Re: [PyOpenGL-Users] No valid context

[Ian M. <ia...@ge...>]

On Tue, Apr 21, 2020 at 11:26 PM physkets <phy...@tu...> wrote:

> Offhand, I am surprised that `glVertexAttribPointer(...)` would be the
>> call to fail; there are plenty of other GL calls that should have failed
>> beforehand, if indeed they didn't fail. GLFW creates a context, and it
>> looks like you're setting it as current. If that code does what it looks
>> like, correctly, then the error especially wouldn't make sense. I would
>> suggest to try validating the context, perhaps creating a debug context.
>>
> Can you point me to some resources that explain how I can do that?
>
In general, Google is your friend on this, but I'll give a shoutout to the
OpenGL Wiki.

PyOpenGL conveniently does user-level OpenGL error checking
(`glGetError()`, etc.) for you, but this is separate from a debug context
per-se, which is a driver-level flag that enables additional validation
within the context itself.

I don't know how this works in Python, since if I need to do
robust/fast/reliable graphics, I do it in C++, but in that language it's as
easy as:

void __stdcall callback_err_gl(GLenum source,GLenum type,GLuint id,
GLenum severity,GLsizei length,GLchar const* message,void const* user_param)
 {

         //[Notice errors here . . .]

}

int main(int argc, char* argv[]) {

         //[...]

         glfwWindowHint(GLFW_OPENGL_DEBUG_CONTEXT, GLFW_TRUE);

         glDebugMessageCallback(callback_err_gl,nullptr);

         //[...]

         return 0;

}



This should be a reasonable guide as you search for the Python binding's
equivalent.


> What happens if you just try drawing an empty window without the
>> problematic setup code?
>>
> It works if I simply set a single colour using `glClearColor()`. There is
> no crash, and it displays a window with the right color.
>
This suggests to me that the OpenGL context is working, and that there's
some peculiarity with the `glVertexAttribPointer(...)` function itself. I
have no idea what that might be, though. It working for me on a different
platform, and other GL calls succeeding suggests a possible bug.

Also, why did you comment the context creation hints?
>>
> I did not think those ones were important. I assume it will use that
> latest version of opengl that I have. Is that not true?
>
I don't know whether it's mandated to be a particular way by the OpenGL
specification (I don't think it is, and the spec. is version-specific
anyway), but my experience has been that when you request a context, you'll
get back whatever the driver wants to give you. This is often the highest
OpenGL version your hardware supports (which is not necessarily a good
thing, mind), but it's sometimes not. The point is, you want to *know* what
you're getting. Specifying the parameters is a good way to do that.

You might also try seeing if you can reproduce the problem in RenderDoc
<https://renderdoc.org/>? (Although IDK how well it plays with Python . . .)

Ian