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Dale Kirkland, NVIDIA (dkirkland 'at'
Copyright (c) 2000-2013 The Khronos Group Inc. Copyright terms at
Specification Update Policy
Khronos-approved extension specifications are updated in response to
issues and bugs prioritized by the Khronos OpenGL Working Group. For
extensions which have been promoted to a core Specification, fixes will
first appear in the latest version of that core Specification, and will
eventually be backported to the extension document. This policy is
described in more detail at
Complete. Approved by ARB on March 15, 2000.
Last Modified Date: 03/12/2002
Author Revision: 1.1
Based on: WGL_EXT_pbuffer specification
Date: 3/1/1999 Version: 1.5
ARB Extension #10
WGL_ARB_extensions_string is required.
The association of a separate "read" and "draw" DC with the current
context allows for preprocessing of image data in an "off screen"
DC which is then read into a visible DC for final display.
New Procedures and Functions
BOOL wglMakeContextCurrentARB(HDC hDrawDC,
HDC hReadDC,
HGLRC hglrc);
HDC wglGetCurrentReadDCARB(VOID);
New Tokens
New errors returned by GetLastError:
Additions to Chapter 2 of the 1.2 GL Specification (OpenGL Operation)
Additions to Chapter 3 of the 1.2 GL Specification (Rasterization)
Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations
and the Frame buffer)
Additions to Chapter 5 of the 1.2 Specification (Special Functions)
Additions to Chapter 6 of the 1.2 Specification (State and State Requests)
Additions to the GLX Specification
This specification is written for WGL.
GLX Protocol
This specification is written for WGL.
Additions to the WGL specification
The function wglMakeContextCurrentARB associates the context <hglrc>
with the device <hDrawDC> for draws and the device <hReadDC> for
reads. All subsequent OpenGL calls made by the calling thread are
drawn on the device identified by <hDrawDC> and read on the device
identified by <hReadDC>.
The <hDrawDC> and <hReadDC> parameters must refer to drawing
surfaces supported by OpenGL. These parameters need not be the
same <hdc> that was passed to wglCreateContext when <hglrc> was
created. <hDrawDC> must have the same pixel format and be created
on the same physical device as the <hdc> that was passed into
wglCreateContext. <hReadDC> must be created on the same device as
the <hdc> that was passed to wglCreateContext and it must support
the same pixel type as the pixel format of the <hdc> that was
passed to wglCreateContext.
If wglMakeContextCurrentARB is used to associate a different device
for reads than for draws, the "read" device will be used for the
following OpenGL operations:
1. Any pixel data that are sourced based on the value of READ_BUFFER.
Note, that accumulation operations use the value of READ_BUFFER,
but are not allowed when a different device context is used
for reads. In this case, the accumulation operation will
2. Any depth values that are retrieved by ReadPixels, CopyPixels,
or any OpenGL extension that sources depth images from the frame
buffer in the manner of ReadPixels and CopyPixels.
3. Any stencil values that are retrieved by ReadPixels, CopyPixels,
or any OpenGL extension that sources stencil images from the
framebuffer in the manner of ReadPixels and CopyPixels.
These frame buffer values are taken from the surface associated with
the device context specified by <hReadDC>.
No error will be generated if the value of READ_BUFFER at the time
the wglMakeContextCurrentARB call is made does not correspond to a
valid color buffer in <hReadDC>. Also, no error due to READ_BUFFER
mismatch will be generated by subsequent calls to any of the
operations enumerated above, but the pixels values used will be
undefined until READ_BUFFER is set to a color buffer that is valid
in the <hReadDC>. Operations that query the value of READ_BUFFER
(i.e., Get, PushAttrib) use the value set last in the context,
independent of whether it is a valid buffer in <hReadDC>.
Error conditions set by ReadBuffer and by the operations enumerated
above are with respect to color and ancillary buffers available in
<hReadDC> (i.e., ReadBuffer(BACK_BUFFER) will generate an error
when <hReadDC> is single buffered, and so will an operation that
tries to source stencil images when <hReadDC> does not have a
stencil buffer). When the read buffer is set implicitly via
PopAttrib to a state not supported by the pixel format, an error
may be generated.
If wglMakeContextCurrentARB succeeds, the return value is TRUE.
If the function fails, the return value is FALSE. To get extended
error information, call GetLastError. Possible errors are as follows:
ERROR_INVALID_PIXEL_FORMAT The pixel format associated with
<hDrawDC> does not match the pixel
format associated with the render
ERROR_INVALID_PIXEL_TYPE_ARB The pixel type for <hReadDC> is
different than the pixel type
associated with the <hdc> that was
passed to wglCreateContext.
The device contexts specified by
<hReadDC> and <hDrawDC> are not
compatible. This can occur if the
device contexts are managed by
different drivers or possibly on
different graphics adapters.
ERROR_DC_NOT_FOUND <hReadDC> or <hDrawDC> is not a valid
device context.
ERROR_NO_SYSTEM_RESOURCES The device contexts specified by
<hReadDC> and <hDrawDC> cannot exist
in the framebuffer simultaneously.
wglGetCurrentReadDC returns a handle to the "read" device context that
is associated with the current OpenGL rendering context of the calling
thread. If the calling thread does not have a current context, the
return value is NULL.
Dependencies on WGL_ARB_extensions_string
Because there is no way to extend wgl, these calls are defined in
the ICD and can be called by obtaining the address with
wglGetProcAddress. Because this extension is a WGL extension, it
is not included in the GL_EXTENSIONS string. Its existence can be
determined with the WGL_ARB_extensions_string extension.
New State
New Implementation Dependent State
Conformance Testing
1. Create two non-overlapping windows (windows 1 and 2).
2. Create three contexts (context A, B, and C).
3. Set context A to draw to window 1 and read from window 1.
4. Set context B to draw to window 2 and read from window 1.
5. Set context C to draw to window 2 and read from window 2.
6. For a conformance test (TBD),
a. Draw using context A.
b. Blit from window to window using context B.
c. Test conformance using context C.
7. If pixel buffers are supported, repeat using a pixel buffer.
Revision History
12/16/1999 0.1
- First ARB draft based on the EXT specification.
03/15/2000 0.2
- Removed the changes to Chapter 4.
- Added a discussion that accumulation operations may
- PopAttrib may (not will) generate an error.
- Added an error if the read and draw DCs are not managed
on the same driver.
03/22/2000 1.0
- Changed rendering context to device context.
- Added the new error conditions values.
- Approved by ARB: 10-0-0.
03/12/2002 1.1
- Updated contact information.