'**SUMMARY**
Get the ROI as a list, the values are top left x, to left y,
width and height. These values are scaled to unit values with
respect to the source image..
**RETURNS**
A list of the form [x,y,w,h]
**EXAMPLE**
>>> roi = ROI(10,10,100,100,img)
>>> print roi.toUnitXYWH()'

def toUnitXYWH(self):

    if (self.image is None):
        return None
    srcw = float(self.image.width)
    srch = float(self.image.height)
    (x, y, w, h) = self.toXYWH()
    nx = 0
    ny = 0
    if (x != 0):
        nx = (x / srcw)
    if (y != 0):
        ny = (y / srch)
    return [nx, ny, (w / srcw), (h / srch)]
