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
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.
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.
Table of Contents
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.
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.
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.
Table of Contents
The DBE defines the following requests.
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).
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.
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.
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).
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.
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.
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.
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.
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".
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
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
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 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
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.