Mini GLX Specification

Tungsten Graphics, Inc.

January 20, 2003

Copyright © 2002-2003 by Tungsten Graphics, Inc., Cedar Park, Texas. All Rights Reserved.

Permission is granted to make and distribute verbatim copies of this document provided the copyright notice and this permission notice are preserved on all copies.

1. Introduction

The Mini GLX interface facilitates OpenGL rendering on embedded devices. The interface is a subset of the GLX interface, plus a minimal set of Xlib-like functions.

Programs written to the Mini GLX specification should run unchanged on systems with the X Window System and the GLX extension. The intention is to allow flexibility for prototyping and testing.

This document serves as both the reference guide and programming guide for Mini GLX.

2. Mini GLX Concepts

The OpenGL specification does not describe how OpenGL rendering contexts and drawing surfaces (i.e. the frame buffer) are created and managed. Rather, this is handled by an OpenGL window system interface, such as Mini GLX.

There are three main datatypes or resources managed by Mini GLX. The resources and their corresponding GLX or Xlib data types are:

Resource Data type
pixel formats X Visual and XVisualInfo
drawing surfaces X Window or GLXDrawable
rendering contexts GLXContext

Pixel formats or X Visuals describe the per-pixel attributes of the frame buffer. For example, bits per color component, Z buffer size, stencil size, TrueColor vs PseudoColor, etc.

Drawing surfaces or X Windows typically describe a spatial allocation of the frame buffer (i.e. the position and size of a rectangular region of pixels). Since MiniGLX doesn't really support a window system, the window is effectively the entire frame buffer.

A rendering context represents the current OpenGL state such as current drawing color, line width, blending mode, texture parameters, etc. Several rendering contexts can be created but only one can be in use at any given time.

The Mini GLX interface provides all the functions needed for choosing pixel formats, create drawing surfaces, creating rendering contexts and binding rendering contexts to drawing surfaces.

3. Using Mini GLX

To use the Mini GLX interface in your application, include the GL/miniglx.h header file at compile time:

#include <GL/miniglx.h>
Applications should link with libGL.so (i.e. gcc myprogram.o -lGL -o myprogram).  libGL.so implements the MiniGLX API functions and, in turn, loads a hardware-specific device driver (such as radeon_dri.so) at runtime.  The environment variable LIBGL_DRIVERS_PATH should name the directory where these modules are located.

The remainder of this section describes the MiniGLX API functions.

3.1 Initialization

The XOpenDisplay function is used to initialize the graphics system:

Display *XOpenDisplay(const char *displayname)

The displayName parameter is currently ignored in Mini GLX. It is recommended that NULL be passed as thedisplayName parameter.

If XOpenDisplay is able to initialize the graphics system a pointer to a Display will be returned. Otherwise, NULL will be returned.

3.2 Choosing a Visual

A visual (i.e. pixel format) must be chosen before a drawing surface or rendering context can be created. This is done with the glXChooseVisual function:

XVisualInfo *glXChooseVisual(Display *dpy, int screen, const int *attribList)

dpy is a pointer to the display returned by XOpenDisplay.

screen is currently ignored by Mini GLX and should be zero.

attribList is a list of GLX attributes which describe the desired pixel format. It is terminated by the token None. The attributes are as follows:

GLX_USE_GL
This attribute should always be present in order to maintain compatibility with GLX.
GLX_RGBA
If present, only RGBA pixel formats will be considered. Otherwise, only color index formats are considered.
GLX_DOUBLEBUFFER
if present, only double-buffered pixel formats will be chosen.
GLX_RED_SIZE n
Must be followed by a non-negative integer indicating the minimum number of bits per red pixel component that is acceptable.
GLX_GREEN_SIZE n
Must be followed by a non-negative integer indicating the minimum number of bits per green pixel component that is acceptable.
GLX_BLUE_SIZE n
Must be followed by a non-negative integer indicating the minimum number of bits per blue pixel component that is acceptable.
GLX_ALPHA_SIZE n
Must be followed by a non-negative integer indicating the minimum number of bits per alpha pixel component that is acceptable.
GLX_STENCIL_SIZE n
Must be followed by a non-negative integer indicating the minimum number of bits per stencil value that is acceptable.
None
This token is used to terminate the attribute list.

glXChooseVisual will return a pointer to an XVisualInfo object which most closely matches the requirements of the attribute list. If there is no visual which matches the request, NULL will be returned.

Note that visuals with accumulation buffers and depth buffers are not available.

3.3 Creating a Drawing Surface

Drawing surfaces are created as X windows.  For Mini GLX, windows are full-screen; they cover the entire frame buffer.  Also, Mini GLX imposes a limit of one window. A second window cannot be created until the first one is destroyed.

3.3.1 Window Creation

The XCreateWindow function is used to create a drawing surface:

Window XCreateWindow( Display *display,
                      Window parent,
                      int x, int y,
                      unsigned int width, unsigned int height,
                      unsigned int borderWidth,
                      int depth,
                      unsigned int class,
                      Visual *visual,
                      unsigned long valuemask,
                      XSetWindowAttributes *attributes )

The parameters are as follows:

display
A Display pointer, as returned by XOpenDisplay.
parent
The parent window for the new window. For Mini GLX, this should beRootWindow(dpy, 0).
x, y
The position of the window. For Mini GLX, both values should be zero.
width, height
The size of the window. For Mini GLX, this specifies the desired screen size such as 1024, 768 or 1280, 1024.
borderWidth
This parameter should be zero.
depth
The pixel depth for the window. For Mini GLX this should be the depth found in the XVisualInfo object returned by glxChooseVisual.
class
The window class. For Mini GLX this value should be InputOutput.
visual
This parameter should be the visual field of the XVisualInfo object returned by glxChooseVisual.
valuemask
This parameter indicates which fields of the XSetWindowAttributes are to be used. For Mini GLX this is typically the bitmaskCWBackPixel | CWBorderPixel | CWColormap.
attributes
Initial window attributes. Of the fields in the XSetWindowAttributes structure, thebackground_pixel, border_pixel and colormap fields should be set.  See the discussion below regarding colormaps.

XCreateWindow will return a window handle if it succeeds or zero if it fails.

3.3.2 Window Mapping

To display the window the XMapWindow function must be called:

void XMapWindow(Display *dpy, Window w)

This function does nothing in Mini GLX but is required for Xlib/GLX compatibility

3.3.3 Colormaps

Xlib requires specification of a colormap when creating a window.  For purposes of interoperability, Mini GLX requires this as well, though the colormap is not actually used.  The XCreateColormap function is used to create a colormap:

Colormap XCreateColormap(Display *dpy, Window window, Visual *visual, int alloc)

The parameters are as follows:

dpy
The display handle as returned by XOpenDisplay.
window
This parameter is ignored by Mini GLX but should be the value returned by the RootWindow(dpy, 0) macro.
visual
This parameter is ignored by Mini GLX but should be the visual field of the XVisualInfo object returned by glXChooseVisual.
alloc
This parameter is ignored by Mini GLX but should be set to AllocNone.

3.4 Creating a Rendering Context

An OpenGL rendering context is created with the glXCreateContext function:

GLXContext glXCreateContext(Display *dpy, XVisualInfo *visInfo, GLXContext shareList, Bool direct)

The parameters are as follows:

dpy
The display handle as returned by XOpenDisplay.
visInfo
The visual as returned by glXChooseVisual.
shareList
If non-zero, texture objects and display lists are shared with the named rendering context. If zero, texture objects and display lists will (initially) be private to this context. They may be shared when a subsequent context is created.
direct
Specifies whether direct or indirect rendering is desired. For Mini GLX this value is ignored but it should be set to True.

glXCreateContext will return a GLXContext handle if it succeeds or zero if it fails due to invalid parameter or insufficient resources.

3.5 Binding a Rendering Context

The final step before beginning OpenGL rendering is to bind (i.e. activate) a rendering context and drawing surface with the glXMakeCurrent function:

Bool glXMakeCurrent(Display *dpy, GLXDrawable drawable, GLXContext ctx)

The parameters are as follows:

dpy
The display handle, as returned by XOpenDisplay.
drawable
The window or drawable to bind to the rendering context. This should be the value returned by XCreateWindow.
ctx
The rendering context to bind, as returned by glXCreateContext.

If glXMakeCurrent succeeds True is returned. Otherwise False is returned to indicate an invalid display, window or context parameter.

After the rendering context has been bound to the drawing surface OpenGL rendering can begin.

The current rendering context may be unbound by calling glXMakeCurrent with the window and context parameters set to zero.

An application may create any number of rendering contexts and bind them as needed. Note that binding a rendering context is generally not a light-weight operation.  Most simple OpenGL applications create only one rendering context.

3.6 Color Buffer Swapping

A double buffered window has two color buffers: a front buffer and a back buffer. Normally, rendering is directed to the back buffer while the front buffer is displayed. When rendering of a frame is finished the front and back buffers are swapped to provide the illusion of instanteous screen updates.

The color buffers for a particular window (i.e. drawable) may be swapped with the glXSwapBuffers command:

void glXSwapBuffers(Display *dpy, GLXDrawable drawable)
Any pending rendering commands will be completed before the buffer swap takes place.

Calling glXSwapBuffers on a window which is single-buffered has no effect.

3.7 Releasing Resources

3.7.1 Releasing Rendering Contexts

A rendering context may be destroyed by calling glXDestroyContext:

void glXDestroyContext(Display *dpy, GLXContext ctx)

3.7.2 Releasing Windows

A window may be destroyed by calling XDestroyWindow:

void XDestroyWindow(Display *dpy, Window window)

3.7.3 Releasing Visuals

An XVisualInfo object may be freed by calling XFree:

void XFree(void *data)

3.7.4 Releasing Colormaps

A colormap may be freed by calling XFreeColormap:

void XFreeColormap(Display *dpy, Colormap colormap)

3.7.4 Releasing Display Resources

When the application is about to exit, the resources associated with the graphics system can be released by calling XCloseDisplay:

void XCloseDisplay(Display *dpy)

The display handle becomes invalid at this point.

3.8 Query Functions

3.8.1 Querying Available Visuals

A list of all available visuals can be obtained with the XGetVisualInfo function:

XVisualInfo *XGetVisualInfo(Display *dpy, long vinfo_mask, XVisualInfo *vinfo_template, int *nitems_return)

The parameters are as follows:
dpy
The display handle, as returned by XOpenDisplay.
vinfo_mask
A bitmask indicating which fields of the vinfo_template are to be matched.  The value must be VisualScreenMask.
vinfo_template
A template whose fields indicate which visual attributes must be matched by the results.  The screen field of this structure must be zero.
nitems_return
Returns the number of visuals returned.
The return value is the address of an array of all available visuals.

An example of using XGetVisualInfo to get all available visuals follows:

XVisualInfo visTemplate, *results;
int numVisuals;
Display *dpy = XOpenDisplay(NULL);
visTemplate.screen = 0;
results = XGetVisualInfo(dpy, VisualScreenMask, &visTemplate, &numVisuals);

3.8.2 Querying Visual Attributes

The GLX attributes of an X visual may be queried with the glXGetConfig function:

int glXGetConfig(Display *dpy, XVisualInfo *vis, int attribute, int *value)

The parameters are as follows:

dpy
The display handle, as returned by XOpenDisplay.
vis
The visual, as returned by glXChooseVisual.
attribute
The attribute to query. The attributes are listed below.
value
Pointer to an integer in which the result of the query will be stored.

The return value will be zero if no error occurs.  GLX_INVALID_ATTRIBUTE will be returned if the attribute parameter is invalid.  GLX_BAD_VISUAL will be returned if the XVisualInfo parameter is invalid.

The following attributes may be queried:

GLX_USE_GL
The result will be True or False to indicate if OpenGL rendering is supported with the visual. Mini GLX always return True.
GLX_RGBA
The result will be True for RGBA visuals or False for color index visuals.
GLX_DOUBLEBUFFER
The result will be True if the visual has two color buffers or False if the visual has one color buffer.
GLX_RED_SIZE
The result will be the number of red bits per pixel.
GLX_GREEN_SIZE
The result will be the number of green bits per pixel.
GLX_BLUE_SIZE
The result will be the number of blue bits per pixel.
GLX_ALPHA_SIZE
The result will be the number of alpha bits per pixel.
GLX_DEPTH_SIZE
The result will be the number of bits per Z value.
GLX_STENCIL_SIZE
The result will be the number of bits per stencil value.

3.8.3 Querying the Current Rendering Context

The current rendering context can be queried with glXGetCurrentContext: 

GLXContext glXGetCurrentContext(void)

Zero will be returned if no context is currently bound.

3.8.4 Querying the Current Drawable

The current drawable (i.e. window or drawing surface) can be queried with glXGetCurrentDrawable:

GLXDrawable glXGetCurrentDrawable(void)

Zero will be returned if no drawable is currently bound.

3.8.5 Function Address Queries

The glXGetProcAddress function will return the address of any available OpenGL or Mini GLX function:

void *glXGetProcAddress(const GLubyte *procName)

If procName is a valid function name, a pointer to that function will be returned.  Otherwise, NULL will be returned.

The purpose of glXGetProcAddress is to facilitate using future extensions to OpenGL or Mini GLX. If a future version of the library adds new extension functions they'll be accessible via glXGetProcAddress. The alternative is to hard-code calls to the new functions in the application but doing so will prevent linking the application with older versions of the library.

3.9 Versioning

The Mini GLX version can be queried at run time with glXQueryVersion: 
Bool glXQueryVersion(Display *dpy, int *major, int *minor)

major will be set to the major version number andminor will be set to the minor version number.True will be returned if the function succeeds. False will be returned if the function fails due to invalid parameters. The dpy argument is currently ignored, but should be the value returned by XOpenDisplay.

At compile time, the Mini GLX interface version can be tested with the MINI_GLX_VERSION_1_x preprocessor tokens. For example, if version 1.0 of Mini GLX is supported, then MINI_GLX_VERSION_1_0 will be defined. If version 1.1 of Mini GLX is supported, then MINI_GLX_VERSION_1_1 will be defined.

At the time of writing the current Mini GLX version is 1.0.

4.0 Interoperability with GLX and Xlib

While Mini GLX strives to be compatible with GLX and Xlib there are some unavoidable differences which must be taken into consideration.

4.1 Public vs Private Structures

The structure of many X data types is public.  For example, the Display data type is defined as a structure in /usr/include/X11/Xlib.h and programmers may access any fields of that structure at will.  Mini GLX also defines a Display data type but its fields are hidden and not visiblein miniglx.h.  Duplicating the Xlib declaration for the Display data type in minigl.h would require defining a large number of other superfluous Xlib datatypes.

Mini GLX users are discouraged from directly accessing the fields of Xlib data types to maximize portability - though this is unavoidable to some extent.  For example, the XVisualInfo and XSetWindowAtttributes data types must be completely public.

4.2 Macros

In some cases, Xlib defines macros which are meant to be used instead of direct structure accesses.  For example, the RootWindow(dpy, screen) macro returns the root window for a given screen on a given display.  Unfortunately, macros do nothing to aid in ABI compatibility since they are resolved at compile time instead of at link/run time.

Mini GLX also defines a RootWindow macro since it's essential for creating windows.  But the implementation of this macro by Xlib and Mini GLX is completely different.

4.3 Summary

Because Xlib and Mini GLX define data types and macros differently, Mini GLX applications must be recompiled when retargeting Mini GLX or native Xlib/GLX.  That is, applications can't simply be re-linked because of ABI incompatibilities.

Nevertheless, the fact that Mini GLX programs can be recompiled for Xlib and GLX increases portability and flexibility for testing and prototyping.

5.0 Example Program

This section shows an example program which uses the Mini GLX interface. The program simply draws several frames of a rotating square.

The program may be compiled for use with Xlib/GLX or Mini GLX by setting the USE_MINIGLX token to 0 or 1, respectively.  Note that the only difference is the header files which are included.


#define USE_MINIGLX 1  /* 1 = use Mini GLX, 0 = use Xlib/GLX */

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

#if USE_MINIGLX
#include <GL/miniglx.h>
#else
#include <GL/glx.h>
#include <X11/Xlib.h>
#endif

/*
 * Create a simple double-buffered RGBA window.
 */
static Window
MakeWindow(Display * dpy, unsigned int width, unsigned int height)
{
   int visAttributes[] = {
      GLX_RGBA,
      GLX_RED_SIZE, 1,
      GLX_GREEN_SIZE, 1,
      GLX_BLUE_SIZE, 1,
      GLX_DOUBLEBUFFER,
      None
   };
   XSetWindowAttributes attr;
   unsigned long attrMask;
   Window root;
   Window win;
   GLXContext ctx;
   XVisualInfo *visinfo;

   root = RootWindow(dpy, 0);

   /* Choose GLX visual / pixel format */
   visinfo = glXChooseVisual(dpy, 0, visAttributes);
   if (!visinfo) {
      printf("Error: couldn't get an RGB, Double-buffered visual\n");
      exit(1);
   }

   /* Create the window */
   attr.background_pixel = 0;
   attr.border_pixel = 0;
   attr.colormap = XCreateColormap(dpy, root, visinfo->visual, AllocNone);
   attrMask = CWBackPixel | CWBorderPixel | CWColormap;
   win = XCreateWindow(dpy, root, 0, 0, width, height,
		       0, visinfo->depth, InputOutput,
		       visinfo->visual, attrMask, &attr);
   if (!win) {
      printf("Error: XCreateWindow failed\n");
      exit(1);
   }

   /* Display the window */
   XMapWindow(dpy, win);

   /* Create GLX rendering context */
   ctx = glXCreateContext(dpy, visinfo, NULL, True);
   if (!ctx) {
      printf("Error: glXCreateContext failed\n");
      exit(1);
   }

   /* Bind the rendering context and window */
   glXMakeCurrent(dpy, win, ctx);

   return win;
}


/*
 * Draw a few frames of a rotating square.
 */
static void
DrawFrames(Display * dpy, Window win)
{
   int angle;
   glShadeModel(GL_FLAT);
   glClearColor(0.5, 0.5, 0.5, 1.0);
   for (angle = 0; angle < 360; angle += 10) {
      glClear(GL_COLOR_BUFFER_BIT);
      glColor3f(1.0, 1.0, 0.0);
      glPushMatrix();
      glRotatef(angle, 0, 0, 1);
      glRectf(-0.8, -0.8, 0.8, 0.8);
      glPopMatrix();
      glXSwapBuffers(dpy, win);
   }
}


int
main(int argc, char *argv[])
{
   Display *dpy;
   Window win;

   dpy = XOpenDisplay(NULL);
   if (!dpy) {
      printf("Error: XOpenDisplay failed\n");
      return 1;
   }

   win = MakeWindow(dpy, 300, 300);

   DrawFrames(dpy, win);

   return 0;
}