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

[Matti K. <mat...@gm...>]

Consider using Mesa's software renderer, if your requirements allow it:

$ sudo apt-get install python-opengl libosmesa # maybe libosmesa6?
$ PYOPENGL_PLATFORM=osmesa python your_app.py
$ cat your_app.py
...
   from OpenGL import GL, arrays, platform
   ctx = platform.OSMesaCreateContext(GL.GL_RGBA, None) # requires PYOPENGL_PLATFORM=osmesa
   buf = arrays.GLubyteArray.zeros((height, width, 4))
   p = arrays.ArrayDatatype.dataPointer(buf)
   assert(platform.OSMesaMakeCurrent(ctx, p, GL.GL_UNSIGNED_BYTE, width, height))
   assert(platform.CurrentContextIsValid())
...
   platform.OSMesaDestroyContext(ctx)
...

I understand that you're on Windows. There may be an easy way to install 
Mesa for use with PyOpenGL on your setup, but I'm afraid I'm not 
familiar with the Windows platform so you'll have to do some digging first.

On 9/2/17 2:31 PM, Anton Ovsyannikov wrote:
> 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 
> <http://www.lfd.uci.edu/%7Egohlke/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
>
>
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