'Sets the sketch parameters.
Parameters
scale : float, optional
The amplitude of the wiggle perpendicular to the source
line, in pixels.  If scale is `None`, or not provided, no
sketch filter will be provided.
length : float, optional
The length of the wiggle along the line, in pixels
(default 128.0)
randomness : float, optional
The scale factor by which the length is shrunken or
expanded (default 16.0)'

def set_sketch_params(self, scale=None, length=None, randomness=None):

    if (scale is None):
        self._sketch = None
    else:
        self._sketch = (scale, (length or 128.0), (randomness or 16.0))
    self.stale = True
