c++ - 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("
cannot connect to X server

");
    exit(0);
  }

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

  if (!vi) {
    printf("
no appropriate visual found

");
    exit(0);
  }

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

  printf("vendor: %s
", (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.