canvas 元素用于在网页上绘制图形
什么是 Canvas?
HTML5 的 canvas 元素使用 JavaScript 在网页上绘制图像。
画布是一个矩形区域,您可以控制其每一像素。
canvas 拥有多种绘制路径、矩形、圆形、字符以及添加图像的方法。
canvas 方法:
方法 | 用途 |
---|---|
getContext(contextId) |
公开在 canvas 上绘图需要的 API。惟一(当前)可用的 contextID 是 2d 。 |
height |
设置 canvas 的高度。默认值是 150 像素。 |
width |
设置 canvas 的宽度。默认值是 300 像素。 |
createLinearGradient(x1,y1,x2,y2) |
创建一个线性渐变。起始坐标为 x1,y1 ,结束坐标为 x2,y2 。 |
createRadialGradient(x1,y1,r1,x2,y2,r2) |
创建一个放射状渐变。圆圈的起始坐标是 x1,y1 ,半径为 r1 。圆圈的结束坐标为 x2,y2 ,半径为 r2 。 |
addColorStop(offset, color) |
向一个渐变添加一个颜色停止。颜色停止(color stop) 是渐变中颜色更改发生的位置。offset 必须介于 0 到 1 之间。 |
fillStyle |
设置用于填充一个区域的颜色 — 例如,fillStyle='rgb(255,0,0)' . |
strokeStyle |
设置用于绘制一根直线的颜色 — 例如,fillStyle='rgb(255,0,0)' . |
fillRect(x,y,w,h) |
填充一个定位于 x 和 y ,宽度和高度分别为 w 和 h 的矩形。 |
strokeRect(x,y,w,h) |
绘制一个定位于 x 和 y ,宽度和高度分别为 w 和 h 的矩形的轮廓。 |
moveTo(x,y) |
将绘图位置移动到坐标 x,y 。 |
lineTo(x,y) |
从绘图方法结束的最后位置到 x,y 绘制一条直线。 |
创建 Canvas 元素
向 HTML5 页面添加 canvas 元素。
规定元素的 id、宽度和高度:
<canvas id="myCanvas" width=800" height="800" style="border:1px solid #c3c3c3;">//width和height属性一定要和canvas一起出现,不要用id在css中实现。
Your browser does not support the canvas element.
</canvas>
通过 JavaScript 来绘制
canvas 元素本身是没有绘图能力的。所有的绘制工作必须在 JavaScript 内部完成:
<script type="text/javascript">
var c = document.getElementById("myCanvas");
var cxt = c.getContext("2d");
cxt.fillStyle = "#FF0000";
cxt.fillRect(0, 0, 150, 75);
</script>
JavaScript 使用 id 来寻找 canvas 元素:
var c = document.getElementById("myCanvas");
然后,创建 context 对象:
var cxt = c.getContext("2d");
getContext("2d") 对象是内建的 HTML5 对象,拥有多种绘制路径、矩形、圆形、字符以及添加图像的方法。
下面的两行代码绘制一个红色的矩形:
cxt.fillStyle = "#FF0000";
cxt.fillRect(0, 0, 150, 75);
fillStyle 方法将其染成红色,fillRect 方法规定了形状、位置和尺寸。
理解坐标
上面的 fillRect 方法拥有参数 (0,0,150,75)。
意思是:在画布上绘制 150x75 的矩形,从左上角开始 (0,0)。
如下图所示,画布的 X 和 Y 坐标用于在画布上对绘画进行定位。
更多 Canvas 实例
实例 - 线条
通过指定从何处开始,在何处结束,来绘制一条线:
<canvas id="myCanvas" width="200" height="200" style="border:1px solid #ff1a10;">
Your browser does not support the canvas element.
</canvas>
<script type="text/javascript">
var c=document.getElementById("myCanvas");
var cxt=c.getContext("2d");
cxt.moveTo(20,20);
cxt.lineTo(100,100);
cxt.lineTo(30,150);
cxt.stroke();
</script>
result:
理解坐标
上面的 cxt.moveTo方法拥有参数(20,20);
意思是:在画布上绘制 以(20,20)为起点。
路线依次替丁到cxt.lineTo(100,100);—— cxt.lineTo(30,150);
实例 - 圆形
通过规定尺寸、颜色和位置,来绘制一个圆:
<canvas id="myCanvas" width="200" height="200" style="border:1px solid #ff1a10;">
Your browser does not support the canvas element.
</canvas>
<script type="text/javascript">
var c=document.getElementById("myCanvas");
var cxt=c.getContext("2d");
cxt.fillStyle="#FF0000";
cxt.beginPath();
cxt.arc(100,100,50,0,Math.PI*2,true);
cxt.closePath();
cxt.fill();
</script>
result:
理解坐标
cxt.beginPath(); 开始画圆
cxt.arc(100,100,50,0,Math.PI*2,true); 原点X,原点y,半径,开始狐度数,结束狐度数,顺时针
cxt.closePath(); 画圆结束
实例 - 渐变
使用您指定的颜色来绘制渐变背景:
<canvas id="myCanvas" width="200" height="200" style="border:1px solid #ff1a10;">
Your browser does not support the canvas element.
</canvas>
<script type="text/javascript">
var c=document.getElementById("myCanvas");
var cxt=c.getContext("2d");
var grd=cxt.createLinearGradient(0,0,175,50);
grd.addColorStop(0,"#FF0000");
grd.addColorStop(0.6,"#00FF00");
grd.addColorStop(1,"#f5ff3a");
cxt.fillStyle=grd;
cxt.fillRect(10,10,175,50);
</script>
result:
理解坐标:
context.createLinearGradient(x0,y0,x1,y1);-----x0渐变开始点的 x 坐标;y0渐变开始点的 y 坐标;x1渐变结束点的 x 坐标;y1渐变结束点的 y 坐标。
实例 - 图像
把一幅图像放置到画布上:
<canvas id="myCanvas" width="200" height="200" style="border:1px solid #ff1a10;">
Your browser does not support the canvas element.
</canvas>
<script type="text/javascript">
var c = document.getElementById("myCanvas");
var cxt = c.getContext("2d");
var img = new Image()
img.src = "img/flower.png"
img.onload = function () {
cxt.drawImage(img, 50, 50);
};
cxt.drawImage(img, 50, 50);
</script>
result:
理解坐标:
img.onload = function () {
cxt.drawImage(img, 50, 50);
}; 解决图片不显示问题
笑脸火柴人实战:
<canvas id="myCanvas" width="200" height="400" style="border:1px solid #ff1a10;">
Your browser does not support the canvas element.
</canvas>
<script type="text/javascript">
var c = document.getElementById("myCanvas");
var cxt = c.getContext("2d");
// 脸的轮廓
cxt.beginPath();
cxt.fillStyle = "#F1FF32";
cxt.arc(100,100,50,0,Math.PI*2, true);
cxt.closePath();
cxt.fill();
// 嘴巴轮廓
cxt.beginPath();
cxt.strokeStyle="#c00";
cxt.arc(100,100,30,0,Math.PI,false);
// cxt.closePath(); 若加此代码,则口会封起来
cxt.lineWidth=8;
cxt.stroke();
// 眼睛轮廓
cxt.beginPath();
cxt.fillStyle="#c00";
cxt.arc(84,90,8,0,Math.PI*2,true);
cxt.closePath();
cxt.fill();
cxt.beginPath();
cxt.fillStyle="#c00";
cxt.arc(115,90,8,0,Math.PI*2,true);
cxt.closePath();
cxt.fill();
// 身体轮廓
cxt.beginPath();
cxt.lineWidth=5;
cxt.strokeStyle="000000";
cxt.moveTo(100,150);
cxt.lineTo(100,280);
cxt.moveTo(100,200);
cxt.lineTo(30,250);
cxt.moveTo(100,200);
cxt.lineTo(170,250);
cxt.moveTo(100,280);
cxt.lineTo(40,330);
cxt.moveTo(100,280);
cxt.lineTo(160,330);
cxt.stroke();
</script>
result:
![](data:image/png;base64,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)