javascript - Image gradient on HTML5 canvas - Stack Overflow

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I would like to obtain a radial gradient effect on an image (alpha = 1 in the middle and transparent on the edges).

Do you have any ideeas on how I could do that?

I would like to obtain a radial gradient effect on an image (alpha = 1 in the middle and transparent on the edges).

Do you have any ideeas on how I could do that?

• javascript
• gradient
• html5-canvas

Share Improve this question Follow edited Sep 30, 2011 at 11:12 pimvdb 155k8080 gold badges311311 silver badges356356 bronze badges asked Jun 21, 2011 at 11:57 gabitzishgabitzish 9,69177 gold badges4646 silver badges6767 bronze badges 2

• take a look at this post maybe it will help you: stackoverflow./questions/5525874/… – Tim Commented Jun 21, 2011 at 11:59
• I can make radial gradients on circles, but I need it on images. – gabitzish Commented Jun 21, 2011 at 12:33

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1 Answer 1

Sorted by: Reset to default 7

If I'm not mistaking what you want to do is:

1. Draw the image
2. Draw a radial gradient over it, where the borders are transparent and the middle is opaque and using the globalCompositeOperation setting on the context to blend the transparency gradient with the image.

You can rather easily translate this into code: http://jsfiddle/W8Ywp/1/.

var ctx = $('#cv').get(0).getContext('2d');

var img = new Image();

img.src = 'http://wwwstate./states/'
        + 'symb/flowers/images/oklahoma_rose.jpg';

img.onload = function() {
    ctx.drawImage(img, 0, 0, 300, 300); // Draw image

    // Create gradient, from middle to borders
    var gradient = ctx.createRadialGradient(150, 150, 0,
                                            150, 150, 150);

    // Opaque white in the middle
    gradient.addColorStop(0, 'rgba(255,255,255,0)');

    // Transparent white at the borders
    gradient.addColorStop(1, 'rgba(255,255,255,1)');

    ctx.globalCompositeOperation = 'destination-out';
    ctx.fillStyle = gradient;
    ctx.fillRect(0, 0, 300, 300); // Fill rectangle over image with the gradient
};

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