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HTML5 canvas basic drawing graphic combination

黄舟Original: 2017-02-23 13:56:562388browse

5ba626b379994d53f7acf72a64f9b697c2caaf3fc160dd2513ce82f021917f8b is just a container for drawing graphics. In addition to attributes such as id, class, style, etc., it also has height and width attributes. There are three main steps for drawing on the 5ba626b379994d53f7acf72a64f9b697> element:

1. Get the DOM object corresponding to the 5ba626b379994d53f7acf72a64f9b697 element, which is a Canvas object;
2. Call the getContext( of the Canvas object ) method to get a CanvasRenderingContext2D object;
3. Call the CanvasRenderingContext2D object for drawing.

Graphic combination:

•globalAlpha: Set or return the current alpha or transparency value of the drawing

This method is mainly to set the transparency of the graphic, here I won’t introduce it in detail.

•globalCompositeOperation: Set or return how the new image is drawn to the existing image. This method has the following attribute values:

HTML5 canvas basic drawing graphic combination

The following is a small example, you can change the combination effect by clicking:

<!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>

Readers can click on the label to observe different combination effects, the effect is as follows:

HTML5 canvas basic drawing graphic combination

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HTML5 canvas basic drawing graphic combination

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