HTML5 canvas基本绘图之图形组合

2017-03-27 11:01:16 html
<canvas></canvas>是HTML5中新增的标签,用于绘制图形,这篇文章主要为大家详细介绍了HTML5 canvas基本绘图之图形组合方法,感兴趣的小伙伴们可以参考一下

<canvas></canvas>只是一个绘制图形的容器,除了id、class、style等属性外,还有height和width属性。在<canvas>>元素上绘图主要有三步:

1.获取<canvas>元素对应的DOM对象,这是一个Canvas对象;
2.调用Canvas对象的getContext()方法,得到一个CanvasRenderingContext2D对象;
3.调用CanvasRenderingContext2D对象进行绘图。

图形组合:

•globalAlpha: 设置或返回绘图的当前 alpha 或透明值

该方法主要是设置图形的透明度,这里就不具体介绍。

•globalCompositeOperation: 设置或返回新图像如何绘制到已有的图像上,该方法有以下属性值:
 


下面是一个小示例,可以通过点击改变组合效果:

 

<!DOCTYPE html>  
<html lang="en">  
<head>  
    <meta charset="UTF-8">  
    <title>图形组合</title>  
    <style type="text/css">  
        #canvas{   
            border: 1px solid #1C0EFA;   
            display: block;   
            margin: 20px auto;   
        }   
        #buttons{   
            width: 1000px;   
            margin: 5px auto;   
            clear:both;   
        }   
        #buttons a{   
            font-size: 18px;   
            display: block;   
            float: left;   
            margin-left: 20px;   
        }   
    </style>  
</head>  
<body>  
    <canvas id="canvas" width="1000" height="800">  
            你的浏览器还不支持canvas   
    </canvas>  
    <div id="buttons">  
        <a href="#">source-over</a>  
        <a href="#">source-atop</a>  
        <a href="#">source-in</a>  
        <a href="#">source-out</a>  
        <a href="#">destination-over</a>  
        <a href="#">destination-atop</a>  
        <a href="#">destination-in</a>  
        <a href="#">destination-out</a>  
        <a href="#">lighter</a>  
        <a href="#">copy</a>  
        <a href="#">xor</a>  
    </div>  
</body>  
<script type="text/Javascript">  
  
window.onload = function(){   
    draw("source-over");   
  
    var buttons = document.getElementById("buttons").getElementsByTagName("a");   
    for (var i = 0; i < buttons.length; i++) {   
        buttons[i].onclick = function(){   
            draw(this.text);   
            return false;   
        };   
    }   
};   
  
    function draw(compositeStyle){   
        var canvas = document.getElementById("canvas");   
        var context = canvas.getContext("2d");   
  
        context.clearRect(0, 0, canvas.width, canvas.height);   
  
        //draw title   
        context.font = "bold 40px Arial";   
        context.textAlign = "center";   
        context.textBasedline = "middle";   
        context.fillStyle = "#150E0E";   
        context.fillText("globalCompositeOperation = "+compositeStyle, canvas.width/2, 60);   
  
        //draw a rect   
        context.fillStyle = "#F6082A";   
        context.fillRect(300, 150, 500, 500);   
  
        //draw a triangle   
        context.globalCompositeOperation = compositeStyle;   
        context.fillStyle = "#1611F5";   
        context.beginPath();   
        context.moveTo(700, 250);   
        context.lineTo(1000,750);   
        context.lineTo(400, 750);   
        context.closePath();   
        context.fill();   
    }   
  
</script>  
</html> 

读者可以点击标签来观察不同的组合效果,效果如下:

上面就是这篇文章的全部内容,希望对大家的学习和工作有所帮助