function setCanvasOffset (canvas, offsetLeft, offsetTop) {
    //horizontal offset
    var minXOffset = -canvasSize[0]*zoom+ 164;
    var maxXOffset = window.innerWidth - 148;

    if 	(offsetLeft < minXOffset)
        canvas.style.left = minXOffset +'px';
    else if (offsetLeft > maxXOffset)
        canvas.style.left = maxXOffset +'px';
    else
        canvas.style.left = offsetLeft +'px';

    //vertical offset
    var minYOffset = -canvasSize[1]*zoom + 164;
    var maxYOffset = window.innerHeight-100;

    if 	(offsetTop < minYOffset)
        canvas.style.top = minYOffset +'px';
    else if (offsetTop > maxYOffset)
        canvas.style.top = maxYOffset +'px';
    else
        canvas.style.top = offsetTop +'px';
}