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Use js code to make web scratch cards

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PHP中文网Original
2017-06-22 14:37:044612browse

Share a piece of code for making scratch-off tickets using canvas and JS. The JS part is commented out and is less than 20 lines of code

The effect is as follows

Cover Lun.jpg


scratch.gif

HTML part
<body> <img src="img/gailun.jpg"/> <canvas id="canvas" width="400" height="300"></canvas> </body>

There is nothing to pay special attention to. Added some CSS styles for the effect

CSS part
<style type="text/css">*{margin: 0;padding: 0;
        }img{width: 400px;height: 300px;left: 200px;position: absolute;z-index: -1;
            }canvas{margin-left:200px;
            }</style>
Note

1.Added

*{margin: 0;padding: 0;
        }

2.img in order to clear the browser’s built-in effects You need to add z-index under the gray cloth;
3. Absolute positioning of the picture

js part
Analyze the logic

1. Press the mouse to move accordingly Area scraping
2. Raise the mouse to change the mouse position without continuing to scrape

js code
    <script type="text/javascript">var canvas = document.getElementById("canvas");var  context =  canvas.getContext(&#39;2d&#39;);//画蒙布
        context.beginPath();
        context.fillStyle= &#39;grey&#39;
        context.fillRect(0,0,400,300);//鼠标按下开刮
        canvas.onmousedown=function(){
            canvas.onmousemove = function(){//获取鼠标坐标var x = event.clientX;var y  = event.clientY;//destination-out             显示原来的不在后来区域的部分
                context.globalCompositeOperation = "destination-out";
                context.beginPath();
                context.arc(x-200,y,30,0,Math.PI*2);
                context.fill();        
            }
        }//鼠标抬起不刮开
        canvas.onmouseup=function(){
            canvas.onmousemove = function(){    
            }
        }    </script>

It should be noted that

1. The picture and canvas are moved 200px to the left, so the starting point coordinates of the circle are reduced by 200px relative to the acquisition position;
2.globalCompositeOperation is a function of the canvas that sets or returns how to combine a source (new ) image is drawn onto the target (existing) image, and there are 10 other writing methods

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