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.

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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 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 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 this screen

The XdbeVisualInfo structure has the following fields:


VisualID         visual    one visual ID that supports double-buffering
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 buffers
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 from
XdbeBackBuffer      buffer        resource id
unsigned long       serial        serial number of failed request
unsigned char       error code    error base + XdbeBadBuffer
unsigned char       request code  Major op-code of failed request
unsigned char       minor code    Minor op-code of failed request

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."

