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POTransparency lost with getImageData - HTML5 2d Context
 

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    <p>I noticed a strange problem with <code>getImageData</code>; transparency of the image seems to be ignored when the image data is fetched. </p> <p>Since any image needs to be drawn on to a <code>canvas</code> before its image data can be obtained, I assumed this was a problem with the <code>canvas</code> in question being opaque. But I was wrong, since using the <code>canvas</code> as an argument in <code>drawImage</code> maintains transparency.</p> <p>Here is how I loaded the image;</p> <pre><code>var load_image = function(name, url, holder, callback) { var img = new Image(); img.src = url; img.addEventListener('load', function(e) { var canvas = make_canvas(e.target.width, e.target.height); var ctx = canvas.getContext('2d'); ctx.clearRect(0, 0, canvas.width, canvas.height); ctx.drawImage(e.target, 0, 0); holder[name] = {canvas: canvas, ctx: ctx}; delete e.target; callback.call(); }, false); }; </code></pre> <p>The <code>callback</code> is simply the draw function, which invokes <code>draw_image</code> to draw the image.</p> <p>The usual version;</p> <pre><code>var draw_image = function(ctx, img, sx, sy, w, h, dx, dy) { ctx.drawImage(img.canvas, sx, sy, w, h, dx, dy, w, h); }; </code></pre> <p>It simply takes the canvas as an argument for <code>drawImage</code>, and the result is as intended with transparency maintained. <strong><a href="http://jsfiddle.net/MnG7a/3/" rel="nofollow">Example</a></strong>.</p> <p>The image data version;</p> <pre><code>var draw_image = function(ctx, img, sx, sy, w, h, dx, dy) { var imagedata = img.ctx.getImageData(sx, sy, w, h); ctx.putImageData(imagedata, dx, dy); }; </code></pre> <p>This one obtains the image data of the required rectangle from the same canvas as the one used in the usual version, and puts the image data on the canvas I want to draw to. I believe transparency should be maintained, but this is not so. <strong><a href="http://dl.dropbox.com/u/80133449/imagedataTest/index.html" rel="nofollow">Example</a></strong>. (This is a Dropbox link because of the <code>origin-clean</code> flag.)</p> <p>Am I wrong in assuming that transparency should be maintained with <code>getImageData</code>? Or am I just using it in a wrong manner?</p> <p>Either way, help would really be appreciated.</p>
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