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.
     __________________________________________________________

   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 synonomous 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:

   Undefined

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

   Background

   The unobscured region of the new back buffer will be tiled with
   the 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 l
ist
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                  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."
