function thumb(img, width, height)
{
    var x_ratio = width / img.width;
    var y_ratio = height / img.height;

    var ratio = Math.min(x_ratio, y_ratio);
    var use_x_ratio = x_ratio<y_ratio ? 1 : 0;

    var w = use_x_ratio ? width : Math.ceil(img.width * ratio);
    var h = !use_x_ratio ? height : Math.ceil(img.height * ratio);

    img.width = w;
    img.height = h;
}