Double Buffer Extension Protocol

X Consortium Standard

Ian Elliott

Hewlett-Packard Company

David P. Wiggins

X Consortium

X Version 11, Release 7.7

Version 1.0

Copyright © 1989, 1992, 1993, 1994 X Consortium, Inc.

Copyright © 1989 Digital Equipment Corporation

Copyright © 1992 Intergraph Corporation

Copyright © 1993 Silicon Graphics, Inc.

Copyright © 1994 Hewlett-Packard Company

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, Intergraph Corporation, Silicon Graphics, Hewlett-Packard, and the
X Consortium make 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.

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Table of Contents

1. Introduction
2. Goals
3. Concepts
    Window Management Operations
    Complex Swap Actions
4. Requests
    DBEGetVersion
    DBEGetVisualInfo
    DBEAllocateBackBufferName
    DBEDeallocateBackBufferName
    DBESwapBuffers
    DBEBeginIdiom
    DBEEndIdiom
    DBEGetBackBufferAttributes
5. Encoding
    Type
    Error
    Request
6. Acknowledgements
7. References

Chapter 1. Introduction

The Double Buffer Extension (DBE) provides a standard way to utilize
double-buffering within the framework of the X Window System. Double-buffering
uses two buffers, called front and back, which hold images. The front buffer is
visible to the user; the back buffer is not. Successive frames of an animation
are rendered into the back buffer while the previously rendered frame is
displayed in the front buffer. When a new frame is ready, the back and front
buffers swap roles, making the new frame visible. Ideally, this exchange
appears to happen instantaneously to the user and with no visual artifacts.
Thus, only completely rendered images are presented to the user, and they
remain visible during the entire time it takes to render a new frame. The
result is a flicker-free animation.

Chapter 2. Goals

This extension should enable clients to:

  • Allocate and deallocate double-buffering for a window.

  • Draw to and read from the front and back buffers associated with a window.

  • Swap the front and back buffers associated with a window.

  • Specify a wide range of actions to be taken when a window is swapped. This
    includes explicit, simple swap actions (defined below), and more complex
    actions (for example, clearing ancillary buffers) that can be put together
    within explicit "begin" and "end" requests (defined below).

  • Request that the front and back buffers associated with multiple
    double-buffered windows be swapped simultaneously.

In addition, the extension should:

  • Allow multiple clients to use double-buffering on the same window.

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

  • Add no new event types.

  • Be reasonably easy to integrate with a variety of direct graphics hardware
    access (DGHA) architectures.

Chapter 3. Concepts

Table of Contents

Window Management Operations
Complex Swap Actions

Normal windows are created using the core CreateWindow request, which allocates
a set of window attributes and, for InputOutput windows, a front buffer, into
which an image can be drawn. The contents of this buffer will be displayed when
the window is visible.

This extension enables applications to use double-buffering with a window. This
involves creating a second buffer, called a back buffer, and associating one or
more back buffer names (XIDs) with the window for use when referring to (that
is, drawing to or reading from) the window's back buffer. The back buffer name
is a DRAWABLE of type BACKBUFFER.

DBE provides a relative double-buffering model. One XID, the window, always
refers to the front buffer. One or more other XIDs, the back buffer names,
always refer to the back buffer. After a buffer swap, the window continues to
refer to the (new) front buffer, and the back buffer name continues to refer to
the (new) back buffer. Thus, applications and toolkits that want to just render
to the back buffer always use the back buffer name for all drawing requests to
the window. Portions of an application that want to render to the front buffer
always use the window XID for all drawing requests to the window.

Multiple clients and toolkits can all use double-buffering on the same window.
DBE does not provide a request for querying whether a window has
double-buffering support, and if so, what the back buffer name is. Given the
asynchronous nature of the X Window System, this would cause race conditions.
Instead, DBE allows multiple back buffer names to exist for the same window;
they all refer to the same physical back buffer. The first time a back buffer
name is allocated for a window, the window becomes double-buffered and the back
buffer name is associated with the window. Subsequently, the window already is
a double-buffered window, and nothing about the window changes when a new back
buffer name is allocated, except that the new back buffer name is associated
with the window. The window remains double-buffered until either the window is
destroyed or until all of the back buffer names for the window are deallocated.

In general, both the front and back buffers are treated the same. In
particular, here are some important characteristics:

  • Only one buffer per window can be visible at a time (the front buffer).

  • Both buffers associated with a window have the same visual type, depth,
    width, height, and shape as the window.

  • Both buffers associated with a window are "visible" (or "obscured") in the
    same way. When an Expose event is generated for a window, both buffers
    should be considered to be damaged in the exposed area. Damage that occurs
    to either buffer will result in an Expose event on the window. When a
    double-buffered window is exposed, both buffers are tiled with the window
    background, exactly as stated by the core protocol. Even though the back
    buffer is not visible, terms such as obscure apply to the back buffer as
    well as to the front buffer.

  • It is acceptable at any time to pass a BACKBUFFER in any request, notably
    any core or extension drawing request, that expects a DRAWABLE. This
    enables an application to draw directly into BACKBUFFERs in the same
    fashion as it would draw into any other DRAWABLE.

  • It is an error (Window) to pass a BACKBUFFER in a core request that expects
    a Window.

  • A BACKBUFFER will never be sent by core X in a reply, event, or error where
    a Window is specified.

  • If core X11 backing-store and save-under applies to a double-buffered
    window, it applies to both buffers equally.

  • If the core ClearArea request is executed on a double-buffered window, the
    same area in both the front and back buffers is cleared.

The effect of passing a window to a request that accepts a DRAWABLE is
unchanged by this extension. The window and front buffer are synonymous with
each other. This includes obeying the GetImage semantics and the subwindow-mode
semantics if a core graphics context is involved. Regardless of whether the
window was explicitly passed in a GetImage request, or implicitly referenced
(that is, one of the windo's ancestors was passed in the request), the front
(that is, visible) buffer is always referenced. Thus, DBE-naive screen dump
clients will always get the front buffer. GetImage on a back buffer returns
undefined image contents for any obscured regions of the back buffer that fall
within the image.

Drawing to a back buffer always uses the clip region that would be used to draw
to the front buffer with a GC subwindow-mode of ClipByChildren. If an ancestor
of a double-buffered window is drawn to with a core GC having a subwindow-mode
of IncludeInferiors, the effect on the double-buffered window's back buffer
depends on the depth of the double-buffered window and the ancestor. If the
depths are the same, the contents of the back buffer of the double-buffered
window are not changed. If the depths are different, the contents of the back
buffer of the double-buffered window are undefined for the pixels that the
IncludeInferiors drawing touched.

DBE adds no new events. DBE does not extend the semantics of any existing
events with the exception of adding a new DRAWABLE type called BACKBUFFER. If
events, replies, or errors that contain a DRAWABLE (for example,
GraphicsExpose) are generated in response to a request, the DRAWABLE returned
will be the one specified in the request.

DBE advertises which visuals support double-buffering.

DBE does not include any timing or synchronization facilities. Applications
that need such facilities (for example, to maintain a constant frame rate)
should investigate the Synchronization Extension, an X Consortium standard.

Window Management Operations

The basic philosophy of DBE is that both buffers are treated the same by core X
window management operations.

When the core DestroyWindow is executed on a double-buffered window, both
buffers associated with the window are destroyed, and all back buffer names
associated with the window are freed.

If the core ConfigureWindow request changes the size of a window, both buffers
assume the new size. If the windo's size increases, the effect on the buffers
depends on whether the implementation honors bit gravity for buffers. If bit
gravity is implemented, then the contents of both buffers are moved in
accordance with the windo's bit gravity (see the core ConfigureWindow request),
and the remaining areas are tiled with the window background. If bit gravity is
not implemented, then the entire unobscured region of both buffers is tiled
with the window background. In either case, Expose events are generated for the
region that is tiled with the window background.

If the core GetGeometry request is executed on a BACKBUFFER, the returned x, y,
and border-width will be zero.

If the Shape extension ShapeRectangles, ShapeMask, ShapeCombine, or ShapeOffset
request is executed on a double-buffered window, both buffers are reshaped to
match the new window shape. The region difference is the following:


   D = newshape - oldshape

It is tiled with the window background in both buffers, and Expose events are
generated for D.

Complex Swap Actions

DBE has no explicit knowledge of ancillary buffers (for example, depth buffers
or alpha buffers), and only has a limited set of defined swap actions. Some
applications may need a richer set of swap actions than DBE provides. Some DBE
implementations have knowledge of ancillary buffers, and/or can provide a rich
set of swap actions. Instead of continually extending DBE to increase its set
of swap actions, DBE provides a flexible "idiom" mechanism. If an application's
needs are served by the defined swap actions, it should use them; otherwise, it
should use the following method of expressing a complex swap action as an
idiom. Following this policy will ensure the best possible performance across a
wide variety of implementations.

As suggested by the term "idiom," a complex swap action should be expressed as
a group/series of requests. Taken together, this group of requests may be
combined into an atomic operation by the implementation, in order to maximize
performance. The set of idioms actually recognized for optimization is
implementation dependent. To help with idiom expression and interpretation, an
idiom must be surrounded by two protocol requests: DBEBeginIdiom and
DBEEndIdiom. Unless this begin-end pair surrounds the idiom, it may not be
recognized by a given implementation, and performance will suffer.

For example, if an application wants to swap buffers for two windows, and use
core X to clear only certain planes of the back buffers, the application would
issue the following protocol requests as a group, and in the following order:

  • DBEBeginIdiom request.

  • DBESwapBuffers request with XIDs for two windows, each of which uses a swap
    action of Untouched.

  • Core X PolyFillRectangle request to the back buffer of one window.

  • Core X PolyFillRectangle request to the back buffer of the other window.

  • DBEEndIdiom request.

The DBEBeginIdiom and DBEEndIdiom requests do not perform any actions
themselves. They are treated as markers by implementations that can combine
certain groups/series of requests as idioms, and are ignored by other
implementations or for nonrecognized groups/series of requests. If these
requests are sent out of order, or are mismatched, no errors are sent, and the
requests are executed as usual, though performance may suffer.

An idiom need not include a DBESwapBuffers request. For example, if a swap
action of Copied is desired, but only some of the planes should be copied, a
core X CopyArea request may be used instead of DBESwapBuffers. If
DBESwapBuffers is included in an idiom, it should immediately follow the
DBEBeginIdiom request. Also, when the DBESwapBuffers is included in an idiom,
that request's swap action will still be valid, and if the swap action might
overlap with another request, then the final result of the idiom must be as if
the separate requests were executed serially. For example, if the specified
swap action is Untouched, and if a PolyFillRectangle using a client clip
rectangle is done to the windo's back buffer after the DBESwapBuffers request,
then the contents of the new back buffer (after the idiom) will be the same as
if the idiom was not recognized by the implementation.

It is highly recommended that Application Programming Interface (API) providers
define, and application developers use, "convenience" functions that allow
client applications to call one procedure that encapsulates common idioms.
These functions will generate the DBEBeginIdiom request, the idiom requests,
and DBEEndIdiom request. Usage of these functions will ensure best possible
performance across a wide variety of implementations.

Chapter 4. Requests

Table of Contents

DBEGetVersion
DBEGetVisualInfo
DBEAllocateBackBufferName
DBEDeallocateBackBufferName
DBESwapBuffers
DBEBeginIdiom
DBEEndIdiom
DBEGetBackBufferAttributes

The DBE defines the following requests.

DBEGetVersion

This request returns the major and minor version numbers of this extension.

DBEGetVersion

client-major-version  CARD8
client-minor-version  CARD8
=>                     
server-major-version  CARD8
server-minor-version  CARD8

The client-major-version and client-minor-version numbers indicate what version
of the protocol the client wants the server to implement. The
server-major-version and the server-minor-version numbers returned indicate the
protocol this extension actually supports. This might not equal the version
sent by the client. An implementation can (but need not) support more than one
version simultaneously. The server-major-version and server-minor-version allow
the creation of future revisions of the DBE protocol that may be necessary. In
general, the major version would increment for incompatible changes, and the
minor version would increment for small, upward-compatible changes. Servers
that support the protocol defined in this document will return a
server-major-version of one (1), and a server-minor-version of zero (0).

The DBE client must issue a DBEGetVersion request before any other double
buffering request in order to negotiate a compatible protocol version;
otherwise, the client will get undefined behavior (DBE may or may not work).

DBEGetVisualInfo

This request returns information about which visuals support double buffering.

DBEGetVisualInfo

screen-specifiers LISTofDRAWABLE
=>                 
visinfo           LISTofSCREENVISINFO

where:

SCREENVISINFO LISTofVISINFO
VISINFO       [ visual: VISUALID
              depth: CARD8
              perflevel: CARD8 ]

Errors: Drawable

All of the values passed in screen-specifiers must be valid DRAWABLEs (or a
Drawable error results). For each drawable in screen-specifiers, the reply will
contain a list of VISINFO structures for visuals that support double-buffering
on the screen on which the drawable resides. The visual member specifies the
VISUALID. The depth member specifies the depth in bits for the visual. The
perflevel is a performance hint. The only operation defined on a perflevel is
comparison to a perflevel of another visual on the same screen. The visual
having the higher perflevel is likely to have better double-buffer graphics
performance than the visual having the lower perflevel. Nothing can be deduced
from any of the following: the magnitude of the difference of two perflevels, a
perflevel value in isolation, or comparing perflevels from different servers.

If the list of screen-specifiers is empty, information for all screens is
returned, starting with screen zero.

DBEAllocateBackBufferName

This request allocates a drawable ID used to refer to the back buffer of a
window.

DBEAllocateBackBufferName

window            WINDOW
back-buffer-name  BACKBUFFER
swap-action-hint  SWAPACTION

Errors: Alloc, Value, IDChoice, Match, Window

If the window is not already a double-buffered window, the window becomes
double-buffered, and the back-buffer-name is associated with the window. The
swap-action-hint tells the server which swap action is most likely to be used
with the window in subsequent DBESwapBuffers requests. The swap-action-hint
must have one of the values specified for type SWAPACTION (or a Value error
results). See the description of the DBESwapBuffers request for a complete
discussion of swap actions and the SWAPACTION type.

If the window already is a double-buffered window, nothing about the window
changes, except that an additional back-buffer-name is associated with the
window. The window remains double-buffered until either the window is
destroyed, or until all of the back buffer names for the window are
deallocated.

The window passed into the request must be a valid WINDOW (or a Window error
results). The window passed into the request must be an InputOutput window (or
a Match error results). The visual of the window must be in the list returned
by DBEGetVisualInfo (or a Match error results). The back-buffer-name must be in
the range assigned to the client, and must not already be in use (or an
IDChoice error results). If the server cannot allocate all resources associated
with turning on double-buffering for the window, an Alloc error results, the
windo's double-buffer status (whether it is already double-buffered or not)
remains unchanged, and the back-buffer-name is freed.

DBEDeallocateBackBufferName

This request frees a drawable ID that was obtained by
DBEAllocateBackBufferName.

DBEDeallocateBackBufferName

back-buffer-name  BACKBUFFER

Errors: Buffer

The back-buffer-name passed in the request is freed and no longer associated
with the window. If this is the last back-buffer-name associated with the
window, then the back buffer is no longer accessible to clients, and all
double-buffering resources associated with the window may be freed. The
window's current front buffer remains the front buffer.

The back-buffer-name must be a valid BACKBUFFER associated with a window (or a
Buffer error results).

DBESwapBuffers

This request swaps the buffers for all windows listed, applying the appropriate
swap action for each window.

DBESwapBuffers

windows LISTofSWAPINFO

where:

SWAPINFO   [ window: WINDOW
           swap-action: SWAPACTION ]
SWAPACTION { Undefined, Background, Untouched, Copied }

Errors: Match, Window, Value

Each window passed into the request must be a valid WINDOW (or a Window error
results). Each window passed into the request must be a double-buffered window
(or a Match error results). Each window passed into the request must only be
listed once (or a Match error results). Each swap-action in the list must have
one of the values specified for type SWAPACTION (or a Value error results). If
an error results, none of the valid double-buffered windows will have their
buffers swapped.

The swap-action determines what will happen to the new back buffer of the
window it is paired with in the list in addition to making the old back buffer
become visible. The defined actions are as follows:

           The contents of the new back buffer become undefined. This may be
Undefined  the most efficient action since it allows the implementation to
           discard the contents of the buffer if it needs to.

           The unobscured region of the new back buffer will be tiled with the
Background window background. The background action allows devices to use a
           fast clear capability during a swap.

Untouched  The unobscured region of the new back buffer will be unmodified by
           the swap.

Copied     The unobscured region of the new back buffer will be the contents of
           the old back buffer.

If DBESwapBuffers is included in a "swap and clear" type of idiom, it must
immediately follow the DBEBeginIdiom request.

DBEBeginIdiom

This request informs the server that a complex swap will immediately follow
this request.

DBEBeginIdiom

As previously discussed, a complex swap action is a group/series of requests
that, taken together, may be combined into an atomic operation by the
implementation. The sole function of this request is to serve as a "marker"
that the server can use to aid in idiom processing. The server is free to
implement this request as a no-op.

DBEEndIdiom

This request informs the server that a complex swap has concluded.

DBEEndIdiom

The sole function of this request is to serve as a "marker" that the server can
use to aid in idiom processing. The server is free to implement this request as
a no-op.

DBEGetBackBufferAttributes

This request returns information about a back buffer.

DBEGetBackBufferAttributes

back-buffer-name BACKBUFFER
=>                
attributes       BUFFER_ATTRIBUTES

where:

BUFFER_ATTRIBUTES: [ window: WINDOW ]

If back-buffer-name is a valid BACKBUFFER, the window field of the attributes
in the reply will be the window which has the back buffer that back-buffer-name
refers to. If back-buffer-name is not a valid BACKBUFFER, the window field of
the attributes in the reply will be None.

Chapter 5. Encoding

Table of Contents

Type
Error
Request

Please refer to the X11 Protocol Encoding document as this section uses
syntactic conventions and data types established there.

The name of this extension is "DOUBLE-BUFFER".

Type

The following new types are used by the extension.

BACKBUFFER: XID

SWAPACTION

#x00  Undefined
#x01  Background
#x02  Untouched
#x03  Copied

SWAPINFO

4     WINDOW                 window
1     SWAPACTION             swap action
3                            unused

VISINFO

4     VISUALID                visual
1     CARD8                   depth
1     CARD8                   perflevel
2                             unused

SCREENVISINFO

4     CARD32                  n, number in list
8n    LISTofVISINFO           n VISINFOs

BUFFER_ATTRIBUTES

4     WINDOW                 window

Error

Buffer

1     0                       error
1     error base + 0          code
2     CARD16                  sequence number
4     CARD32                  bad buffer
2     CARD16                  minor-opcode
1     CARD8                   major-opcode
21                            unused

Request

DBEGetVersion

1     CARD8                  major-opcode
1     0                      minor-opcode
2     2                      request length
1     CARD8                  client-major-version
1     CARD8                  client-minor-version
2                            unused
=>
1                            unused
2     CARD16                 sequence number
4     0                      reply length
1     CARD8                  server-major-version
1     CARD8                  server-minor-version
22                           unused

DBEAllocateBackBufferName

1     CARD8                  major-opcode
1     1                      minor-opcode
2     4                      request length
4     WINDOW                 window
4     BACKBUFFER             back buffer name
1     SWAPACTION             swap action hint
3                            unused

DBEDeallocateBackBufferName

1     CARD8                  major-opcode
1     2                      minor-opcode
2     2                      request length
4     BACKBUFFER             back buffer name

DBESwapBuffers

1     CARD8                   major-opcode
1     3                       minor-opcode
2     2+2n                    request length
4     CARD32                  n, number of window/swap action pairs in list
8n    LISTofSWAPINFO          window/swap action pairs

DBEBeginIdiom

1     CARD8                   major-opcode
1     4                       minor-opcode
2     1                       request length

DBEEndIdiom

1     CARD8                   major-opcode
1     5                       minor-opcode
2     1                       request length

DBEGetVisualInfo

1     CARD8                   major-opcode
1     6                       minor-opcode
2     2+n                     request length
4     CARD32                  n, number of screen specifiers in list
4n    LISTofDRAWABLE          n screen specifiers
=>
1     1                       Reply
1                             unused
2     CARD16                  sequence number
4     CARD32                  j, reply length
4     CARD32                  m, number of SCREENVISINFOs in list
20                            unused
4j    LISTofSCREENVISINFO     m SCREENVISINFOs

DBEGetBackBufferAttributes

1     CARD8                   major-opcode
1     7                       minor-opcode
2     2                       request length
4     BACKBUFFER              back-buffer-name
=>
1                             unused
2     CARD16                  sequence number
4     0                       reply length
4     BUFFER_ATTRIBUTES       attributes
20                            unused

Chapter 6. Acknowledgements

We wish to thank the following individuals who have contributed their time and
talent toward shaping the DBE specification:

T. Alex Chen, IBM; Peter Daifuku, Silicon Graphics, Inc.; Ian Elliott,
Hewlett-Packard Company; Stephen Gildea, X Consortium, Inc.; Jim Graham, Sun;
Larry Hare, AGE Logic; Jay Hersh, X Consortium, Inc.; Daryl Huff, Sun; Deron
Dann Johnson, Sun; Louis Khouw, Sun; Mark Kilgard, Silicon Graphics, Inc.; Rob
Lembree, Digital Equipment Corporation; Alan Ricker, Metheus; Michael
Rosenblum, Digital Equipment Corporation; Bob Scheifler, X Consortium, Inc.;
Larry Seiler, Digital Equipment Corporation; Jeanne Sparlin Smith, IBM; Jeff
Stevenson, Hewlett-Packard Company; Walter Strand, Metheus; Ken Tidwell,
Hewlett-Packard Company; and David P. Wiggins, X Consortium, Inc.

Mark provided the impetus to start the DBE project. Ian wrote the first draft
of the specification. David served as architect.

Chapter 7. References

Jeffrey Friedberg, Larry Seiler, and Jeff Vroom, "Multi-buffering Extension
Specification Version 3.3."

Tim Glauert, Dave Carver, Jim Gettys, and David P. Wiggins, "X Synchronization
Extension Version 3.0."

