Extending X for Double-Buffering, Multi-Buffering, and Stereo

Jeffrey Friedberg

Larry Seiler

Jeff Vroom

X Version 11, Release 7.7

Version 3.3

Copyright © 1989 Digital Equipment Corporation

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.

Copyright © 1989, 1994 X Consortium

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:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

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.

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.

X Window System is a trademark of The OpenGroup.

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Table of Contents

Warning
1. Introduction
2. Goals
3. Image Buffers
4. New Requests
5. Attributes
6. Events
7. Errors
    Buffer
    Double-Buffering Normal Windows
    Multi-Buffering Normal Windows
    Stereo Windows
    Single-Buffered Stereo Windows
    Double-Buffering Stereo Windows
    Multi-Buffering Stereo Windows
    Protocol Encoding
8. TYPES
9. EVENTS
10. ERRORS
11. REQUESTS

Warning

Warning

The Multi-Buffering 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.

Chapter 1. Introduction

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:

  • Extending X for Double-Buffering, Jeffrey Friedberg, Larry Seiler, Randi
    Rost.

  • (Proposal for) Double-Buffering Extensions, Jeff Vroom.

  • An Extension to X.11 for Displays with Multiple Buffers, David S.H.
    Rosenthal.

  • A Multiple Buffering/Stereo Proposal, Mark Patrick.

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.

Chapter 2. Goals

Clients should be able to:

  • Associate multiple buffers with a window.

  • Paint in any buffer associated with a window.

  • Display any buffer associated with a window.

  • Display a series of buffers in a window in rapid succession to achieve a 
    smooth animation.

  • Request simultaneous display of different buffers in different windows.

In addition, the extension should:

  • Allow existing X applications to run unchanged.

  • Support a range of implementation methods that can capitalize on existing
    hardware features.

Chapter 3. Image Buffers

Normal windows are created using the standard CreateWindow request:

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

This request allocates a set of window attributes and a buffer into which an
image can be drawn. The contents of this image buffer will be displayed when
the window is mapped to the screen.

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:

  • All image buffers in a group will have the same visual type, depth, and
    geometry (ie: width and height).

  • Only one image buffer per group can be displayed at a time.

  • Draw operations can occur to any image buffer at any time.

  • Window management requests (MapWindow, DestroyWindow, ConfigureWindow,
    etc...) affect all image buffers associated with a window.

  • Appropriate resize and exposure events will be generated for every image
    buffer that is affected by a window management operation.

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, Buffer, is
introduced. Furthermore, a Buffer is also a Drawable, thus draw operations may
also be performed on buffers simply by passing a buffer ID to the existing
pixmap/window interface.

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 alias for the
ID of the currently displayed image buffer. Any draw requests (eq: GetImage) on
the window will be relative to the displayed image buffer.

In window management requests, only a window ID will be accepted. Requests like
QueryTree, 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.

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.

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.

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.

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.

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.

Chapter 4. New Requests

The new request, CreateImageBuffers, creates a group of image buffers and
associates them with a normal X window:

CreateImageBuffers
     w_id           : WINDOW
     buffers        : LISTofBUFFER
     update_action  : {Undefined,Background,Untouched,Copied}
     update_hint    : {Frequent,Intermittent,Static}
     =>
     number_buffers : CARD16

     (Errors: Window, IDChoice, Value)

One image buffer will be associated with each ID passed in buffers. 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].

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.

If the window already has a group of image buffers associated with it (ie: from
a previous CreateImageBuffers request) the actions described for
DestroyImageBuffers 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).

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 Alloc errors will be generated \- buffer[0] can
always be associated with the existing displayed image buffer.

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.

One might be tempted to provide a request that inquires whether n buffers of a
particular type, size, and depth could 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.

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 
list order (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.

update_action indicates what should happen to a previously displayed buffer
when a different buffer becomes displayed. Possible actions are:

           The contents of the buffer that was last displayed will become
Undefined  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.

           The contents of the buffer that was last displayed will be set to
Background the background of the window after the update. The background action
           allows devices to use a fast clear capability during an update.

           The contents of the buffer that was last displayed will be untouched
Untouched  after the update. Used primarily when cycling through images that
           have already been drawn.

           The contents of the buffer that was last displayed will become the
Copied     same as those that are being displayed after the update. This is
           useful when incrementally adding to an image.

update_hint 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:

Frequent     An animation or movie loop is being attempted and the fastest,
             most efficient means for multi-buffering should be employed.

             The displayed image will be changed every so often. This is common
Intermittent for images that are displayed at a rate slower than a second. For
             example, a clock that is updated only once a minute.

             The displayed image buffer will not be changed any time soon.
Static       Typically set by an application whenever there is a pause in the
             animation.

To display an image buffer the following request can be used:

DisplayImageBuffers
     buffers         : LISTofBUFFER
     min_delay       : CARD16
     max_delay       : CARD16

     (Errors: Buffer, Match)

The image buffers listed will become displayed as simultaneously as possible
and the update action, bound at CreateImageBuffers time, will be performed.

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 (Match) since it violates the rule that only one image
buffer per group can be displayed at a time.

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
Background (and possibly Undefined) 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.

When a DisplayImageBuffers 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 Copied), the contents of target image buffer will
become undefined.

min_delay and max_delay 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 min_delay milli-seconds should elapse since the last time any
of the windows were updated; conversely, no window should have to wait more
than max_delay milli-seconds before being updated.

min_delay allows an application to slow down an animation or movie loop so that
it appears synchronized at a rate the server can support given the current
load. For example, a min_delay 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 
min_delay of zero indicates no waiting is necessary.

max_delay can be thought of as an additional delay beyond min_delay the server
is allowed to wait to facilitate such things as efficient update of multiple
windows. If max_delay would require an update before min_delay is satisfied,
then the server should process the display request as soon as the min_delay
requirement is met. A typical value for max_delay is zero.

To implement the above functionality, the time since the last update by a
DisplayImageBuffers request for each multi-buffered window needs to be saved as
state by the server. The server may delay execution of the DisplayImageBuffers
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.

To explicitly clear a rectangular area of an image buffer to the window
background, the following request can be used:

ClearImageBufferArea
     buffer          : BUFFER
     x, y            : INT16
     w, h            : CARD16
     exposures       : BOOL

     (Errors: Buffer, Value)

Like the X ClearArea request, x and y are relative to the window's origin and
specify the upper-left corner of the rectangle. If width is zero, it is
replaced with the current window width minus x. If height is zero it is
replaced with the current window height minus y. If the window has a defined
background tile, the rectangle is tiled with a plane mask of all ones, a
function of Copy, and a subwindow-mode of ClipByChildren. If the window has
background None, the contents of the buffer are not changed. In either case, if
exposures 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.

The group of image buffers allocated by a CreateImageBuffers request can be
destroyed with the following request:

DestroyImageBuffers
     w_id          : WINDOW

     (Error: Window)

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.

Chapter 5. Attributes

The following attributes will be associated with each window that is
multi-buffered:

     displayed_buffer : CARD16
     update_action    : {Undefined,Background,Untouched,Copied}
     update_hint      : {Frequent,Intermittent,Static}
     window_mode      : {Mono,Stereo}
     buffers          : LISTofBUFFER

displayed_buffer is set to the index 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 index+1). window_mode indicates whether this window
is Mono or Stereo. The ID for each buffer associated with the window is
recorded in the buffers list. The above attributes can be queried with the
following request:

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)

If the window is not multi-buffered, a Access error will be generated. The only
multi-buffer attribute that can be explicitly set is update_hint. Rather than
have a specific request to set this attribute, a generic set request is
provided to allow for future expansion:

SetMultiBufferAttributes
     w_id            : WINDOW
     value_mask      : BITMASK
     value_list      : LISTofVALUE

     (Errors: Window, Match, Value)

If the window is not multi-buffered, a Match error will be generated. The
following attributes are maintained for each buffer of a multi-buffered window:

     window           : WINDOW
     event_mask       : SETofEVENT
     index            : CARD16
     side             : {Mono,Left,Right}

window indicates the window this buffer is associated with. event_mask
specifies which events, relevant to buffers, will be sent back to the client
via the associated buffer ID (initially no events are selected). index is the
list position (0, 1, ...) of the buffer. side 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 Mono. These attributes can be queried
with the following request:

GetBufferAttributes
     buffer          : BUFFER
     =>
     window           : WINDOW
     event_mask       : SETofEVENT
     index            : CARD16
     side             : {Mono,Left,Right}

     (Errors: Buffer, Value)

The only buffer attribute that can be explicitly set is event_mask. The only
events that are valid are Expose and the new ClobberNotify and UpdateNotify
event (see Events section below). A Value 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:

SetBufferAttributes
     buffer          : BUFFER
     value_mask      : BITMASK
     value_list      : LISTofVALUE

     (Errors: Buffer, Value)

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:

GetBufferInfo
     root            : WINDOW
     =>
     info            : LISTofSCREEN_INFO

Where SCREEN_INFO and BUFFER_INFO are defined as:

     SCREEN_INFO     : [ normal_info : LISTofBUFFER_INFO,
                         stereo_info : LISTofBUFFER_INFO ]

     BUFFER_INFO     : [ visual      : VISUALID,
                         max_buffers : CARD16,
                         depth       : CARD8 ]

Information regarding multi-buffering of normal (mono) windows is returned in
the normal_info list. The stereo_info list contains information about stereo
windows. If the stereo_info list is empty, stereo windows are not supported on
the screen. If max_buffers 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.

The following request returns the major and minor version numbers of this
extension:

GetBufferVersion
     =>
     major_number    : CARD8
     minor_number    : CARD8

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.

Chapter 6. Events

All events normally generated for single-buffered windows are also generated
for multi-buffered windows. Most of these events (ie: ConfigureNotify) will
only be generated for the window and not for each buffer. These events will
return a window ID.

Expose 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.

GraphicsExposure and NoExposure 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.

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:

ClobberNotify
     buffer         :  BUFFER
     state          : {Unclobbered,PartiallyClobbered,FullyClobbered}

The ClobberNotify event is reported to clients selecting ClobberNotify on a
buffer. When a buffer that was fully or partially clobbered becomes
unclobbered, an event with Unclobbered is generated. When a buffer that was
unclobbered becomes partially clobbered, an event with PartiallyClobbered is
generated. When a buffer that was unclobbered or partially clobbered becomes
fully clobbered, an event with FullyClobbered is generated.

ClobberNotify events on a given buffer are generated before any Expose events
on that buffer, but it is not required that all ClobberNotify events on all
buffers be generated before all Expose events on all buffers.

The ordering of ClobberNotify events with respect to VisibilityNotify events is
not constrained.

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 DisplayImageBuffers 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:

UpdateNotify
     buffer         :  BUFFER

The UpdateNotify event is reported to clients selecting UpdateNotify on a
buffer. Whenever a buffer becomes updated (e.g. its update action is performed
as part of a DisplayImageBuffers request), an UpdateNotify event is generated.

Chapter 7. Errors

Table of Contents

Buffer
Double-Buffering Normal Windows
Multi-Buffering Normal Windows
Stereo Windows
Single-Buffered Stereo Windows
Double-Buffering Stereo Windows
Multi-Buffering Stereo Windows
Protocol Encoding

The following error type has been added to support this extension:

Buffer

A value for a BUFFER argument does not name a defined BUFFER.

Double-Buffering Normal Windows

The following pseudo-code fragment illustrates how to create and display a
double-buffered image:

/*
 * 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 )

Multi-Buffering Normal Windows

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:

/*
 * 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 )
}

Stereo Windows

How 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 display of a stereo picture should be
independent of how often the contents of the picture are updated by an
application. Double and multi-buffering of images should be possible regardless
of whether the image is displayed normally or in stereo.

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:

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)

This request, modeled after the CreateWindow request, adds just two new
parameters: left and right. 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 displayed.

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: QueryTree) 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 Set/GetBufferAttributes requests.

Using the window ID of a stereo window in a draw request (eg: GetImage) results
in pixels that are undefined. 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).

Having an explicit CreateStereoWindow 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.

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).

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).

Single-Buffered Stereo Windows

The following shows how to create and display a single-buffered stereo image:

/*
 * 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>

Double-Buffering Stereo Windows

Additional image buffers may be added to a stereo window to allow double or
multi-buffering of stereo images. Simply use the the CreateImageBuffers
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.

The CreateImageBuffers 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 Value error is generated. All the
rules mentioned at the start of this proposal still apply to the image buffers
supported by a stereo window.

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 DisplayImageBuffers
request, but not both.

To double-buffer a stereo window:

/*
 * 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 )
}

Multi-Buffering Stereo Windows

To cycle through N stereo images:

/*
 * 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 )
}

Protocol Encoding

The official name of this extension is "Multi-Buffering". When this string
passed to QueryExtension the information returned should be interpreted as
follows:

major-opcode Specifies the major opcode of this extension. The first byte of
             each extension request should specify this value.

first-event  Specifies the code that will be returned when ClobberNotify events
             are generated.

first-error  Specifies the code that will be returned when Buffer errors are
             generated.

The following sections describe the protocol encoding for this extension.

Chapter 8. TYPES

BUFFER_INFO

4       VISUALID     visual
2       CARD16       max-buffers
1       CARD8        depth
1                    unused

SETofBUFFER_EVENT

        #x00008000   Exposure
        #x02000000   ClobberNotify
        #x04000000   UpdateNotify

Chapter 9. EVENTS

ClobberNotify
1       see first-event code
1                                unused
2       CARD16                   sequence number
4       BUFFER                   buffer
1                                state
        0 Unclobbered
        1 PartiallyClobbered
        2 FullyClobbered
23                                unused

UpdateNotify
1 first-event+1 code
1                       unused
2      CARD16           sequence number
4      BUFFER           buffer
24                      unused

Chapter 10. ERRORS

Buffer
1     0                 Error
1     see first-error code
2     CARD16                 sequence number
4     CARD32                 bad resource id
2     CARD16                 minor-opcode
1     CARD8                  major-opcode
21                           unused

Chapter 11. REQUESTS

GetBufferVersion
1       see major-opcode         major-opcode
1       0                  minor-opcode
2       1                  request length
->
1       1                  Reply
1                          unused
2       CARD16             sequencenumber
4       0                  reply length
1       CARD8              majorversion number
1       CARD8              minorversion number
22                         unused


CreateImageBuffers

1        see major-opcode  major-opcode
1        1                 minor-opcode
2        3+n               requestlength
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              sequencenumber
4      0                   reply length
2      CARD16              number-buffers
22                         unused


DestroyImageBuffers

1        see major-opcode  major-opcode
1       2                  minor-opcode
2       2                  request length
4       WINDOW             wid


DisplayImageBuffers


1        see major-opcode  major-opcode
2        2+n               requestlength
2        CARD16            min-delay
2        CARD16            max-delay
4n       LISTofBUFFER      buffer-list


SetMultiBufferAttributes

1        see major-opcode  major-opcode
1        4                 minor-opcode
2        3+n               requestlength
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


GetMultiBufferAttributes

1        see major-opcode  major-opcode
1        5                 minor-opcode
2        2                 request length
4        WINDOW            wid
®
1        1                 Reply
1                          unused
2        CARD16            sequencenumber
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


SetBufferAttributes

1        see major-opcode  major-opcode
1        6                 minor-opcode
2        3+n               requestlength
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

GetBufferAttributes

1        see major-opcode  major-opcode
1        7                 minor-opcode
2        2                 request length
4        BUFFER            buffer
->
1        1                 Reply
1                          unused
2        CARD16            sequencenumber
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

GetBufferInfo

1        see major-opcode  major-opcode
1        8                   minor-opcode
2        2                   request length
4        WINDOW              root
®
1        1                   Reply
1                            unused
2        CARD16              sequencenumber
4        2(n+m)              replylength
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

CreateStereoWindow

1        see major-opcode  major-opcode
1        9 minor-opcode
2        11+n                    requestlength
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)
         encodings are the same
         as for CreateWindow
4n       LISTofVALUE             value-list
         encodings are the same
         as for CreateWindow


ClearImageBufferArea

1 see major-opcode 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


