[OpenLayers-Users] Changing brightness of the map image

[Lubos and Alena Pochman]

I have Map with WMS (geoserver) base layer and I would like to change
the brightness of the map viewport.
I found great pixel library http://www.pixastic.com/lib/
and DEV demo http://dev.openlayers.org/sandbox/camptocamp/canvas/openlayers/examples/processingCanvas.html,
which works great.

Unfortunately I use OL 2.12, and the functions from the demo are not
available even in the latest devel on Github.

Is there a way in OL 2.12 to modify the image of the viewport (tile),
and replace it?

I came with the following code, to get the canvas of the image (note,
I know it will work only for untiled image):

        var gridRows = currLayer.grid[0];
        var img = gridRows[0].getImage();
        var context = gridRows[0].getCanvasContext();
        if (context) {
          var newimg = Pixastic.process(img, "brightness", {brightness
: brightness, contrast : contrast, legacy : false});
        }

but I am getting cross-origin error: canvas context getImageData() -
SecurityError: The operation is insecure.

How do I avoid the cross-origin error?
Will my approach modify the viewport/tile image brightness if I
overcome the cross-origin error?

Thanks for any help/hints, Lubos Pochman