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.\" $XFree86: xc/doc/man/Xext/dbe/DBE.man,v 3.4 2001/01/27 18:20:19 dawes Exp $
.\"
.TH DBE __libmansuffix__ __xorgversion__ "X FUNCTIONS"
.SH NAME
DBE - Double Buffer Extension
.SH SYNOPSIS
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, with no visual
artifacts.  Thus, only completely rendered images are presented to the user, 
and remain visible during the entire time it takes to render a new frame.  The
result is a flicker-free animation.
.SH DESCRIPTION
.B Concepts
.RS
Normal windows are created using
.B XCreateWindow()
or
.B XCreateSimpleWindow(),
which allocate 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
.I (XIDs)
with the window, for use when referring 
to (i.e., drawing to or reading from) the window's back buffer.
The back buffer name is a drawable of type
.I XdbeBackBuffer.

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 ae treated the same.  In 
particular, here are some important characteristics:

.RS
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, this 
event is considered to apply to both buffers equally.  When a 
double-buffered window is exposed, both buffers are tiled with the 
window background.
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 an
.I XdbeBackBuffer
in any function that expects a drawable.
This enables an application to draw directly into
.I XdbeBackBuffer
in the same fashion as it would draw into any other drawable.

It is an error (Window) to pass an
.I XdbeBackBuffer
in a function that expects a Window.

An
.I XdbeBackBuffer
will never be sent in a reply, event, or error where a Window is specified.

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

If the
.B XClearArea()
or
.B XClearWindow()
function is executed on a
double-buffered window, the same area in both the front and back buffers
is cleared.
.RE

The effect of passing a window to a function that accepts a drawable
is unchanged by this extension.  The window and front buffer are synonymous
with each other.  This includes obeying the
.B XGetImage()
and
.B XGetSubImage()
semantics and the subwindow-mode semantics if a graphics context is
involved.  Regardless of whether the window was explicitly passed in an
.B XGetImage()
or
.B XGetSubImage()
call, or implicitly referenced (i.e., one of
the window's ancestors was passed in the function), the front (i.e. visible)
buffer is always referenced.
Thus, DBE-naive screen dump clients will always get the front buffer.
.B XGetImage()
and
.B XGetSubImage()
on a back
buffer return 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 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
.I XdbeBackBuffer.

If events, replies, or errors that contain a drawable
(e.g., 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 (e.g., to maintain a constant frame rate) should
investigate the Synchronization Extension, an X Consortium standard.
.RE

.B Window Management Operations

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

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

If the size of a double-buffered window changes, 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,
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
.B XGetGeometry()
function is executed on an
.I XdbeBackBuffer,
the returned x, y, and border-width will be zero.

If the Shape extension
.B ShapeRectangles, ShapeMask, ShapeCombine,
or
.B ShapeOffset
request is executed on a double-buffered window, both 
buffers are reshaped to match the new window shape.  The region difference
D = new shape - old shape is tiled with the window background in both 
buffers, and Expose events are generated for D.
.RE

.B Complex Swap Actions

.RS
DBE has no explicit knowledge of ancillary buffers (e.g. 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 
applications'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 function calls:  
.B XdbeBeginIdiom()
and
.B XdbeEndIdiom().
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
X to clear only certain planes of the back buffers, the application would
make the following calls as a group, and in the following order:

.RS
.B XdbeBeginIdiom().

.B XdbeSwapBuffers()
with XIDs for two windows, each of which uses a swap action of Untouched.

.B XFillRectangle()
to the back buffer of one window.

.B XFillRectangle()
to the back buffer of the other window.

.B XdbeEndIdiom().
.RE

The
.B XdbeBeginIdiom()
and
.B XdbeEndIdiom()
functions 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 non-recognized groups/series of requests.  If these
function calls are made out of order, or are mismatched, no errors are sent,
and the functions are executed as usual, though performance may suffer.

.B XdbeSwapBuffers()
need not be included in an idiom.  For
example, if a swap action of Copied is desired, but only some of the planes
should be copied,
.B XCopyArea()
may be used instead of 
.B XdbeSwapBuffers().
If
.B XdbeSwapBuffers()
is included in an idiom, it should immediately follow the
.B XdbeBeginIdiom()
call.  Also, when the 
.B XdbeSwapBuffers()
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 
.B XFillRectangle()
using a client clip rectangle is done to the window's back
buffer after the
.B XdbeSwapBuffers()
call, 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 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
.B XdbeBeginIdiom(),
idiom, and 
.B XdbeEndIdiom()
calls.  Usage of these functions will ensure best possible
performance across a wide variety of implementations.
.SH SEE ALSO
.I XdbeAllocateBackBufferName(),
.I XdbeBeginIdiom(),
.I XdbeDeallocateBackBufferName(),
.I XdbeEndIdiom(),
.I XdbeFreeVisualInfo(),
.I XdbeGetBackBufferAttributes(),
.I XdbeGetVisualInfo(),
.I XdbeQueryExtension(),
.I XdbeSwapBuffers().