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POusing globalCompositeOperation with lines?
 

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  1. POusing globalCompositeOperation with lines?
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    <p>I'm trying to make a clock with HTML 5's canvas element. </p> <p>What i'm trying to do is make a line for every second, and then erase the previous line.</p> <p>I want to erase the previous line with drawing another line using the <code>globalCompositeOperation='xor';</code> but it doesn't work! </p> <p>Here is the code:</p> <pre><code> &lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt; &lt;html xmlns="http://www.w3.org/1999/xhtml"&gt; &lt;head&gt; &lt;meta http-equiv="Content-Type" content="text/html; charset=utf-8" /&gt; &lt;title&gt;Clock&lt;/title&gt; &lt;/head&gt; &lt;body onload="spin()"&gt; &lt;canvas id="myCanvas" width="578" height="400"&gt;&lt;/canvas&gt; &lt;script&gt; var firstTime = 0; var prevX=null; var prevY=null; function spin() { //get the canvas element var canvas = document.getElementById('myCanvas'); var context = canvas.getContext('2d'); //get the right angle for the clock hand var date = new Date; var seconds = date.getSeconds(); var a = seconds*6; var angleRadian = a*Math.PI/180; var angle = 1/2*Math.PI - angleRadian; if(a &gt; 360) a = 0; //Erase the previous line, if it has been drawn. if(prevX!=null) erasePrevLine(angle, canvas, context); //draw line drawLine(angle, 100, canvas, context); //repeat for the next second setTimeout(spin, 500); } function drawLine(angle, radius, canvas, context) { var centerX = canvas.width/2; var centerY = canvas.height/2; var xTarget = centerX + Math.cos(angle)*radius; var yTarget = centerY - Math.sin(angle)*radius; //save this state to be erased prevX = xTarget; prevY = yTarget; //draw context.beginPath(); context.moveTo(centerX,centerY); context.lineTo(xTarget, yTarget); context.stroke(); } function erasePrevLine(angle, canvas, context) { context.globalCompositeOperation = 'xor'; var centerX = canvas.width/2; var centerY = canvas.height/2; prevAngle = angle + (Math.PI/180*6); var xTarget = prevX; var yTarget = prevY; //draw on the previous line context.beginPath(); context.moveTo(centerX,centerY); context.lineTo(xTarget, yTarget ); context.stroke(); } &lt;/script&gt; &lt;/body&gt; &lt;/html&gt; </code></pre> <p>And here is the live example: <a href="http://jsfiddle.net/pyerT/1/" rel="nofollow">http://jsfiddle.net/pyerT/1/</a> Anybody knows the answer? it works fine with shapes and texts..</p>
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    1. COWilling to help...any code?
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