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$Date: 1997/09/08 21:31:59 $ $Revision: 1.4 $
OpenGL 1.1 is required.
This extension provides BeginScene/EndScene markers to allow buffering
and sorting of the OpenGL command sequence needed to construct and
entire frame. Scene markers are useful for region (aka tile) rendering
architectures, in which all of the geometry for a region a window must
be known before any can be rasterized. It may also be applicable to
other architectures requiring a scene database, such as scanline depth
* Region architectures usually generate only a color buffer. Depth,
stencil, and other ancillary buffers, if they exist, are instantiated
only for the screen region being rasterized at any given time, and
are not retained after all geometry for that region has been
processed. As a consequence, ancillary buffers are usually
inaccessible via Read/Copy/Draw operations. Implementations may
choose to relax this restriction.
* State changes affecting primitives are buffered and may need to be
processed for every screen region. This makes "large" state changes,
such as DrawPixels or TexImage2D, likely to be prohibitively
expensive within a scene.
* State queries within a scene may also be very expensive, if they are
deferred to hardware which normally would not process them until
EndSceneEXT. As with large state changes, it seems preferable to
allow state queries within a scene, but document that they may be
* Initial buffer state after BeginSceneEXT is undefined, forcing the
application to overwrite all pixels. This implies that multiple
scenes may not be merged into a single visible frame without software
intervention to read back and combine scenes.
* SCENE_REQUIRED_EXT is a property of the visual or pixel format,
rather than an Enable. Making it an Enable would probably force a
switch between hardware and software paths.
* 2D (X or GDI or Quickdraw) rendering is probably only allowed after
EndSceneEXT, to prevent synchronization issues. It's unclear how to
phrase this in the spec, however.
* Reads and copies between BeginSceneEXT and EndSceneEXT are made
illegal, since there are many sensible interpretations of what the
buffer contents might be during this period.
* Add mechanism to allow out-of-order rendering, etc.
New Procedures and Functions
void BeginSceneEXT(void);
void EndSceneEXT(void);
New Tokens
Accepted by the <attribList> parameter of glXChooseVisual, and by the
<attrib> parameter of glXGetConfig:
Accepted by the <pname> parameters of GetBooleanv, GetIntegerv,
GetFloatv, and GetDoublev:
Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation)
Additions to Chapter 3 of the 1.1 Specification (Rasterization)
Additions to Chapter 4 of the 1.1 Specification (Per-Fragment Operations
and the Frame buffer)
Additions to Chapter 5 of the 1.1 Specification (Special Functions)
<Insert before Section 5.6 Hints>
5.5.5 Scene Markers
Some OpenGL implementations may benefit from having delimiters which
mark the start and end of a command sequence which draws an entire
scene. These implementations may process commands in such a way that it
is impractical to allow certain commands such as reading the color
buffer to occur during the course of rendering the scene. When
SCENE_REQUIRED_EXT is TRUE, OpenGL command processing is modified as
A scene is started by issuing the command BeginSceneEXT; the contents of
the selected color, stencil, and depth buffers are undefined after
BeginSceneEXT. A scene is marked complete by issuing the command
EndSceneEXT. BeginSceneEXT and EndSceneEXT must be matched and do not
When issued between BeginSceneEXT and the corresponding EndSceneEXT, the
commands ReadPixels, CopyPixels, CopyTexImage, CopyTexSubImage,
DrawBuffer, Accum, Flush, and Finish generate an INVALID_OPERATION
A ReadPixels or CopyPixels call requesting stencil, alpha, or depth
values may generate an INVALID_OPERATION error if the implementation
does not store those buffers in a form visible to the application.
When outside of a BeginSceneEXT/EndSceneEXT sequence, the fragments
generated from the command DrawPixels and from geometry issued during a
Begin/End sequence have an undefined effect on the color and ancillary
buffers, but their changes to OpenGL state are processed as normal. All
other commands are unaffected.
When SCENE_REQUIRED_EXT is FALSE, BeginSceneEXT and EndSceneEXT have no
effect, and none of the restrictions above are in force.
Additions to Chapter 6 of the 1.1 Specification (State and State Requests)
Additions to the GLX Specification
The parameter GLX_SCENE_REQUIRED_EXT is added to glXGetConfig. When
queried, by calling glXGetConfig with <attrib> set to
GLX_SCENE_REQUIRED_EXT, it returns True if the visual requires scene
markers. For a normal visual, the return value is False.
glXChooseVisual accepts GLX_SCENE_REQUIRED_EXT in <attribList>. Visuals
that require scene markers are preferred.
GLX Protocol
XXX - not done yet
Additions to WGL
The flag PFD_SCENE_REQUIRED_EXT is added to ChoosePixelFormat, to allow
selecting a pixel format which requires scene markers.
XXX - not done yet (no extension query mechanism for WGL at present).
INVALID_OPERATION if BeginSceneEXT or EndSceneEXT is issued between
execution of Begin and the corresponding execution of End.
INVALID_OPERATION if BeginSceneEXT is issued between execution of
BeginSceneEXT and the corresponding execution of EndSceneEXT or
EndSceneEXT is issued when no BeginSceneEXT preceded it.
INVALID_OPERATION if ReadPixels, CopyPixels, CopyTexImage,
CopyTexSubImage, DrawBuffer, Accum, Flush, or Finish is issued between
execution of BeginSceneEXT and the corresponding execution of
INVALID_OPERATION if ReadPixels or CopyPixels is issued requesting
alpha, stencil, or depth components, and the implementation does not
store those buffers in a form visible to the application.
New State
Get Value Get Command Type Initial Value Attribute
--------- ----------- ---- ------------- ---------
New Implementation Dependent State
Revision History
Changes due to IHV feedback, revision 1.3, September 2, 1997 (ljp)
Made scene_marker a property of the visual or pixel format, rather than
an enable. Made DrawBuffer within a scene, or ReadPixels/CopyPixels of
depth or stencil, illegal.