GLXMAKECURRENTREADSGI(3gl)GLXMAKECURRENTREADSGI(3gl)NAME
glXMakeCurrentReadSGI - attach a GLX context to separate write and read
drawables (windows, GLX pixmaps, or GLX pbuffers)
C SPECIFICATION
Bool glXMakeCurrentReadSGI( Display *dpy,
GLXDrawable drawable,
GLXDrawable readable,
GLXContext ctx )
PARAMETERS
dpy Specifies the connection to the X server.
drawable Specifies a GLX drawable. Must be either an X window ID or a
GLX pixmap ID.
readable Specifies a GLX readable. Must be either an X window ID or a
GLX pixmap ID.
ctx Specifies a GLX rendering context that is to be attached to
drawable.
DESCRIPTION
glXMakeCurrentReadSGI specifies the association of the current context
with a drawable is extended from glXMakeCurrent to allow separate write
and read drawables. This allows preprocessing of image data in an off
screen window which is then read into the visible window for final dis‐
play. Similarly it sets the framework for direct transfer of video to
the GL library, by treating the video as a special kind of read draw‐
able (a.k.a, readable).
glXMakeCurrentReadSGI returns True if it succeeds, and False if it
fails.
NOTES
If glXMakeCurrentReadSGI is used to associate two glXDrawables with the
current rendering context, glXDrawable drawable is used for all OpenGL
operations except:
1. Any pixel data that is sourced based on the value of
GL_READ_BUFFER. Note, that accumulation operations use the
value of GL_READ_BUFFER, but are not allowed when drawable in
not identical to readable.
2. Any depth values that are retrieved by glReadPixels, glCopyPix‐
els, or any OpenGL extension that sources depth images from the
frame buffer in the manner of glReadPixels and glCopyPixels.
3. Any stencil values that are retrieved by glReadPixels, glCopyP‐
ixels, or any OpenGL extension that sources stencil images from
the frame buffer in the manner of glReadPixels and glCopyPixels.
These frame buffer values are taken from glXDrawable readable.
No error will be generated if the value of GL_READ_BUFFER at the time
the glXMakeCurrentReadSGI call is made does not correspond to a valid
color buffer in readable. Also, no error due to GL_READ_BUFFER mismatch
will be generated by subsequent calls to any of the operations enumer‐
ated above, but the pixels values used will be undefined until
GL_READ_BUFFER is set to a color buffer that is valid in the readable.
Operations that query the value of GL_READ_BUFFER (i.e., glGet,
glPushAttrib) use the value set last in the context, independent of
whether it is a valid buffer in readable. Subsequent to a glXMakeCur‐
rentRead call, it is possible that the GL_READ_BUFFER may be set to a
color buffer that is not valid with respect to the visual of the con‐
text. This also applies to glXMakeCurrent where readable is replaced
by the single drawable parameter to the call.
Error conditions set by glDrawBuffer (even when called implicitly via
glPopAttrib) and by the operations enumerated above are with respect to
color and ancillary buffers available in readable (i.e., glReadBuffer
(GL_BACK_BUFFER) will generate an error when readable is single
buffered, and so will an operation that tries to source stencil images
when readable does not have a stencil buffer).
Accumulation operations are only allowed when the glXDrawbles drawable
and readable are identical.
glXMakeCurrentReadSGI may fail if a window or a GLXPixmap as the read‐
able GLXDrawable refuses to accept the readable GLXDrawable, or a GLXP‐
buffer with color, depth, stencil, or accumulation storage facilities
that differ from those of drawable.
glXMakeCurrentReadSGI may return False if drawable and readable cannot
exist in framebuffer memory simultaneously.
glXMakeCurrentReadSGI is part of the GLX_SGI_make_current_read exten‐
sion, not part of the core GLX command set. If GLX_SGI_make_cur‐
rent_read is included in the string returned by glXQueryExtension‐
sString, extension GLX_SGI_make_current_read is supported by the con‐
nection.
This function is superseded by glXMakeContextCurrent, which is part of
GLX Version 1.3 command set. glXMakeCurrentReadSGI is only supported
for backward compatibility.
ERRORS
GLXBadDrawable is generated if drawable was not created with the same X
screen and visual as ctx. glXMakeCurrentReadSGI associates two GLXDraw‐
ables with a single GLXContext.
BadMatch is generated if readable was not created with the same X
screen as ctx. The color, depth, stencil, and accumulation buffers of
readable need not match the resources of drawable.
SEE ALSO
glXCreateGLXPixmap, glXMakeCurrent, glXGetCurrentReadDrawableSGI,
glGet, glPushAttrib, glPopAttrib, glAccum, glReadPixels, glCopyPixels
23 May 01 GLXMAKECURRENTREADSGI(3gl)
