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POhtml5 image cropping
 

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  1. POhtml5 image cropping
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    <p>I'm using <a href="https://github.com/Phrogz/context-blender" rel="nofollow">context-blender</a> to apply a multiply effect on the first 192 pixels of the html background-image with a fixed color to achieve a transparency effect on the header of the page.</p> <p>On the html I have 2 canvas. One for the part of the image to apply the multiply effect and one for the color.</p> <p>On the javascript, after setting the color of the color-canvas and the width of both canvas to the window.innerWidth I'm getting the background image with: </p> <pre><code>imageObj.src = $('html').css('background-image').replace(/^url|[\(\)]/g, ''); </code></pre> <p>Now comes the problem. I want to draw a cropped image to the image to the image-canvas so I can apply the multiply effect. I'm trying to do the following:</p> <pre><code>imageObj.onload = function(){ // getting the background-image height var imageHeight = window.innerWidth * imageObj.height / imageObj.width; // get the corresponding pixels of the source image that correspond to the first 192 pixels of the background-image var croppedHeight = 192 * imageObj.height / imageHeight; // draw the image to the canvas imageCanvas.drawImage(imageObj, 0, 0, imageObj.width, croppedHeight, 0, 0, window.innerWidth, 192); // apply the multiply effect colorCanvas.blendOnto( imageCanvas, 'multiply'); } </code></pre> <p>But I'm doing something wrong getting the cropped height. </p> <p>Ex: For an 1536x1152 image and a 1293x679 browser container, the value I'm getting for the source cropped height is 230 but to get the correct crop I need to use something around 296.</p> <p><strong>Edit:</strong></p> <p>I'm using background-size: cover on the css to create the background-image</p> <p><strong>Edit2:</strong></p> <p>I created a <a href="http://jsfiddle.net/254ck/4/" rel="nofollow">fiddle</a> to illustrate the problem. If you uncomment the line <code>//cHeight *= magicConstant;</code> the cropped image looks a lot better but things stop making sense. I removed the multiply effect on the fiddler but that's not required to reproduce the problem. I also noticed that the behavior changed if I remove the second canvas from the URL.</p> <p>Btw, this behavior happened with google chrome, but I think the same thing happens on safari and firefox.</p>
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    1. USdcarneiro
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    1. COWould you be able to create an example on jsfiddle.net? Might help us come up with an answer for you. Also Is the behaviour (getting 230px vs 296px) consistent across multiple browsers?
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      1. USMarc Gagne
    2. CO@MarcGagne I added the example to the question
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