //ctx.translate(0, 0); // unnecessary
   drawBowtie(ctx, "red");
   dot(ctx);
   ctx.restore();
 
   ctx.save();
   ctx.translate(85, 0);
   ctx.rotate(45 * Math.PI / 180);
   drawBowtie(ctx, "green");
   dot(ctx);
   ctx.restore();
 
   ctx.save();
   ctx.translate(0, 85);
   ctx.rotate(135 * Math.PI / 180);
   drawBowtie(ctx, "blue");
   dot(ctx);
   ctx.restore();
 
   ctx.save();
   ctx.translate(85, 85);
   ctx.rotate(90 * Math.PI / 180);
   drawBowtie(ctx, "yellow");
   dot(ctx);
   ctx.restore();
 }
    </script>
  </head>
  <body onload="draw()">
    <canvas id="canvas" width="300" height="300"></canvas>
  </body>
 </html>

This defines two methods, drawBowtie and dot, that are called 4 times. Before each call, translate() and rotate() are used to set up the current transformation matrix, which in turn positions the dot and the bowtie. dot renders a small black square centered at (0, 0). That dot is moved around by the transformation matrix. drawBowtie renders a simple bowtie path using the passed-in fill style.

As matrix operations are cumulative, save() and restore() are used around each set of calls to restore the original canvas state. One thing to watch out for is that rotation always occurs around the current origin; thus a translate() rotate() translate() sequence will yield different results than a translate() translate() rotate() series of calls.

Compatibility With Apple <canvas>

For the most part, <canvas> is compatible with Apple's and other implementations. There are, however, a few issues to be aware of, described here.

Required </canvas> tag

In the Apple Safari implementation, <canvas> is an element implemented in much the same way <img> is; it does not have an end tag. However, for <canvas> to have widespread use on the web, some facility for fallback content must be provided. Therefore, Mozilla's implementation has a required end tag.

If fallback content is not needed, a simple <canvas id="foo" ...></canvas> will be fully compatible with both Safari and Mozilla -- Safari will simply ignore the end tag.

If fallback content is desired, some CSS tricks must be employed to mask the fallback content from Safari (which should render just the canvas), and also to mask the CSS tricks themselves from IE (which should render the fallback content). Todo: get hixie to put the CSS bits in

Additional Features

Rendering Web Content Into A Canvas

This feature is only available for code running with Chrome privileges. It is not allowed in normal HTML pages.

Mozilla's canvas is extended with the drawWindow method. This method draws a snapshot of the contents of a DOM window into the canvas. For example,

ctx.drawWindow(window, 0, 0, 100, 200, "rgb(0,0,0)");

would draw the contents of the current window, in the rectangle (0,0,100,200) in pixels relative to the top-left of the viewport, on a black background, into the canvas. By specifying "rgba(0,0,0,0)" as the color, the contents would be drawn with a transparent background (which would be slower).

With this method, it is possible to fill a hidden IFRAME with arbitrary content (e.g., CSS-styled HTML text, or SVG) and draw it into a canvas. It will be scaled, rotated and so on according to the current transformation.

Ted Mielczarek's tab preview extension uses this technique in chrome to provide thumbnails of web pages, and the source is available for reference.

See also

{{ wiki.languages( { "fr": "fr/Dessiner_avec_canvas", "ja": "ja/Drawing_Graphics_with_Canvas", "pl": "pl/Rysowanie_grafik_za_pomoc\u0105_Canvas", "ko": "ko/Drawing_Graphics_with_Canvas" } ) }}

Revision Source

<p>
</p>
<h3 name="Introduction"> Introduction </h3>
<p>With <a href="en/Firefox_1.5">Firefox 1.5</a>, Firefox includes a new HTML element for programmable graphics.  <code>&lt;canvas&gt;</code> is based on the <a class="external" href="http://www.whatwg.org/specs/web-apps/current-work/#scs-dynamic">WHATWG canvas specification</a>, which itself is based on Apple's <code>&lt;canvas&gt;</code> implemented in Safari.  It can be used for rendering graphs, UI elements, and other
custom graphics on the client.
</p><p><code>&lt;canvas&gt;</code> creates a fixed size drawing surface that exposes one or more <i>rendering contexts</i>.  We'll focus on the 2D rendering context (incidentally, the only currently defined rendering context). In the future, other contexts may provide different types of rendering; for example, it is likely that a 3D context based on OpenGL ES will eventually be added to the <code>&lt;canvas&gt;</code> specification.
</p>
<h3 name="The_2D_Rendering_Context"> The 2D Rendering Context </h3>
<h4 name="A_Simple_Example"> A Simple Example </h4>
<p>To start off, here's a simple example that draws two intersecting rectangles, one of which has alpha transparency:
</p><p><img align="right" alt="Example 1." src="File:en/Media_Gallery/Canvas_ex1.png">
</p>
<pre class="eval">&lt;html&gt;
 &lt;head&gt;
  &lt;script type="application/x-javascript"&gt;
function draw() {
 var canvas = document.getElementById("canvas");
 var ctx = canvas.getContext("2d");

 ctx.fillStyle = "rgb(200,0,0)";
 ctx.fillRect (10, 10, 55, 50);

 ctx.fillStyle = "rgba(0, 0, 200, 0.5)";
 ctx.fillRect (30, 30, 55, 50);
}
  &lt;/script&gt;
 &lt;/head&gt;
 &lt;body onload="draw()"&gt;
   &lt;canvas id="canvas" width="300" height="300"&gt;&lt;/canvas&gt;
 &lt;/body&gt;
&lt;/html&gt;
</pre>
<p>The <code>draw</code> function gets the <code>canvas</code> element, then
obtains the <code>2d</code> context.  The <code>ctx</code> object can then be
used to actually render to the canvas.  The example simply fills two
rectangles, by setting fillStyle to two different colors using CSS
color specifications and calling <code>fillRect</code>.  The second
fillStyle uses <code>rgba()</code> to specify an alpha value along with
the color.
</p><p>The <code>fillRect</code>, <code>strokeRect</code>, and <code>clearRect</code> calls render a filled, outlined, or clear rectangle.  To render more complex
shapes, paths are used.
</p>
<h4 name="Using_Paths"> Using Paths </h4>
<p>The <code>beginPath</code> function starts a new path, and
<code>moveTo</code>, <code>lineTo</code>, <code>arcTo</code>, <code>arc</code>, and similar methods are used to add segments to the path.  The path can be
closed using <code>closePath</code>.  Once a path is created, you
can use <code>fill</code> or <code>stroke</code> to render the path
to the canvas.
</p><p><img align="right" alt="Example 2." src="File:en/Media_Gallery/Canvas_ex2.png">
</p>
<pre class="eval">&lt;html&gt;
 &lt;head&gt;
  &lt;script type="application/x-javascript"&gt;
function draw() {
  var canvas = document.getElementById("canvas");
  var ctx = canvas.getContext("2d");

  ctx.fillStyle = "red";

  ctx.beginPath();
  ctx.moveTo(30, 30);
  ctx.lineTo(150, 150);
  ctx.quadraticCurveTo(60, 70, 70, 150);
  ctx.lineTo(30, 30);
  ctx.fill();
}
   &lt;/script&gt;
 &lt;/head&gt;
 &lt;body onload="draw()"&gt;
   &lt;canvas id="canvas" width="300" height="300"&gt;&lt;/canvas&gt;
 &lt;/body&gt;
&lt;/html&gt;
</pre>
<p>Calling <code>fill()</code> or <code>stroke()</code> causes the current path
to be used.  To be filled or stroked again, the path must be recreated.
</p>
<h4 name="Graphics_State"> Graphics State </h4>
<p>Attributes of the context such as <code>fillStyle</code>,
<code>strokeStyle</code>, <code>lineWidth</code>, and <code>lineJoin</code> are
part of the current <i>graphics state</i>.  The context provides two
methods, <code>save()</code> and <code>restore()</code>, that can be used to
move the current state to and from the state stack.
</p>
<h4 name="A_More_Complicated_Example"> A More Complicated Example </h4>
<p>Here's a little more complicated example, that uses paths, state, and
also introduces the current transformation matrix.  The context
methods <code>translate()</code>, <code>scale()</code>, and <code>rotate()</code>
all transform the current matrix.  All rendered points are first
transformed by this matrix.
</p><p><img align="right" alt="Example 3." src="File:en/Media_Gallery/Canvas_ex3.png">
</p>
<pre> &lt;html&gt;
  &lt;head&gt;
   &lt;script type="application/x-javascript"&gt;
 function drawBowtie(ctx, fillStyle) {
 
   ctx.fillStyle = "rgba(200,200,200,0.3)";
   ctx.fillRect(-30, -30, 60, 60);
 
   ctx.fillStyle = fillStyle;
   ctx.globalAlpha = 1.0;
   ctx.beginPath();
   ctx.moveTo(25, 25);
   ctx.lineTo(-25, -25);
   ctx.lineTo(25, -25);
   ctx.lineTo(-25, 25);
   ctx.closePath();
   ctx.fill();
 }
 
 function dot(ctx) {
   ctx.save();
   ctx.fillStyle = "black";
   ctx.fillRect(-2, -2, 4, 4);
   ctx.restore();
 }
 
 function draw() {
   var canvas = document.getElementById("canvas");
   var ctx = canvas.getContext("2d");

   // note that all other translates are relative to this
   // one
   ctx.translate(45, 45);

   ctx.save();
   //ctx.translate(0, 0); // unnecessary
   drawBowtie(ctx, "red");
   dot(ctx);
   ctx.restore();
 
   ctx.save();
   ctx.translate(85, 0);
   ctx.rotate(45 * Math.PI / 180);
   drawBowtie(ctx, "green");
   dot(ctx);
   ctx.restore();
 
   ctx.save();
   ctx.translate(0, 85);
   ctx.rotate(135 * Math.PI / 180);
   drawBowtie(ctx, "blue");
   dot(ctx);
   ctx.restore();
 
   ctx.save();
   ctx.translate(85, 85);
   ctx.rotate(90 * Math.PI / 180);
   drawBowtie(ctx, "yellow");
   dot(ctx);
   ctx.restore();
 }
    &lt;/script&gt;
  &lt;/head&gt;
  &lt;body onload="draw()"&gt;
    &lt;canvas id="canvas" width="300" height="300"&gt;&lt;/canvas&gt;
  &lt;/body&gt;
 &lt;/html&gt;
</pre>
<p>This defines two methods, <code>drawBowtie</code> and <code>dot</code>, that
are called 4 times.  Before each call, <code>translate()</code> and
<code>rotate()</code> are used to set up the current transformation
matrix, which in turn positions the dot and the bowtie.  <code>dot</code>
renders a small black square centered at <code>(0, 0)</code>.  That dot is
moved around by the transformation matrix.  <code>drawBowtie</code>
renders a simple bowtie path using the passed-in fill style.
</p><p>As matrix operations are cumulative, <code>save()</code> and
<code>restore()</code> are used around each set of calls to restore the
original canvas state.  One thing to watch out for is that rotation
always occurs around the current origin; thus a <code>translate()
rotate() translate()</code> sequence will yield different results than a
<code>translate() translate() rotate()</code> series of calls.
</p>
<h3 name="Compatibility_With_Apple_.3Ccanvas.3E"> Compatibility With Apple &lt;canvas&gt; </h3>
<p>For the most part, <code>&lt;canvas&gt;</code> is compatible with Apple's and other implementations.  There are, however, a few issues to be aware of, described here.
</p>
<h4 name="Required_.3C.2Fcanvas.3E_tag"> Required <code>&lt;/canvas&gt;</code> tag </h4>
<p>In the Apple Safari implementation, <code>&lt;canvas&gt;</code> is an element implemented in much the same way <code>&lt;img&gt;</code> is; it does not have an end tag.  However, for <code>&lt;canvas&gt;</code> to have widespread use on the web, some facility for fallback content must be provided.  Therefore, Mozilla's implementation has a <i>required</i> end tag.
</p><p>If fallback content is not needed, a simple <code>&lt;canvas id="foo" ...&gt;&lt;/canvas&gt;</code> will be fully compatible with both Safari and Mozilla -- Safari will simply ignore the end tag.
</p><p>If fallback content is desired, some CSS tricks must be employed to mask the fallback content from Safari (which should render just the canvas), and also to mask the CSS tricks themselves from IE (which should render the fallback content).  <b>Todo: get hixie to put the CSS bits in</b>
</p>
<h3 name="Additional_Features"> Additional Features </h3>
<h4 name="Rendering_Web_Content_Into_A_Canvas"> Rendering Web Content Into A Canvas </h4>
<div class="note">This feature is only available for code running with Chrome privileges. It is not allowed in normal HTML pages.</div>
<p>Mozilla's <code>canvas</code> is extended with the <code>drawWindow</code>
method. This method draws a snapshot of the contents of a DOM <code>window</code> into the canvas. For example,
</p>
<pre class="eval">ctx.drawWindow(window, 0, 0, 100, 200, "rgb(0,0,0)");
</pre>
<p>would draw the contents of the current window, in the rectangle (0,0,100,200)
in pixels relative to the top-left of the viewport, on a black background,
into the canvas. By specifying "rgba(0,0,0,0)" as the color, the
contents would be drawn with a transparent background (which would be slower).
</p><p>With this method, it is possible to fill a hidden IFRAME with arbitrary content (e.g., CSS-styled HTML text, or SVG) and draw it into a canvas. It will be scaled, rotated and so on according to the current transformation.
</p><p>Ted Mielczarek's <a class="external" href="http://ted.mielczarek.org/code/mozilla/tabpreview/">tab preview</a> extension uses this technique in chrome to provide thumbnails of web pages, and the source is available for reference.
</p>
<h3 name="See_also"> See also </h3>
<ul><li> <a href="en/HTML/Canvas">Canvas topic page</a>
</li><li> <a href="en/Canvas_tutorial">Canvas tutorial</a>
</li><li> <a class="external" href="http://www.whatwg.org/specs/web-apps/current-work/#scs-dynamic">WHATWG specification</a>
</li><li> <a class="external" href="http://developer.apple.com/documentation/AppleApplications/Reference/SafariJSRef/Classes/Canvas.html">Apple Canvas Documentation</a>
</li><li> <a class="external" href="http://weblogs.mozillazine.org/roc/archives/2005/05/rendering_web_p.html">Rendering Web Page Thumbnails</a>
</li><li> Some <a href="Special:Tags?tag=Canvas_examples&amp;language=en">examples</a>:
<ul><li> <a href="en/A_Basic_RayCaster">A Basic RayCaster</a>
</li><li> <a class="external" href="http://awordlike.com/">The Lightweight Visual Thesaurus</a>
</li><li> <a class="external" href="http://caimansys.com/painter/">Canvas Painter</a>
</li></ul>
</li><li> <a href="Special:Tags?tag=HTML:Canvas&amp;language=en">And more...</a>
</li></ul>
{{ wiki.languages( { "fr": "fr/Dessiner_avec_canvas", "ja": "ja/Drawing_Graphics_with_Canvas", "pl": "pl/Rysowanie_grafik_za_pomoc\u0105_Canvas", "ko": "ko/Drawing_Graphics_with_Canvas" } ) }}
Revert to this revision