HTML5 Canvas Overview_html5 Tutorial Tips

is a new HTML element that can be used to draw graphics using Script language (usually JavaScript). For example, you can use it to draw pictures, composite images, or do simple (and not so simple) animations. The image on the right shows some examples of implementations that we will see throughout this tutorial.

was first introduced in Apple’s Mac OS X Dashboard and then used in Safari. Browsers based on Gecko 1.8, such as Firefox 1.5, also support this new element. The element is part of the WhatWG Web applications 1.0, also known as the HTML 5 standard specification.

In this tutorial, I will try to explain how to use the element in your own web pages. The examples provided should give you a clear idea of ​​what can be done with . These examples can also serve as a starting point for your own applications of .

Before you get started
Using the element is not difficult, as long as you have basic knowledge of HTML and JavaScript.

As mentioned above, not all modern browsers support the element, so you need Firefox 1.5 or newer, or another Gecko-based browser such as Opera 9, or a recent version of Safari to see everything Example actions.

element

Let's start this tutorial by looking at the element itself.
Let's start with the definition of the element.

<canvas id="tutorial" width="150" height="150"></canvas>

This looks a lot like the element, the only difference is that it doesn't have the src and alt attributes. < ;canvas> looks very similar to , the only difference is that it does not contain the src and alt attributes. The element has only two attributes - width and height. These are both optional and can also be set using DOM properties or CSS rules. There are only two properties, width and height, both of which are optional and can be set using DOM or CSS. When no width and height attributes are specified, the canvas will initially be 300 pixels wide and 150 pixels high. If width and height attributes are not specified, the default is 300 pixels wide. , 150 pixels high . The element can be sized arbitrarily by CSS, but during rendering the image is scaled to fit its layout size. (If your renderings seem distorted, try specifying your width and height attributes explicitly in the attributes , and not with CSS.) Although the size of the canvas can be adjusted through CSS, the rendered image will be scaled to fit the layout (if you find that the rendering result looks deformed, you don’t have to rely solely on CSS. You can try to explicitly specify the width of the canvas. and height attribute value).

The id attribute isn't specific to the element but is one of default HTML attributes which can be applied to (almost) every HTML element (like class for instance). It's always a good idea to supply an id because this makes it much easier to identify it in our script.
id attribute is not Exclusive, just like the standard HTML tag, any HTML element can specify its id value. Generally, it's a good idea to assign id to elements, which makes it easier to use them in scripts.

The element can be styled just like any normal image (margin, border, background, etc). These rules however don't affect the actual drawing on the canvas. We'll see how this is done later in this tutorial. When no styling rules are applied to the canvas it will initially be fully transparent. The element can be styled like a normal image (margins, borders , background, etc.). However, these styles will not have any impact on the actual image generated by canvas. Next we'll see how to apply styles. If no style is specified, the canvas is fully transparent by default.

Alternate content

Because the element is still relatively new and isn't implemented in some browsers (such as Firefox 1.0 and Internet Explorer), we need a means of providing fallback content when a browser doesn't 't support the element.

因为 相对较新，有些浏览器并没实现，如Firefox 1.0 和 Internet Explorer，所以我们需要为那些不支持canvas的浏览器提供替用显示内容。

Luckily this is very straightforward: we just provide alternative content inside the canvas element. Browsers who don't support it will ignore the element completely and render the fallback content, others will just render the canvas normally.
For instance we could provide a text description of the canvas content or provide a static image of the dynamically rendered content. This can look something like this:

我们只需要直接在canvas元素内插入替用内容即可。不支持canvas的浏览器会忽略canvas元素而直接渲染替用内容，而支持的浏览器则会正常地渲染canvas。例如，我们可以把一些文字或图片填入canvas内，作为替用内容：

<canvas id="stockGraph" width="150" height="150">
  current stock price: $3.15 +0.15
</canvas>

<canvas id="clock" width="150" height="150">
  <img  src="/static/imghwm/default1.png"  data-src="images/clock.png"  class="lazy"    style="max-width:90%"  style="max-width:90%"/ alt="HTML5 Canvas Overview_html5 Tutorial Tips" >
</canvas>

结束标签 是必须的

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

在Apple Safari里，的实现跟很相似，它并不没有结束标签。然而，为了使 能在web的世界里广泛适用，需要给替用内容提供一个容身之所，因此，在Mozilla的实现里结束标签()是必须的。

If fallback content is not needed, a simple will be fully compatible with both Safari and Mozilla -- Safari will simply ignore the end tag.

如果没有替用内容， 对 Safari 和 Mozilla 是完全兼容的—— Safari 会简单地忽略结束标签。

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).

如果有替用内容，那么可以用一些 CSS 技巧来为并且仅为 Safari 隐藏替用内容，因为那些替用内容是需要在 IE 里显示但不需要在 Safari 里显示。

渲染上下文（Rendering Context）

creates a fixed size drawing surface that exposes one or more rendering contexts, which are used to create and manipulate the content shown. We'll focus on the 2D rendering context, which is 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 be added.

创建的固定尺寸的绘图画面开放了一个或多个渲染上下文（rendering context），我们可以通过它们来控制要显示的内容。我们专注于2D 渲染上，这也是目前唯一的选择，可能在将来会添加基于OpenGL ES 的 3D 上下文。

The is initially blank, and to display something a script first needs to access the rendering context and draw on it. The canvas element has a DOM method called getContext, used to obtain the rendering context and its drawing functions. getContext() takes one parameter, the type of context.

初始化是空白的，要在上面用脚本画图首先需要其渲染上下文（rendering context），它可以通过 canvas 元素对象的 getContext 方法来获取，同时得到的还有一些画图用的函数。getContext() 接受一个用于描述其类型的值作为参数。

var canvas = document.getElementById('tutorial');
var ctx = canvas.getContext('2d');

In the first line we retrieve the canvas DOM node using the getElementById method. We can then access the drawing context using the getContext method.

上面第一行通过 getElementById 方法取得 canvas 对象的 DOM 节点。然后通过其 getContext 方法取得其画图操作上下文。

检查浏览器的支持

The fallback content is displayed in browsers which do not support ; scripts can also check for support when they execute. This can easily be done by testing for the getContext method. Our code snippet from above becomes something like this:

除了在那些不支持  的浏览器上显示替用内容，还可以通过脚本的方式来检查浏览器是否支持 canvas 。方法很简单，判断 getContext 是否存在即可。

var canvas = document.getElementById('tutorial');
if (canvas.getContext){
  var ctx = canvas.getContext('2d');
  // drawing code here
} else {
  // canvas-unsupported code here
}

代码模板

Here is a minimalistic template, which we'll be using as a starting point for later examples. You can download this file to work with on your system.

我们会用下面这个最简化的代码模板来（后续的示例需要用到）作为开始，你可以 下载文件 到本地备用。

  
    
    
    
  
  
    <canvas id="tutorial" width="150" height="150"></canvas>
  

If you look at the script you'll see I've made a function called draw, which will get executed once the page finishes loading (via the onload attribute on the body tag). This function could also have been called from a setTimeout, setInterval, or any other event handler function just as long the page has been loaded first.

细心的你会发现我准备了一个名为 draw 的函数，它会在页面装载完毕之后执行一次（通过设置 body 标签的 onload 属性），它当然也可以在 setTimeout，setInterval，或者其他事件处理函数中被调用。

一个简单的例子

To start off, here's a simple example that draws two intersecting rectangles, one of which has alpha transparency. We'll explore how this works in more detail in later examples.

作为开始，来一个简单的吧——绘制两个交错的矩形，其中一个是有alpha透明效果。我们会在后面的示例中详细的让你了解它是如何运作的。

<html>
 <head>
  <script type="application/x-javascript">
    function draw() {
      var canvas = document.getElementById("canvas");
      if (canvas.getContext) {
        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="150" height="150"></canvas>
 </body>
</html>