[PyOpenGL-Users] Bug? Importing WGL crashes glReadPixel call with "wrong argument type" exception

[Anton O. <ant...@gm...>]

Hello.

I am trying to use PyOpengGL for off-screen rendering, so I need WGL to
create gl context and then glReadPixels to retrieve data

First I wrote simple app with GLUT, but right after I import OpenGL.WGL
(just import, no more), the glReadPixels starts to crash with

ctypes.ArgumentError: argument 7: <type 'exceptions.TypeError'>: wrong type

The app, OpenGL version and stack trace are below. Now some of my
investigations

=======================================================
The crash starts in \OpenGL\GL\images.py line 369

GL_1_1.glReadPixels(
    x,y,width,height,
    format,type,
    imageData
)

imageData in both cases is completely the same

The only difference in further workflow happens in
\OpenGL\platform\baseplatform.py line 125

def finalArgType( self, typ ):
    """Retrieve a final type for arg-type"""
    if typ == ctypes.POINTER( None ) and not getattr( typ, 'final',False):
        from OpenGL.arrays import ArrayDatatype
        return ArrayDatatype
    else:
        return typ

With no WGL the 7th argument (while typ is <class 'ctypes.c_void_p'>) have
no "final" attribute set, so final type becomes ArrayDatatype and
everything works fine

But if to import WGL "final" flag is set, so <class 'ctypes.c_void_p'>
returned leading to "wrong type" exception in GL_1_1.glReadPixels call

I searched for "final" in WGL and found the following in
\OpenGL\raw\WGL\_types.py line 72

HANDLE = POINTER(None) # /home/mcfletch/pylive/OpenGL-ctypes/src/wgl.h:60
# TODO: figure out how to make the handle not appear as a void_p within the
code...
HANDLE.final = True

So we found who set up "final" flag for <class 'ctypes.c_void_p'>

But then I stuck up completely, have no single idea how to solve this type
conflict, suppose it's some issue in design

Have any idea how to solve it?


=======================================================
PyOpenGL package is
http://www.lfd.uci.edu/~gohlke/pythonlibs/#pyopengl
PyOpenGL‑3.1.1‑cp27‑cp27m‑win_amd64.whl

OpenGL.__version__
3.1.1
=======================================================

=======================================================
The App
=======================================================
# -*- coding: utf-8 -*-

# Just uncomment following line to reproduce the bug!
#import OpenGL.WGL

from OpenGL.GL import *
from OpenGL.GLU import *
from OpenGL.GLUT import *

from PIL import Image
import sys

width, height = 300, 300

def init():
    glClearColor(0.5, 0.5, 0.5, 1.0)
    glColor(0.0, 1.0, 0.0)
    gluOrtho2D(-1.0, 1.0, -1.0, 1.0)
    glViewport(0, 0, width, height)

def render():

    glClear(GL_COLOR_BUFFER_BIT)

    glBegin(GL_LINES)
    glVertex2d(-0.5, -0.5)
    glVertex2d(0.5, 0.5)
    glEnd()

    glFlush()

def draw():
    render()
    glutSwapBuffers()

def main():
    glutInit(sys.argv)

    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB)
    glutInitWindowSize(300, 300)
    glutCreateWindow(b"OpenGL vs WGL")

    init()
    render()

    glPixelStorei(GL_PACK_ALIGNMENT, 1)
    data = glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE)
    image = Image.frombytes("RGBA", (width, height), data)
    image.save('output.png', 'PNG')

    glutDisplayFunc(draw)
    glutMainLoop()

main()
=======================================================


=======================================================
Stack trace with import WGL uncommented
=======================================================

Traceback (most recent call last):
  File "C:/Users/Anton/Documents/Dev/PycharmProjects/OpenGL/OpenGLTest.py",
line 54, in <module>
    main()
  File "C:/Users/Anton/Documents/Dev/PycharmProjects/OpenGL/OpenGLTest.py",
line 47, in main
    data = glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE)
  File "C:\ProgramData\Anaconda2\lib\site-packages\OpenGL\GL\images.py",
line 372, in glReadPixels
    imageData
  File
"C:\ProgramData\Anaconda2\lib\site-packages\OpenGL\platform\baseplatform.py",
line 402, in __call__
    return self( *args, **named )
ctypes.ArgumentError: argument 7: <type 'exceptions.TypeError'>: wrong type