My solution to the non-rectangular StencilView

StencilMesh.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

#My solution to the non-rectangular StencilView
#Released by Tim Sheerman-Chase under CC0

from kivy.uix.widget import Widget
from kivy.graphics import Color, Rectangle, ClearColor
from kivy.graphics.vertex_instructions import Triangle, Mesh, Line
from kivy.core.image import Image
from kivy.graphics.stencil_instructions import StencilPush, StencilPop, StencilUse, StencilUnUse
from kivy.graphics.tesselator import Tesselator

class StencilMesh(Widget):
	def __init__(self, **kwargs):
		super(StencilMesh, self).__init__(**kwargs)
		
		self.canvas.after.add(StencilUnUse())
		self.canvas.after.add(StencilPop())
	
		self.rectShape = True
		self.meshes=[]

		self.bind(pos=self.update_bg)
		self.bind(size=self.update_bg)

		self.refresh_instructions()

	def update_bg(self, *args):
		if self.rectShape:
			self.update_rect_shape()

	def update_rect_shape(self):
		self.set_mesh_shape([self.pos[0], self.pos[1],
			self.pos[0]+self.size[0], self.pos[1],
			self.pos[0]+self.size[0], self.pos[1]+self.size[1],
			self.pos[0], self.pos[1]+self.size[1]])

	def refresh_instructions(self):
		self.canvas.before.clear()
		self.canvas.before.add(StencilPush())
		for vertices, indices in self.meshes:
			self.canvas.before.add(Mesh(
					vertices=vertices,
					indices=indices,
					mode="triangle_fan",
				))
		self.canvas.before.add(StencilUse())

	def set_mesh_shape(self, shape):
		tess = Tesselator()
		tess.add_contour(shape)
		if not tess.tesselate():
			print "Tesselator didn't work :("
			self.meshes = []
			return

		self.meshes = tess.meshes
		self.refresh_instructions()