1991-2006
Silicon Graphics, Inc.
glXIntro
3G

# glXIntro

Introduction to OpenGL in the X window system

## Overview

OpenGL (called GL in other pages) is a high-performance 3D-oriented renderer. It is available in the X window system through the GLX extension. To determine whether the GLX extension is supported by an X server, and if so, what version is supported, call glXQueryExtension and glXQueryVersion.

GLX extended X servers make a subset of their visuals available for OpenGL rendering. Drawables created with these visual can also be rendered into using the core X renderer and or any other X extension that is compatible with all core X visuals.

GLX extends a drawable's standard color buffer with additional buffers. These buffers include back and auxiliary color buffers, a depth buffer, a stencil buffer, and a color accumulation buffer. Some or all of the buffers listed are included in each X visual that supports OpenGL.

GLX supports rendering into three types of drawables: windows, pixmaps, and pbuffers (pixel buffers). GLX windows and pixmaps are X resources, and capable of accepting core X rendering as well as OpenGL rendering. GLX-pbuffers are GLX only resources and might not accept core X rendering.

To render using OpenGL into a GLX drawable, you must determine the appropriate GLXFBConfig that supports the rendering features your application requires. glXChooseFBConfig returns a GLXFBConfig matching the required attributes or `NULL` if no match is found. A complete list of GLXFBConfigs supported by a server can be obtained by calling glXGetFBConfigs. Attributes of a particular GLXFBConfig can be queried by calling glXGetFBConfigAttrib.

For GLX windows and pixmaps, a suitable X drawable (using either `XCreateWindow` or `XCreatePixmap`, respectively) with a matching visual must be created first. Call glXGetVisualFromFBConfig to obtain the necessary XVisualInfo structure for creating the X drawable. For pbuffers, no underlying X drawable is required.

To create a GLX window from an X window, call glXCreateWindow. Likewise, to create a GLX pixmap, call glXCreatePixmap. Pbuffers are created by calling glXCreatePbuffer. Use glXDestroyWindow, glXDestroyPixmap, and glXDestroyPbuffer to release previously allocated resources.

A GLX context is required to bind OpenGL rendering to a GLX resource. A GLX resource and rendering context must have compatible GLXFBConfigs. To create a GLX context, call glXCreateNewContext. A context may be bound to a GLX drawable by using glXMakeContextCurrent. This context/drawable pair becomes the current context and current drawable, and is used by all OpenGL rendering commands until glXMakeContextCurrent is called with different arguments.

Both core X and OpenGL commands can be used to operate on drawables; however, the X and OpenGL command streams are not synchronized. Synchronization can be explicitly specified using by calling glXWaitGL, glXWaitX, `XSync`, and `XFlush`.

## Examples

Below is a minimal example of creating an RGBA-format X window that's compatible with OpenGL using GLX 1.3 commands. The window is cleared to yellow when the program runs. The program does minimal error checking; all return values should be checked.

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

    int singleBufferAttributess[] = {
        GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT,
        GLX_RENDER_TYPE,   GLX_RGBA_BIT,
        GLX_RED_SIZE,      1,   /* Request a single buffered color buffer */
        GLX_GREEN_SIZE,    1,   /* with the maximum number of color bits  */
        GLX_BLUE_SIZE,     1,   /* for each component                     */
        None
    };

    int doubleBufferAttributes[] = {
        GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT,
        GLX_RENDER_TYPE,   GLX_RGBA_BIT,
        GLX_DOUBLEBUFFER,  True,  /* Request a double-buffered color buffer with */
        GLX_RED_SIZE,      1,     /* the maximum number of bits per component    */
        GLX_GREEN_SIZE,    1,
        GLX_BLUE_SIZE,     1,
        None
    };

    static Bool WaitForNotify( Display *dpy, XEvent *event, XPointer arg ) {
        return (event->type == MapNotify) && (event->xmap.window == (Window) arg);
    }
    int main( int argc, char *argv[] )
    {
        Display              *dpy;
        Window                xWin;
        XEvent                event;
        XVisualInfo          *vInfo;
        XSetWindowAttributes  swa;
        GLXFBConfig          *fbConfigs;
        GLXContext            context;
        GLXWindow             glxWin;
        int                   swaMask;
        int                   numReturned;
        int                   swapFlag = True;

        /* Open a connection to the X server */
        dpy = XOpenDisplay( NULL );
        if ( dpy == NULL ) {
            printf( "Unable to open a connection to the X server\n" );
            exit( EXIT_FAILURE );
        }

        /* Request a suitable framebuffer configuration - try for a double
        ** buffered configuration first */
        fbConfigs = glXChooseFBConfig( dpy, DefaultScreen(dpy),
                                       doubleBufferAttributes, &numReturned );

        if ( fbConfigs == NULL ) {  /* no double buffered configs available */
          fbConfigs = glXChooseFBConfig( dpy, DefaultScreen(dpy),
                                         singleBufferAttributess, &numReturned );
          swapFlag = False;
        }

        /* Create an X colormap and window with a visual matching the first
        ** returned framebuffer config */
        vInfo = glXGetVisualFromFBConfig( dpy, fbConfigs[0] );

        swa.border_pixel = 0;
        swa.event_mask = StructureNotifyMask;
        swa.colormap = XCreateColormap( dpy, RootWindow(dpy, vInfo->screen),
                                        vInfo->visual, AllocNone );

        swaMask = CWBorderPixel | CWColormap | CWEventMask;

        xWin = XCreateWindow( dpy, RootWindow(dpy, vInfo->screen), 0, 0, 256, 256,
                              0, vInfo->depth, InputOutput, vInfo->visual,
                              swaMask, &swa );

        /* Create a GLX context for OpenGL rendering */
        context = glXCreateNewContext( dpy, fbConfigs[0], GLX_RGBA_TYPE,
    				 NULL, True );

        /* Create a GLX window to associate the frame buffer configuration
        ** with the created X window */
        glxWin = glXCreateWindow( dpy, fbConfigs[0], xWin, NULL );

        /* Map the window to the screen, and wait for it to appear */
        XMapWindow( dpy, xWin );
        XIfEvent( dpy, &event, WaitForNotify, (XPointer) xWin );

        /* Bind the GLX context to the Window */
        glXMakeContextCurrent( dpy, glxWin, glxWin, context );

        /* OpenGL rendering ... */
        glClearColor( 1.0, 1.0, 0.0, 1.0 );
        glClear( GL_COLOR_BUFFER_BIT );

        glFlush();

        if ( swapFlag )
            glXSwapBuffers( dpy, glxWin );

        sleep( 10 );
        exit( EXIT_SUCCESS );
    }

## Notes

An X color map must be created and passed to `XCreateWindow`.

A GLX context must be created and bound to a GLX drawable before OpenGL commands can be executed. OpenGL commands executed while no context/drawable pair is current result in undefined behavior.

Exposure events indicate that all buffers associated with the specified window may be damaged and should be repainted. Although certain buffers of some visuals on some systems may never require repainting (the depth buffer, for example), it is incorrect to write a program assuming that these buffers will not be damaged.

GLX commands utilize XVisualInfo structures rather than pointers to visuals or visualIDs directly. XVisualInfo structures contain visual, visualID, screen, and depth elements, as well as other X-specific information.

## Using GLX Extensions

All supported GLX extensions will have a corresponding definition in glx.h and a token in the extension string returned by glXQueryExtensionsString. For example, if the `EXT_visual_info` extension is supported, then this token will be defined in glx.h and `EXT_visual_info` will appear in the extension string returned by glXQueryExtensionsString. The definitions in glx.h can be used at compile time to determine if procedure calls corresponding to an extension exist in the library.

OpenGL itself is capable of being extended.

## GLX 1.1, GLX 1.2, and GLX 1.3

GLX 1.3 is now supported and is backward compatible with GLX 1.1 and GLX 1.2. It introduces new functionality (namely GLXFBConfigs) that supersedes the GLX 1.2 functionality. GLX 1.2 commands are supported, but their use in new application development is not recommended.

GLX 1.3 corresponds to OpenGL versions 1.2 and introduces the following new calls: glXGetFBConfigs, glXGetFBConfigAttrib, glXGetVisualFromFBConfig, glXCreateWindow, glXDestroyWindow, glXCreatePixmap, glXDestroyPixmap, glXCreatePbuffer, glXDestroyPbuffer, glXQueryDrawable, glXCreateNewContext, glXMakeContextCurrent, glXGetCurrentReadDrawable, glXGetCurrentDisplay, glXQueryContext, and glXSelectEvent, glXGetSelectedEvent.

GLX 1.2 corresponds to OpenGL version 1.1 and introduces the following new call: glXGetCurrentDisplay.

GLX 1.1 corresponds to OpenGL version 1.0 and introduces the following new calls: glXQueryExtensionsString, glXQueryServerString, and glXGetClientString.

Call glXQueryVersion to determine at runtime what version of GLX is available. glXQueryVersion returns the version that is supported on the connection. Thus, if 1.3 is returned, both the client and server support GLX 1.3. You can also check the GLX version at compile time: GLX_VERSION_1_1 will be defined in glx.h if GLX 1.1 calls are supported, GLX_VERSION_1_2 will be defined if GLX 1.2 calls are supported, and GLX_VERSION_1_3 will be defined if GLX 1.3 calls are supported.

## See Also

glFinish, glFlush, glXChooseVisual, glXCopyContext, glXCreateContext, glXCreateGLXPixmap, glXCreateNewContext, glXCreatePbuffer, glXCreatePixmap, glXCreateWindow, glXDestroyContext, glXDestroyPbuffer, glXDestroyPixmap, glXDestroyWindow, glXGetClientString, glXGetConfig, glXGetCurrentDisplay, glXGetCurrentReadDrawable, glXGetFBConfigAttrib, glXGetFBConfigs, glXGetProcAddress, glXGetSelectedEvent, glXGetVisualFromFBConfig, glXIsDirect, glXMakeContextCurrent, glXMakeCurrent, glXQueryContext, glXQueryDrawable, glXQueryExtension, glXQueryExtensionsString, glXQueryServerString, glXQueryVersion, glXSelectEvent, glXSwapBuffers, glXUseXFont, glXWaitGL, glXWaitX. `XCreateColormap`, `XCreateWindow`, `XSync`

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.