create a bitmap mask from a shapely polygon

polygon_mask.py

import numpy
import cairo

def polygon_mask(polygon):
	assert polygon.geom_type == 'Polygon'

	minx, miny, maxx, maxy = polygon.bounds
	minx, miny = numpy.floor([minx, miny]).astype(numpy.int32)
	maxx, maxy = numpy.ceil([maxx, maxy]).astype(numpy.int32)

	w = int(maxx - minx)
	h = int(maxy - miny)

	pts = list(polygon.exterior.coords) - numpy.array([minx, miny])

	with cairo.ImageSurface(cairo.FORMAT_ARGB32, w, h) as surface:
		ctx = cairo.Context(surface)
		ctx.move_to(*pts[0])
		for p in pts[1:]:
			ctx.line_to(*p)
		ctx.set_source_rgb(1, 1, 1)
		ctx.fill()

		surface.finish()

		data = numpy.ndarray(
			shape=(surface.get_height(), surface.get_stride() // 4),
			dtype=numpy.uint32,
			buffer=surface.get_data())

		data = numpy.right_shift(data, 24).astype(numpy.uint8)

	return data > 0, (minx, miny, w, h)