Demonstration of a condition where a Fbo can draw outside the StencilView it's in

texture_stack_plane_bug_demo.py

from kivy.core.image import Image
from kivy.graphics.fbo import Fbo
from kivy.properties import ObjectProperty, BooleanProperty, ListProperty
from kivy.graphics import (
	InstructionGroup,
	Translate,
	Rectangle,
)
from kivy.clock import Clock, mainthread
from kivy.uix.widget import Widget
from kivy.logger import Logger


BACKGROUND = True  # change this to False, and the bug doesn't happen
DEFAULT_IMAGE_PATH = "kivy/examples/canvas/mtexture2.png"
TEST_DATA = [{"name": (x, y), "x": x * 0.04083333333333333, "y": y * 0.04083333333333333, "width": 32, "height": 32, "textures": [DEFAULT_IMAGE_PATH]} for x in range(100) for y in range(100)]


class TextureStackPlane(Widget):
	data = ListProperty()

	def __init__(self, **kwargs):
		super().__init__(**kwargs)
		self._trigger_redraw = Clock.create_trigger(self.redraw)
		self._redraw_bind_uid = self.fbind("data", self._trigger_redraw)

	def on_parent(self, *_):
		if not self.canvas:
			Clock.schedule_once(self.on_parent, 0)
			return
		with self.canvas:
			self._fbo = Fbo(size=self.size)
			self._translate = Translate(x=self.x, y=self.y)
			self._rectangle = Rectangle(
				size=self.size, texture=self._fbo.texture
			)
		self.bind(pos=self._trigger_redraw, size=self._trigger_redraw)
		self._trigger_redraw()

	def on_pos(self, *_):
		if not hasattr(self, "_translate"):
			return
		self._translate.x, self._translate.y = self.pos
		self.canvas.ask_update()

	def on_size(self, *_):
		if not hasattr(self, "_rectangle") or not hasattr(self, "_fbo"):
			return
		self._rectangle.size = self._fbo.size = self.size
		self.redraw()

	@mainthread
	def _redraw_upd_fbo(self, changed_instructions):
		# If I move this code into redraw(), the bug doesn't happen.
		# Might be timing related?
		# If I just remove the @mainthread decorator here, the bug happens *for an instant*
		# and then goes away, I think on the next frame.
		# These changes don't have any effect on the real app this is taken from, though.
		# I think it's because the real app has a bunch of other graphics going on in other Fbos
		# that are also in the same StencilView.
		fbo = self._fbo
		for insts in changed_instructions:
			group = insts["group"]
			group.clear()
			for rect in insts["rectangles"]:
				group.add(rect)
			if "color0" in insts:
				group.add(insts["color0"])
				group.add(insts["line"])
				group.add(insts["color1"])
			if group not in fbo.children:
				fbo.add(group)
		self._rectangle.texture = fbo.texture

	def redraw(self, *_):
		def get_rects(datum):
			width = datum.get("width", 0)
			height = datum.get("height", 0)
			x = datum["x"] * self_width
			y = datum["y"] * self_height
			rects = []
			for source in datum["textures"]:
				texture = Image.load(source).texture
				rects.append(
					Rectangle(pos=(x, y), size=(width, height), texture=texture)
				)
			return rects

		if not hasattr(self, "_rectangle"):
			self._trigger_redraw()
			return
		instructions = {}
		self_width = self.width
		self_height = self.height
		todo = []
		for datum in self.data:
			name = datum["name"]
			rects = get_rects(datum)
			grp = InstructionGroup()
			instructions[name] = insts = {
				"rectangles": rects,
				"group": grp,
			}
			todo.append(insts)
		self._fbo.bind()
		self._fbo.clear_buffer()
		self._fbo.release()
		self._redraw_upd_fbo(todo)


# The following is a demonstration of a graphical error involving TextureStackPlane
# and its interaction with StencilView.
# When BACKGROUND is True, TextureStackPlane overflows the StencilView it's in,
# even though the background image doesn't.
# When BACKGROUND is False, TextureStackPlane obeys the StencilView.
# The bug doesn't seem to come up when the graphics within TextureStackPlane
# have whole-number coordinates.

if __name__ == "__main__":
	import os
	import json
	from kivy.uix.boxlayout import BoxLayout
	from kivy.uix.floatlayout import FloatLayout
	from kivy.uix.image import Image as ImageWidget
	from kivy.uix.widget import Widget
	from kivy.uix.stencilview import StencilView
	from kivy.base import runTouchApp

	root = BoxLayout()
	root.add_widget(Widget())
	texstac = TextureStackPlane(data=TEST_DATA, size_hint=(None, None), size=(1024,1024))
	flot = FloatLayout()
	if BACKGROUND:
		texstacbg = ImageWidget(size_hint=(None, None), size=(1024,1024))
		flot.add_widget(texstacbg)
	flot.add_widget(texstac)
	stenc = StencilView()
	stenc.add_widget(flot)
	root.add_widget(stenc)
	root.add_widget(Widget())
	runTouchApp(root)