glXMakeCurrentReadSGI(3G)   OpenGL Reference - GLX   glXMakeCurrentReadSGI(3G)



NAME
     glXMakeCurrentReadSGI - attach a GLX context to separate read and write
     drawables


C SPECIFICATION
     Bool glXMakeCurrentReadSGI( Display *dpy,
				 GLXDrawable draw,
				 GLXDrawable read,
				 GLXContext gc )


PARAMETERS
     dpy   Specifies the connection to the X server.


     draw  A GLX drawable that receives	the results of OpenGL drawing
	   operations.


     read  A GLX drawable that provides	pixels for glReadPixels	and
	   glCopyPixels	operations.


     gc	   A GLX rendering context to be attached to draw and read.


DESCRIPTION
     glXMakeCurrentReadSGI performs the	same functions as glXMakeCurrent, with
     one important difference:	it attaches a GLX context to two drawables,
     rather than one.  The second drawable serves as the source	for any	pixel
     data that are read	by glReadPixels, glCopyPixels, and OpenGL extension
     commands that read	color, depth, or stencil data from the framebuffer.
     It	may be an ordinary GLX drawable, or it may be a	GLX video source (see
     glXCreateGLXVideoSourceSGIX).  Accumulation operations (see glAccum) are
     not allowed when draw is not identical to read.

     It	is not necessary for draw and read to have the same ancillary buffers.
     It	is also	not necessary for read to contain a buffer corresponding to
     the current GL_READ_BUFFER	of gc.	For example, the current
     GL_READ_BUFFER of gc might	be GL_BACK, and	read might be single-buffered.
     If	there is no corresponding buffer in read, then the pixel values
     extracted from read will be undefined, but	no error will be generated.
     Operations	that query the value of	GL_READ_BUFFER use the value set last
     in	gc, regardless of whether read has the corresponding buffer.

     glXMakeCurrentReadSGI returns True	if it is successful, False otherwise.
     If	False is returned, the previously current rendering context and
     drawables (if any)	remain unchanged.






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glXMakeCurrentReadSGI(3G)   OpenGL Reference - GLX   glXMakeCurrentReadSGI(3G)



ERRORS
     BadMatch is generated if any of the following conditions hold:

	  1.   draw was	not created on the same	X screen and visual as gc.

	  2.   read was	not created on the same	X screen as gc.

	  3.   draw is None and	gc is not None.

	  4.   draw is a GLX video source.

     BadAccess is generated if gc was current to another thread	at the time
     glXMakeCurrentReadSGI was called.

     GLXBadDrawable is generated if either draw	or read	is not a valid GLX
     drawable.

     GLXBadContext is generated	if gc is not a valid GLX context.

     GLXBadContextState	is generated if	glXMakeCurrentReadSGI is called
     between a glBegin and the corresponding call to glEnd.

     GLXBadContextState	is also	generated if the rendering context current to
     the calling thread	has OpenGL renderer state GL_FEEDBACK or GL_SELECT.

     GLXBadCurrentWindow is generated if there are pending OpenGL commands for
     the previous context and the current drawable is a	window that is no
     longer valid.

     BadAlloc may be generated if the server has delayed allocation of
     ancillary buffers until glXMakeCurrentReadSGI is called, only to find
     that it has insufficient resources	to complete the	allocation.



MACHINE	DEPENDENCIES
     The SGI_make_current_read extension is supported on RealityEngine,
     RealityEngine2, and VTX systems with the following	restrictions:

	  1.  A	BadMatch error is generated if

	      a.   draw	is RGBA	and read is not, or vice versa.	 A video
		   source (see glXCreateGLXVideoSourceSGIX) is considered to
		   be RGBA.

	      b.   read	is not a video source and it does not have the same
		   color component resolution or the same number of index bits
		   as draw;

	      c.   Either draw or read is not at buffer	level 0	(the main
		   frame buffer	planes,	rather than overlay planes).




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glXMakeCurrentReadSGI(3G)   OpenGL Reference - GLX   glXMakeCurrentReadSGI(3G)



	      d.   draw	is multisample capable and readP is a pbuffer (see
		   glXCreateGLXPbufferSGIX).

	  2.  When draw	is a pixmap then read must be the same pixmap.	A
	      BadAlloc error is	generated otherwise.

     On	RealityEngine, RealityEngine2, and VTX systems and on InfiniteReality
     systems it	is not possible	to use a context for rendering to both windows
     and pixmaps.


SEE ALSO
     glXMakeCurrent










































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