GLXINTRO(3)GLXINTRO(3)NAMEglXIntro - 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
March 1, 2011
