[RESOLVED] putImageData always blank after pixel-by-pixel processing

[vinylcyril]

Hello all.
I'm quite new to HTML5, so i guess i'm just doing something wrong. What exactly is wrong is beyond me
So, i have a canvas element with an image already put onto it by the "drawImage" function.
Then i implement this seemingly easy piece of code:

function imgInvert(canvas_ID)
{
    var imgD = document.getElementById(canvas_ID).getContext("2d").createImageData
                (
                    document.getElementById(canvas_ID).width, 
                    document.getElementById(canvas_ID).height
                );
                
    for (var i = 0; i < imgD.data.length; i += 4) 
    {
        imgD.data[i  ] = 255 - imgD.data[i  ]; // red
        imgD.data[i+1] = 255 - imgD.data[i+1]; // green
        imgD.data[i+2] = 255 - imgD.data[i+2]; // blue
        // leaving the alpha channel as it is
    }
    
    document.getElementById(canvas_ID).getContext("2d").putImageData(imgD, 0, 0);
} 

and after the "putImageData" method is implemented, i'm left with a blank canvas instead of at least something.

Moreover, this part of code is basically ripped off this tutorial.
Somehow, there it works. On my page, it doesn't so far.

Any help will be much appreciated!