Question

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?

Answer 1

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.net/W8Ywp/1/.

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

var img = new Image();

img.src = 'http://www.netstate.com/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
};