How do i tint an image with HTML5 Canvas?

Question

You have compositing operations, and one of them is destination-atop. If you composite an image onto a solid color with the ‘context.globalCompositeOperation = “destination-atop”‘, it will have the alpha of the foreground image, and the color of the background image. I used this to make a fully tinted copy of an image, and then drew that fully tinted copy on top of the original at an opacity equal to the amount that I want to tint.

Here is the full code:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
    <head>
        <title>HTML5 Canvas Test</title>
        <script type="text/javascript">
var x; //drawing context
var width;
var height;
var fg;
var buffer

window.onload = function() {
    var drawingCanvas = document.getElementById('myDrawing');
    // Check the element is in the DOM and the browser supports canvas
    if(drawingCanvas && drawingCanvas.getContext) {
        // Initaliase a 2-dimensional drawing context
        x = drawingCanvas.getContext('2d');
        width = x.canvas.width;
        height = x.canvas.height;

        // grey box grid for transparency testing
        x.fillStyle="#666666";
        x.fillRect(0,0,width,height);
        x.fillStyle="#AAAAAA";
        var i,j;
        for (i=0; i<100; i++){
            for (j=0; j<100; j++){
                if ((i+j)%2==0){
                    x.fillRect(20*i,20*j,20,20);
                }
            }
        }

        fg = new Image();
        fg.src="http://uncc.ath.cx/LayerCake/images/16/3.png";

        // create offscreen buffer, 
        buffer = document.createElement('canvas');
        buffer.width = fg.width;
        buffer.height = fg.height;
        bx = buffer.getContext('2d');

        // fill offscreen buffer with the tint color
        bx.fillStyle="#FF0000"
        bx.fillRect(0,0,buffer.width,buffer.height);

        // destination atop makes a result with an alpha channel identical to fg, but with all pixels retaining their original color *as far as I can tell*
        bx.globalCompositeOperation = "destination-atop";
        bx.drawImage(fg,0,0);


        // to tint the image, draw it first
        x.drawImage(fg,0,0);

        //then set the global alpha to the amound that you want to tint it, and draw the buffer directly on top of it.
        x.globalAlpha = 0.5;
        x.drawImage(buffer,0,0);
    }
}
        </script>
    </head>
    </body>
        <canvas id="myDrawing" width="770" height="400">
            <p>Your browser doesn't support canvas.</p>
        </canvas>
    </body>
</html>

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