NAME
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 servers make a subset of their visuals
available for OpenGL rendering. Drawables created with
these visuals can also be rendered using the core X renderer
and with the renderer of any other X extension that is
compatible with all core X visuals.
GLX extends drawables with several buffers other than the
standard color buffer. These buffers include back and
auxiliary color buffers, a depth buffer, a stencil buffer,
and a color accumulation buffer. Some or all are included
in each X visual that supports OpenGL.
To render using OpenGL into an X drawable, you must first
choose a visual that defines the required OpenGL buffers.
glXChooseVisual can be used to simplify selecting a
compatible visual. If more control of the selection process
is required, use XGetVisualInfo and glXGetConfig to select
among all the available visuals.
Use the selected visual to create both a GLX context and an
X drawable. GLX contexts are created with glXCreateContext,
and drawables are created with either XCreateWindow or
glXCreateGLXPixmap. Finally, bind the context and the
drawable together using glXMakeCurrent. This
context/drawable pair becomes the current context and
current drawable, and it is used by all OpenGL commands
until glXMakeCurrent is called with different arguments.
Both core X and OpenGL commands can be used to operate on
the current drawable. The X and OpenGL command streams are
not synchronized, however, except at explicitly created
boundaries generated by calling glXWaitGL, glXWaitX, XSync,
and glFlush.
EXAMPLES
Below is the minimum code required to create an RGBA-format,
X window that's compatible with OpenGL and to clear it to
yellow. The code is correct, but it does not include any
error checking. Return values dpy, vi, cx, cmap, and win
should all be tested.
#include <GL/glx.h> #include <GL/gl.h> #include <unistd.h>
static int attributeListSgl[] = { GLX_RGBA,
GLX_RED_SIZE, 1, /*get the deepest buffer with 1 red
bit*/ GLX_GREEN_SIZE, 1, GLX_BLUE_SIZE, 1,
None };
static int attributeListDbl[] = { GLX_RGBA,
GLX_DOUBLE_BUFFER, /*In case single buffering is not
supported*/ GLX_RED_SIZE, 1, GLX_GREEN_SIZE, 1,
GLX_BLUE_SIZE, 1, None };
static Bool WaitForNotify(Display *d, XEvent *e, char *arg)
{
return (e->type == MapNotify) && (e->xmap.window ==
(Window)arg); }
int main(int argc, char **argv) {
Display *dpy;
XVisualInfo *vi;
Colormap cmap;
XSetWindowAttributes swa;
Window win;
GLXContext cx;
XEvent event;
int swap_flag = FALSE;
/* get a connection */
dpy = XOpenDisplay(0);
/* get an appropriate visual */
vi = glXChooseVisual(dpy, DefaultScreen(dpy),
attributeListSgl);
if (vi == NULL) {
vi = glXChooseVisual(dpy, DefaultScreen(dpy),
attributeListDbl);
swap_flag = TRUE;
}
/* create a GLX context */
cx = glXCreateContext(dpy, vi, 0, GL_TRUE);
/* create a color map */
cmap = XCreateColormap(dpy, RootWindow(dpy, vi->screen),
vi->visual, AllocNone);
/* create a window */
swa.colormap = cmap;
swa.border_pixel = 0;
swa.event_mask = StructureNotifyMask;
win = XCreateWindow(dpy, RootWindow(dpy, vi->screen), 0,
0, 100, 100,
0, vi->depth, InputOutput, vi-
>visual,
CWBorderPixel|CWColormap|CWEventMask,
&swa);
XMapWindow(dpy, win);
XIfEvent(dpy, &event, WaitForNotify, (char*)win);
/* connect the context to the window */
glXMakeCurrent(dpy, win, cx);
/* clear the buffer */
glClearColor(1,1,0,1);
glClear(GL_COLOR_BUFFER_BIT);
glFlush();
if (swap_flag) glXSwapBuffers(dpy,win);
/* wait a while */
sleep(10); }
NOTES
A color map must be created and passed to XCreateWindow.
See the preceding example code.
A GLX context must be created and attached to an X drawable
before OpenGL commands can be executed. OpenGL commands
issued 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 manipulate XVisualInfo structures rather than
pointers to visuals or visual IDs. 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 has also been extended. Refer to glIntro for
more information.
GLX 1.1 and GLX 1.2
GLX 1.2 is now supported. It is backward compatible with GLX
1.1 and GLX 1.0.
GLX 1.2 corresponds to OpenGL version 1.1 and introduces the
following new call: glGetCurrentDisplay.
GLX 1.1 corresponds to OpenGL version 1.0 and introduces the
following new calls: glXQueryExtensionsString,
glXQueryServerString, and glXGetClientString.
Call glQueryVersion to determine at runtime what version of
GLX is available. glQueryVersion returns the version that is
supported on the connection. Thus if 1.2 is returned, both
the client and server support GLX 1.2. 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 and
GLX_VERSION_1_2 will be defined if GLX 1.2 calls are
supported.
SEE ALSO
glIntro, glFinish, glFlush, glXChooseVisual, glXCopyContext,
glXCreateContext, glXCreateGLXPixmap, glXDestroyContext,
glXGetClientString, glXGetConfig, glXIsDirect,
glXMakeCurrent,
glXQueryExtension, glXQueryExtensionsString,
glXQueryServerString, glXQueryVersion, glXSwapBuffers,
glXUseXFont, glXWaitGL, glXWaitX, XCreateColormap,
XCreateWindow, XSync