code: private Box makeViewPort(Length x, Length y, Length width, Length height)
     {
        float  _x = (x != null) ? x.floatValueX(this) : 0f;
        float  _y = (y != null) ? y.floatValueY(this) : 0f;
  
        Box viewPortUser = getCurrentViewPortInUserUnits();
        float  _w = (width != null) ? width.floatValueX(this) : viewPortUser.width;  // default 100%
        float  _h = (height != null) ? height.floatValueY(this) : viewPortUser.height;
  
        return new Box(_x, _y, _w, _h);
     }
docs: Derive the viewport from the x, y, width and height attributes of an object