glcontext=None

if (VideoImageWidget.glcontext==None):
  print("VideoImageWidget: initializeGL: creating context for sharing")
  VideoImageWidget.glcontext=QtGui.QOpenGLContext()
  ok=VideoImageWidget.glcontext.create()
  print("VideoImageWidget: initializeGL: created context for sharing",VideoImageWidget.glcontext,ok)

context=self.context()
print("VideoImageWidget: initializeGL: automatically created context:",context)
context.setShareContext(VideoImageWidget.glcontext)
ok=context.create() # must call this ..
print("VideoImageWidget: initializeGL: recreated my context:",ok)

import sys
import time
from PyQt5 import QtWidgets, QtCore, QtGui # Qt5
from OpenGL.GL import *
from PIL import Image

"""
Demonstrating a bug (?) in QOpenGLWidget / Qt OpenGL insfrastructure :

You need:
  * to have two tiff images ("base.tif" and "2.tif") in the same directory
  * to remove/install some libraries:
  sudo apt-get install python3-pyqt5 pip3
  sudo apt-get remove python3-opengl                     # we want the most recent version of the opengl bindings
  sudo pip3 install PyOpenGL PyOpenGL_accelerate
  sudo pip3 install imutils

Usage:
  * look for the tag "TOGGLE HERE" below to switch between nested QWidgets / individual windows
  * run program with
  python3 context_test.py

What's going on here?
  * Press the button a few times : a new image appears to the first widget
  * The image should appear only to the first widget
  * .. but it appears in both widgets if we set "nested=False", i.e. when the widgets constitute individual windows
  * .. confirm this by clicking / resizing the second window after clicking the button a few times

Why this happens?
  * Qt creates some sort of "top-level" (?) opengl context that is referring to the same texture ids = bug ?


This code is licensed under the do-with-it-whatever-you-want license, written by Sampsa Riikonen, 2017
"""

def getImg(fname):
    im =QtGui.QImage(fname)
    im =im.convertToFormat(QtGui.QImage.Format_RGB888)
    ix =im.width()
    iy =im.height()
    ptr=im.bits()
    ptr.setsize(im.byteCount())
    return ptr.asstring(), ix, iy


class VideoImageWidget(QtWidgets.QOpenGLWidget): # http://doc.qt.io/qt-5/qopenglwidget.html # Qt5


  def __init__(self,parent=None):
    super().__init__(parent=parent)
    self.parent=parent

    self.baseimage, self.ix, self.iy =getImg("base.tif")

    self.gl_format=GL_RGB
    self.ratio     =1
    self.picratio  =1


  def changeTexture(self,image,ix,iy):
    glBindTexture(GL_TEXTURE_2D, self.tex) # this is the texture we will manipulate
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR)
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR)
    glTexImage2D(GL_TEXTURE_2D, 0, self.gl_format, ix, iy, 0, self.gl_format, GL_UNSIGNED_BYTE, image) # load bitmap to texture
    self.picratio=self.iy/self.ix


  def resetTexture(self):
    self.changeTexture(self.baseimage,self.ix,self.iy)


  def initializeGL(self):
    # "This function should set up any required OpenGL resources and state"
    glEnable(GL_TEXTURE_2D)
    self.tex = glGenTextures(1) # create a new texture
    # https://www.khronos.org/opengles/sdk/docs/man/xhtml/glGenTextures.xml
    # "it is guaranteed that none of the returned names was in use immediately before the call"
    print("VideoImageWidget: glGenTextures returned:",self.tex)
    self.resetTexture()


  def paintGL(self):
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)  # Clear The Screen And The Depth Buffer
    glLoadIdentity() # Reset The View
    glBindTexture(GL_TEXTURE_2D, self.tex) # this is the texture we will manipulate
    glBegin(GL_QUADS)
    dz=0
    r=self.ratio/self.picratio # screen h/w  // picture h/w
    if (r<1):   # screen wider than image
      dy=1
      dx=r
    elif (r>1): # screen taller than image
      dx=1
      dy=1/r
    else:
      dx=1
      dy=1
    glTexCoord2f(0.0, 0.0); glVertex3f(-dx, dy, dz)
    glTexCoord2f(1.0, 0.0); glVertex3f( dx, dy, dz)
    glTexCoord2f(1.0, 1.0); glVertex3f( dx,-dy, dz)
    glTexCoord2f(0.0, 1.0); glVertex3f(-dx,-dy, dz)
    glEnd()


  def resizeGL(self, width, height):
    """Called upon window resizing: reinitialize the viewport.
    """
    glViewport(0, 0, width, height)
    glLoadIdentity()
    glOrtho(-1, 1, 1, -1, -1, 1)
    self.ratio=height/width


  @QtCore.pyqtSlot(object)
  def frameReceived(self,frame):
    buf   =frame[0]
    width =frame[1]
    height=frame[2]
    print("VideoImageWidget updating with frame",width,height)
    self.changeTexture(buf,width,height)
    self.update()


class MyGui(QtWidgets.QMainWindow):

  f1 = QtCore.pyqtSignal(object)
  f2 = QtCore.pyqtSignal(object)

  def __init__(self,parent=None):
    super().__init__(parent)
    self.cw=QtWidgets.QWidget(self)
    self.setCentralWidget(self.cw)

    self.lay = QtWidgets.QVBoxLayout(self.cw)

    self.b   = QtWidgets.QPushButton("Send frame",self.cw)

    # *** TOGGLE HERE ***
    # nested=True   # *** widgets sitting in the QMainWindow
    nested=False    # *** individual windows

    self.lay.addWidget(self.b)
    if (nested):
      self.v1  = VideoImageWidget(parent=self.cw)
      self.v2  = VideoImageWidget(parent=self.cw)
      self.lay.addWidget(self.v1)
      self.lay.addWidget(self.v2)
    else:
      self.v1  = VideoImageWidget(parent=None)
      self.v2  = VideoImageWidget(parent=None)
      self.v1.show()
      self.v2.show()

    self.b.clicked. connect(self.clicked)
    self.f1.        connect(self.v1.frameReceived)

    self.newimage, self.ix, self.iy =getImg("2.tif")


  @QtCore.pyqtSlot()
  def clicked(self):
    print("emitting frame")
    self.f1.emit([self.newimage, self.ix, self.iy])  # update _only_ the first VideoImageWidget


if (__name__=="__main__"):
  app=QtWidgets.QApplication([])

  # *** Set this to apply context sharing ***
  # app.setAttribute(QtCore.Qt.AA_ShareOpenGLContexts)
  """
  .. but that does not work for textures

  See the last comment on this post:
  http://stackoverflow.com/questions/29838356/is-it-possible-to-use-the-same-opengl-context-between-top-level-windows-in-qt
  "I've realised that this does not solve the problem in the question, as it does not actually share the context, but enables sharing of a subset of resources .."

  That is said also in the qt docs, but in an extremely implicit way..
  http://doc.qt.io/qt-5/qopenglwidget.html
  "Creating extra QOpenGLContext instances that share resources like textures .. "
  """

  print("OpenGL context sharing status:",app.testAttribute(QtCore.Qt.AA_ShareOpenGLContexts))
  mg=MyGui()
  mg.show()
  app.exec_()