How to generate mask image and use the mask image as the ControlNetUnit class parameter?

[derekcbr]

class ControlNetUnit:
def init(
self,
input_image: Image = None,
mask: Image = None,
....
)
How to generate mask image and use the mask image as the ControlNetUnit class parameter?
r = api.controlnet_detect(images=[img], module='canny')
r.image
Is using detect function right to generate the mask image?