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   <TITLE>Appendix D - OpenGL Programming Guide (Addison-Wesley Publishing Company)</TITLE>
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<H1>
Appendix D<BR>
The OpenGL Extension to the X Window System</H1>
This appendix briefly discusses the routines defined as part of the OpenGL
Extension to the X Window System (GLX). These routines are discussed in
more detail in the <I>OpenGL Reference Manual</I>. You need to have some
knowledge of X to fully understand the following and to use GLX successfully.
This appendix has the following major sections:
<UL>"Initialization"
<BR>&nbsp;
<P>"Controlling Rendering"
<BR>&nbsp;
<P>"GLX Prototypes"</UL>
In the X Window System, OpenGL rendering is made available as an extension
to X in the formal X sense: Connection and authentication are accomplished
with the normal X mechanisms. As with other X extensions, there is a defined
network protocol for OpenGL's rendering commands encapsulated within the
X byte stream. Since performance is critical in three-dimensional rendering,
the OpenGL extension to X allows OpenGL to bypass the X server's involvement
in data encoding, copying, and interpretation and instead render directly
to the graphics pipeline.
<P>
<HR>
<H2>
<A NAME="X"></A>Initialization</H2>
Use <B>glXQueryExtension()</B> and <B>glXQueryVersion()</B> to determine
whether the GLX extension is defined for an X server, and if so, which
version is present. The <B>glXChooseVisual()</B> routine returns a pointer
to an XVisualInfo structure describing the visual that best meets the client's
specified attributes. You can query a visual about its support of a particular
OpenGL attribute with <B>glXGetConfig()</B>.
<P>
<HR>
<H2>
Controlling Rendering</H2>
Several GLX routines are provided for creating and managing an OpenGL rendering
context. You can use such a context to render off-screen if you want. Routines
are also provided for such tasks as synchronizing execution between the
X and OpenGL streams, swapping front and back buffers, and using an X font.
<H3>
Managing an OpenGL Rendering Context</H3>
An OpenGL rendering context is created with <B>glXCreateContext()</B>.
One of the arguments to this routine allows you to request a direct rendering
context that bypasses the X server as described previously. (Note that
to do direct rendering, the X server connection must be local, and the
OpenGL implementation needs to support direct rendering.) You can determine
whether a GLX context is direct with <B>glXIsDirect()</B>.
<P>To make a rendering context current, use <B>glXMakeCurrent()</B>; <B>glXGetCurrentContext()</B>
returns the current context. You can also obtain the current drawable with
<B>glXGetCurrentDrawable()</B>. Remember that only one context can be current
for any thread at any one time. If you have multiple contexts, you can
copy selected groups of OpenGL state variables from one context to another
with <B>glXCopyContext()</B>. When you're finished with a particular context,
destroy it with <B>glXDestroyContext()</B>.
<H3>
Off-Screen Rendering</H3>
To render off-screen, first create an X Pixmap and then pass this as an
argument to <B>glXCreateGLXPixmap()</B>. Once rendering is completed, you
can destroy the association between the X and GLX Pixmaps with <B>glXDestroyGLXPixmap()</B>.
(Off-screen rendering isn't guaranteed to be supported for direct renderers.)
<H3>
Synchronizing Execution</H3>
To prevent X requests from executing until any outstanding OpenGL rendering
is completed, call <B>glXWaitGL()</B>. Then, any previously issued OpenGL
commands are guaranteed to be executed before any X rendering calls made
after <B>glXWaitGL()</B>. Although the same result can be achieved with
<B>glFinish()</B>, <B>glXWaitGL()</B> doesn't require a round trip to the
server and thus is more efficient in cases where the client and server
are on separate machines.
<P>To prevent an OpenGL command sequence from executing until any outstanding
X requests are completed, use <B>glXWaitX()</B>. This routine guarantees
that previously issued X rendering calls are executed before any OpenGL
calls made after <B>glXWaitX()</B>.
<H3>
Swapping Buffers</H3>
For drawables that are double-buffered, the front and back buffers can
be exchanged by calling <B>glXSwapBuffers()</B>. An implicit <B>glFlush()</B>
is done as part of this routine.
<H3>
Using an X Font</H3>
A shortcut for using X fonts in OpenGL is provided with the command <B>glXUseXFont()</B>.
<P>
<HR>
<H2>
GLX Prototypes</H2>

<H3>
Initialization</H3>
Determine whether the GLX extension is defined on the X server:
<DL>
<DT>
Bool <B>glXQueryExtension</B> (Display *<I>dpy</I>, int *<I>errorBase</I>,
int *<I>eventBase</I>);</DT>

<DD>
</DD>

<DT>
</DT>

<BR>Bool <B>glXQueryVersion</B> (Display *<I>dpy</I>, int *<I>major</I>,
int *<I>minor</I>);
<DD>
</DD>
</DL>
Obtain the desired visual:
<DL>
<DT>
XVisualInfo* <B>glXChooseVisual</B> (Display *<I>dpy</I>, int <I>screen</I>,
int *<I>attribList</I>);</DT>

<DD>
</DD>

<DT>
</DT>

<BR>int <B>glXGetConfig</B> (Display *<I>dpy</I>, XVisualInfo *<I>vis</I>,
int <I>attrib</I>, int *<I>value</I>);
<DD>
</DD>
</DL>

<H3>
Controlling Rendering</H3>
Manage or query an OpenGL rendering context:
<DL>
<DT>
GLXContext <B>glXCreateContext</B> (Display <I>*dpy</I>, XVisualInfo <I>*vis</I>,
GLXContext <I>shareList</I>, Bool <I>direct</I>);</DT>

<DD>
</DD>

<DT>
</DT>

<BR>void <B>glXDestroyContext</B> (Display <I>*dpy</I>, GLXContext <I>ctx</I>);
<DD>
</DD>

<DT>
</DT>

<BR>void <B>glXCopyContext</B> (Display *<I>dpy</I>, GLXContext <I>src</I>,
GLXContext <I>dst</I>, GLuint <I>mask</I>);
<DD>
</DD>

<DT>
</DT>

<BR>Bool <B>glXIsDirect</B> (Display *<I>dpy</I>, GLXContext <I>ctx</I>);
<DD>
</DD>

<DT>
</DT>

<BR>Bool <B>glXMakeCurrent</B> (Display *<I>dpy</I>, GLXDrawable <I>draw</I>,
GLXContext <I>ctx</I>);
<DD>
</DD>

<DT>
</DT>

<BR>GLXContext <B>glXGetCurrentContext</B> (void);
<DD>
</DD>

<DT>
</DT>

<BR>GLXDrawable <B>glXGetCurrentDrawable</B> (void);
<DD>
</DD>
</DL>
Perform off-screen rendering:
<DL>
<DT>
GLXPixmap <B>glXCreateGLXPixmap</B> (Display *<I>dpy</I>, XVisualInfo *<I>vis</I>,
Pixmap <I>pixmap</I>);</DT>

<DD>
</DD>

<DT>
</DT>

<BR>void <B>glXDestroyGLXPixmap</B> (Display *<I>dpy</I>, GLXPixmap <I>pix</I>);
<DD>
</DD>
</DL>
Synchronize execution:
<DL>
<DT>
void <B>glXWaitGL</B> (void);</DT>

<DD>
</DD>

<DT>
</DT>

<BR>void <B>glXWaitX</B> (void);
<DD>
</DD>
</DL>
Exchange front and back buffers:
<DL>
<DT>
void <B>glXSwapBuffers</B> (Display *<I>dpy</I>, Window <I>window</I>);</DT>

<DD>
</DD>
</DL>
Use an X font:
<DL>
<DT>
void <B>glXUseXFont</B> (Font <I>font</I>, int <I>first</I>, int <I>count</I>,
int <I>listBase</I>);</DT>

<DD>
</DD>
</DL>

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