Double Buffer Extension Library

X Consortium Standard

Ian Elliot

   Hewlett-Packard Company

David P. Wiggins

   X Consortium, Inc

   X Version 11, Release 7.7

   Version 1.0

   Copyright © 1989 X Consortium Inc, Digital Equipment
   Corporation

   Copyright © 1992 X Consortium Inc, Intergraph Corporation

   Copyright © 1993 X Consortium Inc, Silicon Graphics, Inc.

   Copyright © 1994, 1995 X Consortium Inc, 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. C Language Binding

        Types
        C Functions
        Errors

   5. Acknowledgements
   6. 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. 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 window'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 window'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 window'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 window'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. C Language Binding

   Table of Contents

   Types
   C Functions
   Errors

   All identifier The header for this extension is
   <X11/extensions/Xdbe.h>. names provided by this header begin
   with Xdbe.

Types

   The type XdbeBackBuffer is a Drawable.

   The type XdbeSwapAction can be one of the constants
   XdbeUndefined, XdbeBackground, XdbeUntouched, or XdbeCopied.

C Functions

   The C functions provide direct access to the protocol and add
   no additional semantics. For complete details on the effects of
   these functions, refer to the appropriate protocol request,
   which can be derived by replacing Xdbe at the start of the
   function name with DBE. All functions that have return type
   Status will return nonzero for success and zero for failure.

   Status XdbeQueryExtension(Display *dpy, int
   *major_version_return, int *minor_version_return);

   XdbeQueryExtension sets major version return and minor version
   return to the major and minor DBE protocol version supported by
   the server. If the DBE library is compatible with the version
   returned by the server, it returns nonzero. If dpy does not
   support the DBE extension, or if there was an error during
   communication with the server, or if the server and library
   protocol versions are incompatible, it returns zero. No other
   Xdbe functions may be called before this function. If a client
   violates this rule, the effects of all subsequent Xdbe calls
   that it makes are undefined.

   XdbeScreenVisualInfo *XdbeGetVisualInfo(Display *dpy, Drawable
   *screen_specifiers, int *num_screens);

   XdbeGetVisualInfo returns information about which visuals
   support double buffering. The argument num_screens specifies
   how many elements there are in the screen_specifiers list. Each
   drawable in screen_specifiers designates a screen for which the
   supported visuals are being requested. If num_screens is zero,
   information for all screens is requested. In this case, upon
   return from this function, num_screens will be set to the
   number of screens that were found. If an error occurs, this
   function returns NULL; otherwise, it returns a pointer to a
   list of XdbeScreenVisualInfo structures of length num_screens.
   The nth element in the returned list corresponds to the nth
   drawable in the screen_specifiers list, unless element in the
   returned list corresponds to the nth screen of the server,
   starting with screen zero.

   The XdbeScreenVisualInfo structure has the following fields:

   int                     count      number of items in visinfo
   XdbeVisualInfo*    visinfo     list of visuals and depths for t
   his screen

   The XdbeVisualInfo structure has the following fields:

   VisualID         visual    one visual ID that supports double-b
   uffering
   int              depth     depth of visual in bits
   int              perflevel  performance level of visual

   void XdbeFreeVisualInfo XdbeGetVisualInfo(XdbeScreenVisualInfo
   *visual_info);

   XdbeFreeVisualInfo frees the list of XdbeScreenVisualInfo
   returned by XdbeGetVisualInfo.

   XdbeBackBuffer XdbeAllocateBackBufferName(Display *dpy, Window
   *window, XdbeSwapAction swap_action);

   XdbeAllocateBackBufferName returns a drawable ID used to refer
   to the back buffer of the specified window. The swap_action is
   a hint to indicate the swap_action that will likely be used in
   subsequent calls to XdbeSwapBuffers. The actual swap_action
   used in calls to XdbeSwapBuffers does not have to be the same
   as the swap_action passed to this function, though clients are
   encouraged to provide accurate information whenever possible.

   Status XdbeDeallocateBackBufferName(Display *dpy,
   XdbeBackBuffer buffer);

   XdbeDeallocateBackBufferName frees the specified drawable ID,
   buffer, that was obtained via XdbeAllocateBackBufferName. The
   buffer must be a valid name for the back buffer of a window, or
   an XdbeBadBuffer error results.

   Status XdbeSwapBuffers(Display *dpy, XdbeSwapInfo *swap_info,
   int num_windows);

   XdbeSwapBuffers swaps the front and back buffers for a list of
   windows. The argument num_windows specifies how many windows
   are to have their buffers swapped; it is the number of elements
   in the swap_info array. The argument swap_info specifies the
   information needed per window to do the swap.

   The XdbeSwapInfo structure has the following fields:

   Window              swap_window    window for which to swap buf
   fers
   XdbeSwapAction      swap_action    swap action to use for this
   swap window

   Status XdbeBeginIdiom(Display *dpy);

   XdbeBeginIdiom marks the beginning of an idiom sequence. See
   the section called "Complex Swap Actions" for a complete
   discussion of idioms.

   Status XdbeEndIdiom(Display *dpy);

   XdbeEndIdiom marks the end of an idiom sequence.

   XdbeBackBufferAttributes *XdbeGetBackBufferAttributes(Display
   *dpy, XdbeBackBuffer buffer);

   XdbeGetBackBufferAttributes returns the attributes associated
   with the specified buffer.

   The XdbeBackBufferAttributes structure has the following
   fields:

   Window           window           window that buffer belongs to

   If buffer is not a valid XdbeBackBuffer, window is set to None.

   The returned XdbeBackBufferAttributes structure can be freed
   with the Xlib function XFree.

Errors

   The XdbeBufferError structure has the following fields:

   int                 type
   Display *           display       Display the event was read fr
   om
   XdbeBackBuffer      buffer        resource id
   unsigned long       serial        serial number of failed reque
   st
   unsigned char       error code    error base + XdbeBadBuffer
   unsigned char       request code  Major op-code of failed reque
   st
   unsigned char       minor code    Minor op-code of failed reque
   st

Chapter 5. 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 6. 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."
