'Return the i, j cell index at `pos` in local coordinates.'

def _cellAt(self, pos):

    if (self.image is None):
        return ((-1), (-1))
    else:
        (h, w) = self.image.shape
        (i, j) = numpy.floor([pos.y(), pos.x()])
        if ((0 <= i < h) and (0 <= j < w)):
            return (int(i), int(j))
        else:
            return ((-1), (-1))
