This is one of the example scripts included with Shady. These scripts can be run conventionally like any normal Python script, or you can choose to run them as interactive tutorials, for example with python -m Shady demo artifacts

#!/usr/bin/env python
# This file is part of the Shady project, a Python framework for
# real-time manipulation of psychophysical stimuli for vision science.
# Copyright (c) 2017-2020 Jeremy Hill, Scott Mooney
# Shady is free software: you can redistribute it and/or modify it
# under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# GNU General Public License for more details.
# You should have received a copy of the GNU General Public License
# along with this program. If not, see http://www.gnu.org/licenses/ .

#: Geometric transformations can lead to interpolation artifacts
This demo illustrates the artifacts that may be
created (especially in unlinearized stimuli) when a
texture stimulus is rotated, scaled or translated by
an amount that has not been appropriately rounded.

See `Shady.Documentation.PreciseControlOfLuminance`
for a section that describes when such artifacts
can occur and how to avoid them.
if __name__ == '__main__':

	import os
	import sys
	import Shady

	Wrangle command-line options:
	cmdline = Shady.WorldConstructorCommandLine()
	cmdline.Option( 'gamma', -1, type=( int, float ), min=-1, doc="Gamma-correction parameter for some of the stimuli (-1 means sRGB)." )
	adjust = cmdline.Option( 'adjust', False, type=bool, container=None, doc="Whether or not to adjust gamma of the lower stimuli using the mouse/touchscreen." )
	Shady.Require( 'numpy' ) # die with an informative error if this is missing
	Create a World:
	w = Shady.World( bg=0.5, **cmdline.opts )
	Some gamma-corrected stimuli:
	import numpy
	size = min( w.width // 3 - 100, w.height // 2 - 100 )
	noise = numpy.random.uniform( size=[ size, size ] )
	xshift = size * 1.1
	yshift = ( w.height + size ) // 6
	s1 = w.Stimulus( noise, x=-xshift, y=-yshift, atmosphere=w )
	s2 = w.Stimulus( noise, x=0,       y=-yshift, atmosphere=w )
	s3 = w.Stimulus( noise, x=+xshift, y=-yshift, atmosphere=w )
	And some definitely-uncorrected stimuli:
	s4 = w.Stimulus( noise, x=-xshift, y=+yshift, gamma=1.0 )
	s5 = w.Stimulus( noise, x=0,       y=+yshift, atmosphere=s4 )
	s6 = w.Stimulus( noise, x=+xshift, y=+yshift, atmosphere=s4  )

	Now let's make the stimuli on the left rotate very slightly
	back and forth; the ones in the middle will shrink and grow
	very slightly; and the ones on the right will get translated
	diagonally back and forth by a sub-pixel amount. All of these
	transformations will create interpolation artifacts, which may
	be very noticeable in the unlinearized stimuli.
	freq = 0.5
	s4.envelopeRotation = s1.envelopeRotation = Shady.Oscillator( freq ) * 2
	s5.envelopeScaling  = s2.envelopeScaling  = Shady.Oscillator( freq ) * 0.02 + 1.0
	s6.envelopeOrigin   = s3.envelopeOrigin   = Shady.Oscillator( freq ) * 1.0
	# Unlike ordinary "repositioning" translations (due to the .envelopeTranslation
	# and .anchor properties), the translation values in .envelopeOrigin are not
	# rounded to the nearest pixel. Therefore .envelopeOrigin allows sub-pixel
	# translations, which cause interpolation artifacts.
	@w.EventHandler( slot=1 )
	def eh( self, event ):
		if event.type == 'text' and event.text == '0': 
	if adjust:
		If the "gamma-corrected" stimuli are, in fact, not well
		gamma-corrected on your particular screen, then perhaps
		`gamma=-1` (i.e. the sRGB profile) was not the correct choice.
		You could always experiment with adjusting `w.gamma` by hand.
		Or adjust it using the mouse/touch-screen, with::
		Shady.Utilities.AdjustGamma( w )   # we don't frequently use this function
										   # which is why it is not in the top-level
										   # `Shady.` namespace. Normally we would
										   # use `Shady.FindGamma` which wraps it but
										   # which also renders a linearization pattern.
		print( """
Adjusting gamma with mouse/touchscreen. Press escape
TWICE to close: once to exit the adjustment procedure,
and once to close the window.
""" )

	Shady.AutoFinish( w )