Windowless OpenGL

Question

I would like to have a windowless OpenGL context (on both GNU/linux with Xorg and Windows). I'm not going to render anything but only call functions like glGetString, glCompileShader and similar.

I've done some goggling but not come up with anything useful, except creating a hidden window; which seems like a hack to me.

So does anyone have a better idea (for any platform)?

EDIT: With Xorg I was able to create and attach an OpenGL context to the root-window:

#include<stdio.h>
#include<stdlib.h>
#include<X11/X.h>
#include<X11/Xlib.h>
#include<GL/gl.h>
#include<GL/glx.h>

int main(int argc, const char* argv[]){
  Display *dpy;
  Window root;
  GLint att[] = { GLX_RGBA, GLX_DEPTH_SIZE, 24, GLX_DOUBLEBUFFER, None };
  XVisualInfo *vi;
  GLXContext glc;

  dpy = XOpenDisplay(NULL);

  if ( !dpy ) {
    printf("\n\tcannot connect to X server\n\n");
    exit(0);
  }

  root = DefaultRootWindow(dpy);
  vi = glXChooseVisual(dpy, 0, att);

  if (!vi) {
    printf("\n\tno appropriate visual found\n\n");
    exit(0);
  }

  glc = glXCreateContext(dpy, vi, NULL, GL_TRUE);
  glXMakeCurrent(dpy, root, glc);

  printf("vendor: %s\n", (const char*)glGetString(GL_VENDOR));

  return 0;
}

EDIT2: I've written a short article about windowless opengl (with sample code) based on the accepted answer.

Answer 1

Actually, it is necessary to have a window handle to create a "traditional" rendering context (the root window on X11 or the desktop window on Windows are good for this). It is used to fetch OpenGL information and extentions availability.

Once you got that information, you can destroy the render context and release the "dummy" window!

You should test for the extensions ARB_extensions_string and ARB_create_context_profile, (described in these page: ARB_create_context). Then, you can create a render context by calling CreateContextAttribs, in a platform independent way, without having a system window associated and requiring only the system device context:

        int[] mContextAttrib = new int[] {
            Wgl.CONTEXT_MAJOR_VERSION, REQUIRED_OGL_VERSION_MAJOR,
            Wgl.CONTEXT_MINOR_VERSION, REQUIRED_OGL_VERSION_MINOR,
            Wgl.CONTEXT_PROFILE_MASK, (int)(Wgl.CONTEXT_CORE_PROFILE_BIT),
            Wgl.CONTEXT_FLAGS, (int)(Wgl.CONTEXT_FORWARD_COMPATIBLE_BIT),
            0
        };


        if ((mRenderContext = Wgl.CreateContextAttribs(mDeviceContext, pSharedContext, mContextAttrib)) == IntPtr.Zero)
            throw new Exception("unable to create context");

Then, you could associate a frame buffer object or a system window to the created render context, if you wish to render (but as I understand, you want to compile only shaders).

Using CreateContextAttribs has many advantages:

• It is platform independent
• It's possible to request specific OpenGL implementation
• It's possible to request a > 3.2 OpenGL implementation
• It's possible to force the forward compatibility option (shader only rendering, that's the future way)
• It's possible to select (in a forward compatible context only) a specific OpenGL implementation profile (actually there is only the CORE profile, but there could be more in the future.
• It's possible to enable a debugging option, even if it isn't defined how this option could be used by the actual driver implementation

---

However, older hardware/drivers could not implements this extension, indeed I suggest to write a fallback code in order to create a backward compatible context.

Answer 2

Until you create a window, OpenGL has no idea what implementation you use. For example, there's a very different driver (and different hardware acceleration) for OpenGL in a remote X-Windows session vs OpenGL in an DRI X-Windows session. Shader language support might be different between these cases, and the output of the shader compiler is definitely going to be implementation-dependent, as well as any errors generated based on resource exhaustion.

So while actually creating a window may not be 100% necessary, you have to associate your context with the graphics hardware (or lack thereof) somehow, and since this can be done with a window no one bothered implementing an alternate method.

Answer 3

You need a window to host the context and you need a context to be able to do anything.

Source

If you don't want to display anything make the window invisible.

If there was another way to do this, it would be documented somewhere and easily found as it's not an uncommon problem.

Answer 4

One of the things I have done - which is admittedly a bit of a hack - to avoid the overhead of creating my own GL window - is to leverage open process windows.

The key to understanding OpenGL is this: All you needs to create a GL context with the call to wglCreateContext is a valid DC.

There's NOTHING in the documentation which says it has to be one you own.

For testing this out, I popped up Worlds Of Warcraft, and leveraging SPY++ to obtain a window handle, I then manually plugged that handle into a call to GetDC, which returns a valid Device Context, and from there, I ran the rest of my GL code like normal.

No GL window creation of my own.

Here's what happened when I did this with both Worlds of Warcraft and Star Trek Online https://universalbri.wordpress.com/2015/06/05/experiment-results

So to answer your question, YES you do need a window, but there's nothing in the documentation which states that window needs to be owned by you.

Now be advised: I couldn't get this method to provide valid visual output using the desktop window, but I was able to successfully create a DC using getDeskTopWindow API for the HWND and then a call to GetDC. So if there's non visual processing you want to use OpenGL for - let me know what you're doing, i am curious, and if you DO happen to get the GetDesktopWindow method working with the visuals - PLEASE repost on this thread what you did.

Good luck.

And don't let anyone tell you it can't be done.

When there's a will there's a way.