Re: [PyOpenGL-Users] "Attempt to retrieve context when no valid context" when calling glVertexAttribPointer in GL ES 2

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

On Tue, Jan 2, 2018 at 12:33 AM, JP LeBreton <jpl...@gm...> wrote:

> I have a program (http://vectorpoem.com/playscii) that's been running
> nicely with PyOpenGL on x86 Linux, macOS, and Windows for a few years now,
> and I'm now attempting to get it running on a Raspberry Pi 3 (using the
> Raspbian OpenGL driver). Until now this process was pretty straightforward,
> tweaking shaders and providing a render path for non-VAO hardware, but I've
> now hit a crash I don't understand and can't figure out how to debug.
>
> Deep into program initialization, I'm setting up some vertex buffers, and
> get a crash whose final line is "OpenGL.error.Error: Attempt to retrieve
> context when no valid context" when calling glVertexAttribPointer. See the
> full log here; my program first logs info about the hardware and the OpenGL
> context it creates (via PySDL2), followed by the crash:
>
> https://gist.github.com/JPLeBreton/782b92872380ee6ab3b11b3954ca56cd
>
> The OpenGL render context is definitely present; I tried logging stuff
> about it just before the glVertexAttribPointer call and as far as I can
> tell it's valid right up until that point. For what it's worth that call
> does appear to be the first (of many) in the program's execution.
>

​It's not enough for the context to be present; it must also be "current"
on the thread and window from which the GL command is issued. I'm not
especially familiar with SDL2 and its requirements, but I should think
`SDL_GL_MakeCurrent(...)` after creating your window and before calling any
GL commands would do it.

If this turns out not to be the issue, I'd suggest some well-placed
`glGetError(...)`s. I don't know if mixing PyOpenGL with PySDL2 is okay,
either—in C++, at least, SDL provides its own GL binding . . . but Mike
would know. Also your repo <https://bitbucket.org/JPLeBreton/playscii>
doesn't seem to match the error perfectly; I assume you made a few changes
since.

GL context creation is ludicrously fraught, even in the more-modern API
revisions—and I've found it to be particularly buggy on *nix graphics
drivers, especially Debian-based Linuxes. So.

Ian