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.TL
Extending X for Double-Buffering, Multi-Buffering, and Stereo
.AU
Jeffrey Friedberg
Larry Seiler
Jeff Vroom
.AI
Version 3.3
January 11, 1990

\fIThe \fRMulti-Buffering\fP extension described here was a draft standard of the
X Consortium prior to Release 6.1.  It has been superseded by the Double Buffer
Extension (DBE).  DBE is an X Consortium Standard as of Release 6.1.\fP
.SH
Introduction
.LP
Several proposals have been written that address some of the
issues surrounding the support of double-buffered, multi-buffered,
and stereo windows in the X Window System:
.IP \(bu
\fIExtending X for Double-Buffering,\fP
Jeffrey Friedberg, Larry Seiler, Randi Rost.
.IP \(bu
\fI(Proposal for) Double-Buffering Extensions\fP,
Jeff Vroom.
.IP \(bu
\fIAn Extension to X.11 for Displays with Multiple Buffers,\fP
David S.H. Rosenthal.
.IP \(bu
\fIA Multiple Buffering/Stereo Proposal\fP,
Mark Patrick.
.FS
Copyright \(co 1989 Digital Equipment Corporation.
.br
Permission to use, copy, modify, and distribute this documentation for any
purpose and without fee is hereby granted, provided that the above copyright
notice and this permission notice appear in all copies.
Digital Equipment Corporation makes no representations
about the suitability for any purpose of the information in
this document.  This documentation is provided "as is"
without express or implied warranty.  This document
is subject to change.
.LP
Copyright \(co 1989, 1994 X Consortium
.LP
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the ``Software''), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
.LP
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
.LP
THE SOFTWARE IS PROVIDED ``AS IS'', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
X CONSORTIUM BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
.LP
Except as contained in this notice, the name of the X Consortium shall not be
used in advertising or otherwise to promote the sale, use or other dealings
in this Software without prior written authorization from the X Consortium.
.LP
\fIX Window System\fP is a trademark of X Consortium, Inc.
.FE
.LP
The authors of this proposal have tried to unify the above documents
to yield a proposal that incorporates support for double-buffering,
multi-buffering, and stereo in a way that is acceptable to all concerned.
.SH
Goals
.LP
Clients should be able to:
.IP \(bu
Associate multiple buffers with a window.
.IP \(bu
Paint in any buffer associated with a window.
.IP \(bu
Display any buffer associated with a window.
.IP \(bu
Display a series of buffers in a window in rapid succession
to achieve a \fIsmooth\fP animation.
.IP \(bu
Request simultaneous display of different buffers in different windows.
.LP
In addition, the extension should:
.IP \(bu
Allow existing X applications to run unchanged.
.IP \(bu
Support a range of implementation methods that can capitalize on
existing hardware features.
.bp
.SH
Image Buffers
.LP
Normal windows are created using the standard \fBCreateWindow\fP request:
.DS
.\" .ft 8
CreateWindow
	parent          : WINDOW
	w_id            : WINDOW
	depth           : CARD8
	visual          : VISUALID or CopyFromParent
	x, y            : INT16
	width, height   : INT16
	border_width    : INT16
	value_mask      : BITMASK
	value_list      : LISTofVALUE
.\" .ft
.DE
.LP
This request allocates a set of window attributes and
a buffer into which an image can be drawn.
The contents of this \fIimage buffer\fP will
be displayed when the window is mapped to the screen.
.LP	
To support double-buffering and multi-buffering,
we introduce the notion that additional image buffers can
be created and bound together to form groups.
The following rules will apply:
.IP \(bu
All image buffers in a group will have the same
visual type, depth, and geometry (ie: width and height).
.IP \(bu
Only one image buffer per group can be displayed
at a time.
.IP \(bu
Draw operations can occur to any image buffer at
any time.
.IP \(bu
Window management requests (\fBMapWindow\fP, \fBDestroyWindow\fP,
\fBConfigureWindow\fP, etc...)
affect all image buffers associated with a window.
.IP \(bu
Appropriate resize and exposure events will be generated
for every image buffer that is affected by a window
management operation.
.LP
By allowing draw operations to occur on any image buffer at any time,
a client could, on a multi-threaded multi-processor server, 
simultaneously build up images for display.
To support this, each buffer must have its own resource ID. 
Since buffers are different than windows and pixmaps
(buffers are not hierarchical and pixmaps cannot be displayed)
a new resource, \fBBuffer\fP, is introduced.
Furthermore, a \fBBuffer\fP is also a \fBDrawable\fP, thus 
draw operations may also be performed on buffers simply
by passing a buffer ID to the existing pixmap/window
interface.
.LP
To allow existing X applications to work unchanged, we assume
a window ID passed in a draw request, for a multi-buffered
window, will be an \fIalias\fP for the ID of the currently
displayed image buffer.  Any draw requests (eq: \fBGetImage\fP) on
the window will be relative to the displayed image buffer.
.LP
In window management requests, only a window ID will be
accepted.  Requests like \fBQueryTree\fP, will continue to
return only window ID's.  Most events will return
just the window ID.  Some new events, described in a subsequent
section, will return a buffer ID.
.LP
When a window has backing store the contents of the window
are saved off-screen.  Likewise, when the contents of an image
buffer of a multi-buffer window is saved off-screen, it is
said to have backing store.  This applies to all image buffers,
whether or not they are selected for display.
.LP
In some multi-buffer implementations, undisplayed buffers might be
implemented using pixmaps.  Since the contents of pixmaps exist
off-screen and are not affected by occlusion, these image buffers
in effect have backing store.
.LP
On the other hand, both the displayed and undisplayed image buffers
might be implemented using a subset of the on-screen pixels.  
In this case, unless the contents of an image buffer are saved
off-screen, these image buffers in effect do not have backing store.
.LP
Output to any image buffer of an unmapped multi-buffered window
that does not have backing store is discarded.  Output to any
image buffer of a mapped multi-buffer window will be performed;
however, portions of an image buffer may be occluded or clipped.
.LP
When an unmapped multi-buffered window becomes mapped, the contents
of any image buffer buffer that did not have backing store is
tiled with the background and zero or more exposure events are
generated.  If no background is defined for the window, then
the screen contents are not altered and the contents of any
undisplayed image buffers are undefined.  If backing store was
maintained for an image buffer, then no exposure events are generated.
.SH
New Requests
.LP
The new request, \fBCreateImageBuffers\fP, creates a group of
image buffers and associates them with a normal X window:
.DS
.\" .ft 8
CreateImageBuffers
	w_id           : WINDOW
	buffers        : LISTofBUFFER
	update_action  : {Undefined,Background,Untouched,Copied}
	update_hint    : {Frequent,Intermittent,Static}
	=>
	number_buffers : CARD16

	(Errors: Window, IDChoice, Value)
.\" .ft
.DE
One image buffer will be associated with each ID passed in \fIbuffers\fP.
The first buffer of the list is referred to as buffer[0], the next
buffer[1], and so on.  Each buffer will have the same visual type
and geometry as the window.
Buffer[0] will refer to the image buffer already associated
with the window ID and its contents will not be modified.
The displayed image buffer attribute is set to buffer[0].
.LP
Image buffers for the remaining ID's (buffer[1],...) are allocated.
If the window is mapped, or if these image buffers have backing
store, their contents will be tiled with the window background
(if no background is defined, the buffer contents are undefined),
and zero or more expose events will be generated for each of these
buffers.  The contents of an image buffer is undefined when
the window is unmapped and the buffer does not have backing store.
.LP
If the window already has a group of image buffers
associated with it (ie: from a previous \fBCreateImageBuffers\fP request)
the actions described for \fBDestroyImageBuffers\fP are performed first
(this will delete the association of the previous buffer ID's and
their buffers as well as de-allocate all buffers except for the
one already associated with the window ID).
.LP
To allow a server implementation to efficiently allocate the
buffers, the total number of buffers required and 
the update action (how they will behave during an update)
is specified "up front" in the request.
If the server cannot allocate all the buffers requested, the
total number of buffers actually allocated will be returned.
No \fBAlloc\fP errors will be generated \- buffer[0] can
always be associated with the existing displayed image buffer.
.LP
For example, an application that wants to animate a short movie
loop may request 64 image buffers.  The server may only be able to
support 16 image buffers of this type, size, and depth.
The application can then decide 16 buffers is sufficient and may
truncate the movie loop, or it may decide it really needs
64 and will free the buffers and complain to the user. 
.LP
One might be tempted to provide a request that inquires whether \fIn\fP
buffers of a particular type, size, and depth \fIcould\fP be allocated.
But if the query is decoupled from the actual allocation, 
another client could sneak in and take the buffers before the
original client has allocated them.
.LP
While any buffer of a group can be selected for display, 
some applications may display buffers in a predictable order
(ie: the movie loop application).  The \fIlist order\fP
(buffer[0], buffer[1], ...) will be used as a hint by the
server as to which buffer will be displayed next.
A client displaying buffers in this order may see a
performance improvement.
.LP
\fIupdate_action\fP indicates what should happen to a previously
displayed buffer when a different buffer becomes displayed.
Possible actions are:
.IP \fIUndefined\fP 15
The contents of the buffer that was
last displayed will become undefined after the update.  This
is the most efficient action since it allows the implementation
to trash the contents of the buffer if it needs to.
.IP \fIBackground\fP
The contents of the buffer that was
last displayed will be set to the background of the window after the update.
The background action allows devices to use a fast clear
capability during an update.
.IP \fIUntouched\fP
The contents of the buffer that was
last displayed will be untouched after the update.  Used
primarily when cycling through images that have already
been drawn.
.IP \fICopied\fP
The contents of the buffer that was
last displayed will become the same as those that are being
displayed after the update.  This is useful when incrementally
adding to an image.
.LP
\fIupdate_hint\fP indicates how often the client will 
request a different buffer to be displayed.
This hint will allow smart server implementations to choose the
most efficient means to support a multi-buffered window based
on the current need of the application (dumb implementations
may choose to ignore this hint).  Possible hints are:
.IP \fIFrequent\fP 15
An animation or movie loop is
being attempted and the fastest, most efficient means for
multi-buffering should be employed.
.IP \fIIntermittent\fP
The displayed image will be
changed every so often.  This is common for images that are
displayed at a rate slower than a second.  For example, a
clock that is updated only once a minute.
.IP \fIStatic\fP
The displayed image buffer will
not be changed any time soon.  Typically set by an application
whenever there is a pause in the animation.
.LP
To display an image buffer the following request can be used:
.DS
.\" .ft 8
DisplayImageBuffers
	buffers         : LISTofBUFFER
	min_delay       : CARD16
	max_delay       : CARD16

	(Errors: Buffer, Match)
.\" .ft
.DE
The image buffers listed will become displayed as simultaneously
as possible and the update action, bound at \fBCreateImageBuffers\fP
time, will be performed.
.LP
A list of buffers is specified to
allow the server to efficiently change the display of more than one
window at a time (ie: when a global screen swap method is used).
Attempting to simultaneously display
multiple image buffers from the same window is an error
(\fBMatch\fP) since it violates the rule that only one
image buffer per group can be displayed at a time. 
.LP
If a specified buffer is already displayed,
any delays and update action will still be
performed for that buffer.  In this instance,
only the update action of \fIBackground\fP (and possibly
\fIUndefined\fP) will have any affect on the contents
of the displayed buffer.  These semantics allow
an animation application to successfully execute
even when there is only a single buffer available
for a window.
.LP
When a \fBDisplayImageBuffers\fP request is made to an unmapped
multi-buffered window, the effect of the update action depends
on whether the image buffers involved have backing store.
When the target of the update action is an image buffer that
does not have backing store, output is discarded.  When the
target image buffer does have backing store, the update is performed;
however, when the source of the update is an image buffer does not
have backing store (as in the case of update action \fICopied\fP), the
contents of target image buffer will become undefined.
.LP
\fImin_delay\fP and \fImax_delay\fP put a bound on how long the
server should wait before processing the display request.
For each of the windows to be updated by this request, at least
\fImin_delay\fP milli-seconds should elapse since the last
time any of the windows were updated; conversely, no window
should have to wait more than \fImax_delay\fP milli-seconds
before being updated.
.LP
\fImin_delay\fP allows an application to
\fIslow down\fP an animation or movie loop so that it appears
synchronized at a rate the server can support given the current load.
For example, a \fImin_delay\fP of 100 indicates the server should
wait at least 1/10 of a second since the last time any of the
windows were updated.  A \fImin_delay\fP of zero indicates
no waiting is necessary.
.LP
\fImax_delay\fP can be thought of as an additional
delay beyond \fImin_delay\fP the server is allowed to wait
to facilitate such things as efficient update of multiple windows.
If \fImax_delay\fP would require an update before \fImin_delay\fP
is satisfied, then the server should process the display request as
soon as the \fImin_delay\fP requirement is met.  A typical
value for \fImax_delay\fP is zero.
.LP
To implement the above functionality, the time since the last
update by a \fBDisplayImageBuffers\fP request for each multi-buffered
window needs to be saved as state by the server.
The server may delay execution of the \fBDisplayImageBuffers\fP
request until the appropriate time (e.g. by requeuing the
request after computing the timeout); 
however, the entire request must be processed in one operation.
Request execution indivisibility must be maintained.  When
a server is implemented with internal concurrency, the
extension must adhere to the same concurrency semantics
as those defined for the core protocol.
.LP 
To explicitly clear a rectangular area of an image buffer to
the window background, the following request can be used:
.DS
.\" .ft 8
ClearImageBufferArea
	buffer          : BUFFER
	x, y            : INT16
	w, h            : CARD16
	exposures       : BOOL

	(Errors: Buffer, Value)
.\" .ft P
.DE
.LP
Like the X \fBClearArea\fP request, \fIx\fP and \fIy\fP are relative to
the window's origin and specify the upper-left corner of the rectangle.
If \fIwidth\fP is zero, it is replaced with the current window width
minus \fIx\fP.  If \fIheight\fP is zero it is replaced with the current
window height minus \fIy\fP.  If the window has a defined background
tile, the rectangle is tiled with a plane mask of all ones,
a function of \fICopy\fP, and a subwindow-mode of \fIClipByChildren\fP.
If the window has background \fINone\fP, the contents of the buffer
are not changed.  In either case, if \fIexposures\fP is true, then one or
more exposure events are generated for regions of the rectangle that are
either visible or are being retained in backing store.
.LP
The group of image buffers allocated by a \fBCreateImageBuffers\fP
request can be destroyed with the following request:
.DS
.\" .ft 8
DestroyImageBuffers
	w_id		: WINDOW

	(Error: Window)
.\" .ft
.DE
.LP
The association between the buffer ID's and their corresponding
image buffers are deleted.  Any image buffers not selected for
display are de-allocated.  If the window is not multi-buffered, 
the request is ignored.
.SH
Attributes
.LP
The following attributes will be associated with each window that
is multi-buffered:
.DS
.\" .ft 8
	displayed_buffer : CARD16
	update_action    : {Undefined,Background,Untouched,Copied}
	update_hint      : {Frequent,Intermittent,Static}
	window_mode      : {Mono,Stereo}
	buffers          : LISTofBUFFER
.\" .ft
.DE
.LP
\fIdisplayed_buffer\fP is set to the \fIindex\fP of the currently
displayed image buffer (for stereo windows, this will be
the index of the left buffer \- the index of the right buffer
is simply \fIindex\fP+1).
\fIwindow_mode\fP indicates whether this window is \fIMono\fP or \fIStereo\fP.
The ID for each buffer associated with the window is recorded
in the \fIbuffers\fP list.
The above attributes can be queried with the following request:
.DS
.\" .ft 8
GetMultiBufferAttributes
	w_id             : WINDOW
	=>
	displayed_buffer : CARD16
	update_action    : {Undefined,Background,Untouched,Copied}
	update_hint      : {Frequent,Intermittent,Static}
	window_mode      : {Mono,Stereo}
	buffers          : LISTofBUFFER

	(Errors: Window, Access, Value)
.\" .ft
.DE
.LP
If the window is not multi-buffered, a \fBAccess\fP error will be generated.
The only multi-buffer attribute that can be explicitly set 
is \fIupdate_hint\fP.  Rather than have a specific request
to set this attribute, a generic set request is provided to
allow for future expansion:
.DS
.\" .ft 8
SetMultiBufferAttributes
	w_id            : WINDOW
	value_mask      : BITMASK
	value_list      : LISTofVALUE

	(Errors: Window, Match, Value)
.\" .ft
.DE
.LP
If the window is not multi-buffered, a \fBMatch\fP error will be generated.
The following attributes are maintained for each buffer of a
multi-buffered window:
.DS
.\" .ft 8
	window           : WINDOW
	event_mask       : SETofEVENT
	index            : CARD16
	side             : {Mono,Left,Right}
.\" .ft
.DE
.LP
\fIwindow\fP indicates the window this buffer is associated with.
\fIevent_mask\fP specifies which events, relevant to
buffers, will be sent back to the client via the associated buffer ID
(initially no events are selected).
\fIindex\fP is the list position (0, 1, ...) of the buffer.
\fIside\fP indicates whether this buffer is associated with 
the left side or right side of a stereo window. 
For non-stereo windows, this attribute will be set to \fIMono\fP.
These attributes can be queried with the following request:
.DS
.\" .ft 8
GetBufferAttributes
	buffer          : BUFFER
	=>
	window           : WINDOW
	event_mask       : SETofEVENT
	index            : CARD16
	side             : {Mono,Left,Right}

	(Errors: Buffer, Value)
.\" .ft
.DE
.LP
The only buffer attribute that can be explicitly set 
is \fIevent_mask\fP.
The only events that are valid are
\fBExpose\fP and the new \fBClobberNotify\fP and \fBUpdateNotify\fP event
(see Events section below).
A \fBValue\fP error will be generated if an event not selectable
for a buffer is specified in an event mask.
Rather than have a specific request
to set this attribute, a generic set request is provided to
allow for future expansion:
.DS
.\" .ft 8
SetBufferAttributes
	buffer          : BUFFER
	value_mask      : BITMASK
	value_list      : LISTofVALUE

	(Errors: Buffer, Value)
.\" .ft
.DE
.LP
Clients may want to query the server about basic multi-buffer
and stereo capability on a per screen basis.  The following request
returns a large list of information
that would most likely be read once by Xlib for each screen, and used as a data base for
other Xlib queries:
.DS
.\" .ft 8
GetBufferInfo
	root            : WINDOW
	=>
	info            : LISTofSCREEN_INFO
.\" .ft
.DE
.LP
Where \fBSCREEN_INFO\fP and \fBBUFFER_INFO\fP are defined as:
.DS
.\" .ft 8

	SCREEN_INFO     : [ normal_info : LISTofBUFFER_INFO,
	                    stereo_info : LISTofBUFFER_INFO ]

	BUFFER_INFO     : [ visual      : VISUALID, 
	                    max_buffers : CARD16,
	                    depth       : CARD8 ]
.\" .ft
.DE
.LP
Information regarding multi-buffering of normal (mono) windows
is returned in the \fInormal_info\fP list.  The \fIstereo_info\fP
list contains information about stereo windows.
If the \fIstereo_info\fP list is empty, stereo windows are
not supported on the screen.  If \fImax_buffers\fP is zero,
the maximum number of buffers for the depth and visual is
a function of the size of the created window and current
memory limitations.
.LP
The following request returns the major and minor version numbers
of this extension:
.DS
.\" .ft 8
GetBufferVersion
	=>
	major_number    : CARD8
	minor_number    : CARD8
.\" .ft
.DE
.LP
The version numbers are an escape hatch in case future revisions of
the protocol are necessary.  In general, the major version would
increment for incompatible changes, and the minor version would
increment for small upward compatible changes.  Barring changes, the
major version will be 1, and the minor version will be 1. 
.SH
Events
.LP
All events normally generated for single-buffered
windows are also generated for multi-buffered windows.
Most of these events (ie: \fBConfigureNotify\fP) will
only be generated for the window and not for each buffer.
These events will return a window ID.
.LP
\fBExpose\fP events will be generated for both the window
and any buffer affected.  When this event is generated for
a buffer, the same event structure will be used
but a buffer ID is returned instead of a window ID.
Clients, when processing these events, will know whether an
ID returned in an event structure is for a window or a buffer
by comparing the returned ID to the ones returned when the
window and buffer were created.
.LP
\fBGraphicsExposure\fP and \fBNoExposure\fP are generated
using whatever ID is specified in the graphics operation.
If a window ID is specified, the event will contain the
window ID.  If a buffer ID is specified, the event will
contain the buffer ID.
.LP
In some implementations, moving a window
over a multi-buffered window may cause one or more of its buffers
to get overwritten or become unwritable.  To allow a
client drawing into one of these buffers the opportunity
to stop drawing until some portion of the buffer is
writable, the following event is added:
.DS
.\" .ft 8
ClobberNotify
	buffer         :  BUFFER
	state          : {Unclobbered,PartiallyClobbered,FullyClobbered}
.\" .ft
.DE
.LP
The \fBClobberNotify\fP event is reported to clients selecting
\fIClobberNotify\fP on a buffer.  When a buffer that was fully
or partially clobbered becomes unclobbered, an event with \fIUnclobbered\fP
is generated.  When a buffer that was unclobbered becomes
partially clobbered, an event with \fIPartiallyClobbered\fP
is generated.  When a buffer that was unclobbered or
partially clobbered becomes fully clobbered, an event with
\fIFullyClobbered\fP is generated.
.LP
\fBClobberNotify\fP events on a given buffer are
generated before any \fBExpose\fP events on that buffer,
but it is not required that all \fBClobberNotify\fP
events on all buffers be generated before all
\fBExpose\fP events on all buffers.
.LP 
The ordering of \fBClobberNotify\fP events with respect
to \fBVisibilityNotify\fP events is not constrained.
.LP
If multiple buffers were used as an image FIFO between an image
server and the X display server, then the FIFO manager would like
to know when a buffer that was previously displayed, has been
undisplayed and updated, as the side effect of a \fBDisplayImageBuffers\fP
request.  This allows the FIFO manager to load up a future frame as
soon as a buffer becomes available.  To support this,
the following event is added:
.DS
.\" .ft 8
UpdateNotify
	buffer         :  BUFFER
.\" .ft
.DE
.LP
The \fBUpdateNotify\fP event is reported to clients selecting
\fIUpdateNotify\fP on a buffer.  Whenever a buffer becomes \fIupdated\fP
(e.g. its update action is performed as part of a \fBDisplayImageBuffers\fP
request), an \fBUpdateNotify\fP event is generated.
.SH
Errors
.LP
The following error type has been added to support
this extension:
.IP \fBBuffer\fP 15
A value for a BUFFER argument does not name a defined
BUFFER.
.bp
.SH
Double-Buffering Normal Windows
.LP
The following pseudo-code fragment illustrates how to create and display
a double-buffered image:
.DS
.\" .ft 8
/*
 * Create a normal window
 */
CreateWindow( W, ... )

/*
 * Create two image buffers.  Assume after display, buffer
 * contents become "undefined".  Assume we will "frequently"
 * update the display.  Abort if we don't get two buffers,
 */
n = CreateImageBuffers( W, [B0,B1], Undefined, Frequent )
if (n != 2) <abort>

/*
 * Map window to the screen 
 */
MapWindow( W )

/*
 * Draw images using alternate buffers, display every
 * 1/10 of a second.  Note we draw B1 first so it will
 * "pop" on the screen
 */
while animating
{
	<draw picture using B1>
	DisplayImageBuffers( [B1], 100, 0 )

	<draw picture using B0>
	DisplayImageBuffers( [B0], 100, 0 )
}

/*
 * Strip image buffers and leave window with
 * contents of last displayed image buffer.
 */
DestroyImageBuffers( W )
.\" .ft
.DE
.bp
.SH
Multi-Buffering Normal Windows
.LP
Multi-buffered images are also supported by these requests.
The following pseudo-code fragment illustrates how to create a
a multi-buffered image and cycle through the images to
simulate a movie loop:
.DS
.\" .ft 8
/*
 * Create a normal window
 */
CreateWindow( W, ... )

/*
 * Create 'N' image buffers.  Assume after display, buffer
 * contents are "untouched".  Assume we will "frequently"
 * update the display.  Abort if we don't get all the buffers.
 */
n = CreateImageBuffers( W, [B0,B1,...,B(N-1)], Untouched, Frequent )
if (n != N) <abort>

/*
 * Map window to screen
 */
MapWindow( W )

/*
 * Draw each frame of movie one per buffer
 */
foreach frame
	<draw frame using B(i)>

/*
 * Cycle through frames, one frame every 1/10 of a second.
 */
while animating
{
	foreach frame
		DisplayImageBuffers( [B(i)], 100, 0 )
}
.\" .ft
.DE
.bp
.SH
Stereo Windows
.LP
\fIHow\fP stereo windows are supported on a server is implementation
dependent.  A server may contain specialized hardware that allows
left and right images to be toggled at field or frame rates.  The
stereo affect may only be perceived with the aid of special
viewing glasses.  The \fIdisplay\fP of a stereo picture should
be independent of how often the contents of the picture are
\fIupdated\fP by an application.  Double and multi-buffering
of images should be possible regardless of whether the image
is displayed normally or in stereo.
.LP
To achieve this goal, a simple extension to normal windows
is suggested.  Stereo windows are just like normal windows
except the displayed image is made up of a left image
buffer and a right image buffer.  To create a stereo window,
a client makes the following request:
.DS
.\" .ft 8
CreateStereoWindow
	parent          : WINDOW
	w_id            : WINDOW
	left, right     : BUFFER
	depth           : CARD8
	visual          : VISUALID or CopyFromParent
	x, y            : INT16
	width, height   : INT16
	border_width    : INT16
	value_mask      : BITMASK
	value_list      : LISTofVALUE

	(Errors: Alloc, Color, Cursor, Match,
	         Pixmap, Value, Window)
.\" .ft
.DE
.LP
This request, modeled after the \fBCreateWindow\fP request,
adds just two new parameters: \fIleft\fP and \fIright\fP.
For stereo, it is essential that one can distinguish whether
a draw operation is to occur on the left image or right image.
While an internal mode could have been added to achieve this,
using two buffer ID's allows clients to simultaneously build up
the left and right components of a stereo image.  These
ID's always refer to (are an alias for) the left and right
image buffers that are currently \fIdisplayed\fP.
.LP
Like normal windows, the window ID is used whenever a window
management operation is to be performed.  Window queries would
also return this window ID (eg: \fBQueryTree\fP) as would most
events.  Like the window ID, the left and right buffer ID's
each have their own event mask.  They can be set and queried
using the \fBSet/GetBufferAttributes\fP requests.
.LP
Using the window ID of a stereo window in a draw request
(eg: \fBGetImage\fP) results in pixels that are \fIundefined\fP.
Possible semantics are that both left and right images get
drawn, or just a single side is operated on (existing applications
will have to be re-written to explicitly use the left and right
buffer ID's in order to successfully create, fetch, and store
stereo images).
.LP
Having an explicit \fBCreateStereoWindow\fP request is helpful
in that a server implementation will know from the onset whether
a stereo window is desired and can return appropriate status
to the client if it cannot support this functionality.
.LP
Some hardware may support separate stereo and non-stereo modes,
perhaps with different vertical resolutions.  For example, the
vertical resolution in stereo mode may be half that of non-stereo
mode.  Selecting one mode or the other must be done through some
means outside of this extension (eg: by providing a separate
screen for each hardware display mode).  The screen attributes
(ie: x/y resolution) for a screen that supports normal windows,
may differ from a screen that supports stereo windows;
however, all windows, regardless of type, displayed on the
same screen must have the same screen attributes
(ie: pixel aspect ratio).
.LP
If a screen that supports stereo windows also supports
normal windows, then the images presented to the left and
right eyes for normal windows should be the same
(ie: have no stereo offset).
.KS
.SH
Single-Buffered Stereo Windows
.LP
The following shows how to create and display a single-buffered
stereo image:
.DS
.\" .ft 8
/*
 * Create the stereo window, map it the screen,
 * and draw the left and right images
 */
CreateStereoWindow( W, L, R, ... )

MapWindow( W )

<draw picture using L,R>
.\" .ft
.DE
.KE
.bp
.SH
Double-Buffering Stereo Windows
.LP
Additional image buffers may be added to a stereo window
to allow double or multi-buffering of stereo images.
Simply use the the \fBCreateImageBuffers\fP request.
Even numbered buffers (0,2,...) will be left buffers.
Odd numbered buffers (1,3,...) will be right buffers.
Displayable stereo images are formed by consecutive
left/right pairs of image buffers.  For example,
(buffer[0],buffer[1]) form the first displayable
stereo image; (buffer[2],buffer[3]) the next;
and so on.
.LP
The \fBCreateImageBuffers\fP request will only create
pairs of left and right image buffers for stereo windows.
By always pairing left and right image
buffers together, implementations might be able to
perform some type of optimization.  If an odd number
of buffers is specified, a \fBValue\fP error is generated.
All the rules mentioned at the start of this proposal
still apply to the image buffers supported by a stereo window.
.LP
To display a image buffer pair of a multi-buffered stereo image,
either the left buffer ID or right buffer ID may be specified in a
\fBDisplayImageBuffers\fP request, but not both.
.LP
To double-buffer a stereo window:
.DS
.\" .ft 8
/*
 * Create stereo window and map it to the screen
 */
CreateStereoWindow( W, L, R, ... )

/*
 * Create two pairs of image buffers.  Assume after display,
 * buffer contents become "undefined".  Assume we will "frequently"
 * update the display.  Abort if we did get all the buffers.
 */
n = CreateImageBuffers( W, [L0,R0,L1,R1], Undefined, Frequently )
if (n != 4) <abort>

/*
 * Map window to the screen
 */
MapWindow( W )

/*
 * Draw images using alternate buffers,
 * display every 1/10 of a second.
 */
while animating
{
	<draw picture using L1,R1>
	DisplayImageBuffers( [L1], 100, 0 )

	<draw picture using L0,R0>
	DisplayImageBuffers( [L0], 100, 0 )
}
.\" .ft
.DE
.bp
.SH
Multi-Buffering Stereo Windows
.LP
To cycle through \fIN\fP stereo images:
.DS
.\" .ft 8
/*
 * Create stereo window
 */
CreateStereoWindow( W, L, R, ... )

/*
 * Create N pairs of image buffers.  Assume after display,
 * buffer contents are "untouched".  Assume we will "frequently"
 * update the display.  Abort if we don't get all the buffers.
 */
n = CreateImageBuffers( W, [L0,R0,...,L(N-1),R(N-1)], Untouched, Frequently )
if (n != N*2) <abort>

/*
 * Map window to screen
 */
MapWindow( W )

/*
 * Draw the left and right halves of each image
 */
foreach stereo image
	<draw picture using L(i),R(i)>

/*
 * Cycle through images every 1/10 of a second
 */
while animating
{
	foreach stereo image
		DisplayImageBuffers( [L(i)], 100, 0 )
}
.\" .ft
.DE
.bp
.SH
Protocol Encoding
.LP
The official name of this extension is "Multi-Buffering".
When this string passed to \fBQueryExtension\fP the
information returned should be interpreted as follows:
.IP \fImajor-opcode\fP 15
Specifies the major opcode of this extension.
The first byte of each extension request should
specify this value.
.IP \fIfirst-event\fP
Specifies the code that will be returned when
\fBClobberNotify\fP events are generated.
.IP \fIfirst-error\fP
Specifies the code that will be returned when
\fBBuffer\fP errors are generated.
.LP
The following sections describe the protocol
encoding for this extension.
.SH
TYPES
.LP
BUFFER_INFO
.TS
lw(.5i) lw(1.5i) lw(2i).
4	VISUALID	visual
2	CARD16	max-buffers
1	CARD8	depth
1		unused
.TE
.LP
SETofBUFFER_EVENT
.TS
lw(.5i) lw(1.5i) lw(2i).
\0	#x00008000	Exposure
\0	#x02000000	ClobberNotify
\0	#x04000000	UpdateNotify
.TE
.SH
EVENTS
.LP
\fBClobberNotify\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fIfirst-event\fP	code
1		unused
2	CARD16	sequence number
4	BUFFER	buffer
1		state
	0 Unclobbered
	1 PartiallyClobbered
	2 FullyClobbered
23		unused
.TE
.LP
\fBUpdateNotify\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	\fIfirst-event\fP+1	code
1		unused
2	CARD16	sequence number
4	BUFFER	buffer
24		unused
.TE
.SH
ERRORS
.LP
\fBBuffer\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	0	Error
1	see \fIfirst-error\fP	code
2	CARD16	sequence number
4	CARD32	bad resource id
2	CARD16	minor-opcode
1	CARD8	major-opcode
21		unused
.TE
.bp
.SH
REQUESTS
.KS
.LP
\fBGetBufferVersion\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	0	minor-opcode
2	1	request length
\(->
1	1	Reply
1		unused
2	CARD16	sequence number
4	0	reply length
1	CARD8	major version number
1	CARD8	minor version number
22		unused
.TE
.KE
.KS
.LP
\fBCreateImageBuffers\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	1	minor-opcode
2	3+n	request length
4	WINDOW	wid
1		update-action
	0 Undefined
	1 Background
	2 Untouched
	3 Copied
1		update-hint
	0 Frequent
	1 Intermittent
	2 Static
2		unused
4n	LISTofBUFFER	buffer-list
\(->
1	1	Reply
1		unused
2	CARD16	sequence number
4	0	reply length
2	CARD16	number-buffers
22		unused
.TE
.KE
.KS
.LP
\fBDestroyImageBuffers\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	2	minor-opcode
2	2	request length
4	WINDOW	wid
.TE
.KE
.KS
.LP
\fBDisplayImageBuffers\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	3	minor-opcode
2	2+n	request length
2	CARD16	min-delay
2	CARD16	max-delay
4n	LISTofBUFFER	buffer-list
.TE
.KE
.KS
.LP
\fBSetMultiBufferAttributes\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	4	minor-opcode
2	3+n	request length
4	WINDOW	wid

4	BITMASK	value-mask (has n bits set to 1)
	#x00000001 update-hint

4n	LISTofVALUE	value-list

VALUEs
1		update-hint
	0 Frequent
	1 Intermittent
	2 Static
.TE
.KE
.KS
.LP
\fBGetMultiBufferAttributes\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	5	minor-opcode
2	2	request length
4	WINDOW	wid
\(->
1	1	Reply
1		unused
2	CARD16	sequence number
4	n	reply length
2	CARD16	displayed-buffer
1		update-action
	0 Undefined
	1 Background
	2 Untouched
	3 Copied
1		update-hint
	0 Frequent
	1 Intermittent
	2 Static
1		window-mode
	0 Mono
	1 Stereo
19		unused
4n	LISTofBUFFER	buffer list
.TE
.KE
.KS
.LP
\fBSetBufferAttributes\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	6	minor-opcode
2	3+n	request length
4	BUFFER	buffer

4	BITMASK	value-mask (has n bits set to 1)
	#x00000001 event-mask

4n	LISTofVALUE	value-list

VALUEs
4	SETofBUFFER_EVENT	event-mask
.TE
.KE
.KS
.LP
\fBGetBufferAttributes\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	7	minor-opcode
2	2	request length
4	BUFFER	buffer
\(->
1	1	Reply
1		unused
2	CARD16	sequence number
4	0	reply length
4	WINDOW	wid
4	SETofBUFFER_EVENT	event-mask
2	CARD16	index
1		side
	0 Mono
	1 Left
	2 Right
13		unused
.TE
.KE
.KS
.LP
\fBGetBufferInfo\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	8	minor-opcode
2	2	request length
4	WINDOW	root
\(->
1	1	Reply
1		unused
2	CARD16	sequence number
4	2(n+m)	reply length
2	n	number BUFFER_INFO in normal-info
2	m	number BUFFER_INFO in stereo-info
20		unused
8n	LISTofBUFFER_INFO	normal-info
8m	LISTofBUFFER_INFO	stereo-info
.TE
.KE
.KS
.LP
\fBCreateStereoWindow\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	9	minor-opcode
2	11+n	request length
3		unused
1	CARD8	depth
4	WINDOW	wid
4	WINDOW	parent
4	BUFFER	left
4	BUFFER	right
2	INT16	x
2	INT16	y
2	CARD16	width
2	CARD16	height
2	CARD16	border-width
2		class
	0 CopyFromParent
	1 InputOutput
	2 InputOnly

4	VISUALID	visual
	0 CopyFromParent

4	BITMASK	value-mask (has n bits set to 1)
	\fIencodings are the same\fP
	\fIas for CreateWindow\fP

4n	LISTofVALUE	value-list
	\fIencodings are the same\fP
	\fIas for CreateWindow\fP
.TE
.KE
.KS
.LP
\fBClearImageBufferArea\fP
.TS
lw(.5i) lw(1.5i) lw(2i).
1	see \fImajor-opcode\fP	major-opcode
1	10	minor-opcode
2	5	request length
4	WINDOW	buffer
2	INT16	x
2	INT16	y
2	CARD16	width
2	CARD16	height
3		unused
1	BOOL	exposures
.TE
.KE