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HTML5canvas基本绘图之图形组合

时间:2021-07-01 10:21:17 帮助过:24人阅读

<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>  

        <p 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>  

        </p>  

    </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:



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