Using an existing OpenGL context with GLFW

soegaard · March 12, 2022, 7:58pm

Hi All,

As you probably know Raylib uses GLFW as its backend.
I am interested in using Raylib to draw onto a canvas.
My programming language of choice (Racket) has a very nice cross platform window and event
system - so I want to use that.

My plan was to create a window containing a canvas.
Get the OpenGL context from the canvas.
Then pass the OpenGL context to Raylib.
Which then passes the OpenGL context to GLFW.

Alas, the plan stopped there. I can’t see a way to use an existing OpenGL context with Raylib.
No problem I thought, I can add that my self.

But I can’t find what I need in the GLFW documentation.

First of all - have I missed something obvious (I hope so).

It seems I am not the only one interested in use case above - the issue below has 20 upvotes.

github.com/glfw/glfw

Creating GLFW window objects with existing native handles

opened 04:27AM - 17 May 13 UTC

martinbis11

enhancement macOS Windows X11 Wayland

It's really great that we can access native data from the OpenGL context such as… glfwGetWin32Window() and glfwGetWGLContext(). It would be nice if that part of the API would be extended to wrap a window that has been created outside of GLFW into GLFW structures for OpenGL context creation, or wrap a context that was created outside of GLFW into GLFW strucutres so that we can use GLFW functions on that context (such as glfwSwapInterval()) for instance. An example of where this would useful is for screensaver development. Let's say an OpenGL cross-platform screensaver is to be developed. On Windows and Linux for instance, writing a screensaver starts by the OS giving you a window in a platform-specific way. It would be nice to be able to give that OS-specific window handle to GLFW so that from then on, we can use platform-independent GLFW calls to create the context, swap buffers, etc.

Is there some hidden technical challenge?
Or is it just a case, that none with the required expertise has run into the issue?

dougbinks · March 13, 2022, 11:47am

As you can see from the issue you linked to this is not currently possible with GLFW.

Multi platform window & context creation is non-trivial (which is why libraries like GLFW exist). For example you mention Then pass the OpenGL context to Raylib. Which then passes the OpenGL context to GLFW however more than a context is required - GLFW is a windowing library so GLFW needs the native window handles and it sets up the OS window event handling. Since Racket likely sets up it’s own OS window event handling this would cause problems, with either GLFW or Racket functionality not working (or other errors occurring).

Even if this functionality was present in GLFW, Raylib would also need to be altered. Indeed it would likely be easier to alter Raylib to use a provided OpenGL context for it’s graphics functionality, since the other aspects of GLFW are only needed for the events & windowing etc. and you don’t need those.

A more sensible approach from someone who is able to choose which APIs to use is choose one or the other. If you want to use Raylib, if possible just use that.