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✏️ Editing: compositeproto.txt

Composite Extension
			     Version 0.4
			      2007-7-3
			    Keith Packard
			  keithp@keithp.com
			    Deron Johnson
     		        deron.johnson@sun.com

1. Introduction

Many user interface operations would benefit from having pixel contents of
window hierarchies available without respect to sibling and antecedent
clipping. In addition, placing control over the composition of these pixel
contents into a final screen image in an external application will enable
a flexible system for dynamic application content presentation.

2. Acknowledgements

This small extension has been brewing for several years, contributors to
both early prototypes and the final design include:

+	Bill Haneman for motivating the ability to magnify occluded windows
 	with his work on accessibility

+	Carsten Haitzler for Enlightenment, the original eye-candy window
 	manager which demonstrated that clever hacks are an awfully
	close substitute for changes in the underlying system.

+	Jim Gettys for key insights into the relationship between damage
 	events and per-window pixmap usage

+	Mike Harris and Owen Taylor for figuring out what to call it.

+	Deron Johnson for the Looking Glass implementation and
 	a prototype of the coordinate transformation mechanism.

+	Ryan Lortie for helping figure out reasonable parent clipping
	semantics in the presence of manual redirected children.

3. Architecture

The composite extension provides three related mechanisms:

1.	Per-hierarchy storage. The rendering of an entire hierarchy of windows
 	is redirected to off-screen storage. The pixels of that hierarchy
	are available whenever it is viewable. Storage is automatically
	reallocated when the top level window changes size. Contents beyond
	the geometry of the top window are not preserved.

2.	Automatic shadow update. When a hierarchy is rendered off-screen,
 	the X server provides an automatic mechanism for presenting those
	contents within the parent window. The implementation is free to
	make this update lag behind actual rendering operations by an
	unspecified amount of time. This automatic update mechanism may
	be disabled so that the parent window contents can be completely
	determined by an external application.

3.	External parent - child pointer coordinate transformation.
 	When a hierarchy is under manual compositing, the relationship
	of coordinates within the parent to those in the child may
	not be known within the X server. This mechanism provides
	for redirection of these transformations through a client.

Per-hierarchy storage may be created for individual windows or for all
children of a window. Manual shadow update may be selected by only a single
application for each window; manual update may also be selected on a
per-window basis or for each child of a window. Detecting when to update
may be done with the Damage extension.

The off-screen storage includes the window contents, its borders and the
contents of all descendants.

3.1 NameWindowPixmap

Version 0.2 of the protocol introduces a mechanism for associating an XID
with the off-screen pixmap used to store these contents. This can be used
to hold onto window contents after the window is unmapped (and hence animate
it's disappearance), and also to access the border of the window, which is
not reachable through the Window ID itself. A new pixmap is created each
time the window is mapped or resized; as these events are nicely signalled
with existing events, no additional notification is needed. The old pixmap
will remain allocated as long as the Pixmap ID is left valid, it is
important that the client use the FreePixmap request when it is done with
the contents and to create a new name for the newly allocated pixmap.

In automatic update mode, the X server is itself responsible for presenting
the child window contents within the parent. It seems reasonable, then, for
rendering to the parent window to be clipped so as not to interfere with any
child window content. In an environment with a mixture of manual and
automatic updating windows, rendering to the parent in the area nominally
occupied by a manual update window should be able to affect parent pixel
values in those areas, but such rendering should be clipped to automatic
update windows, and presumably to other manual update windows managed by
other applications. In any of these cases, it should be easy to ensure that
rendering has no effect on any non-redirected windows.

Instead of attempting to define new clipping modes for rendering, the
Composite extension instead defines ClipByChildren rendering to the parent
to exclude regions occupied by redirected windows (either automatic or
manual). The CreateRegionFromBorderClip request can be used along with
IncludeInferiors clipping modes to restrict manual shadow updates to the
apporpriate region of the screen. Bracketing operations with
GrabServer/UngrabServer will permit atomic sequences that can update the
screen without artifact. As all of these operations are asynchronous,
network latency should not adversely affect update latency.

3.2 Composite Overlay Window

Version 0.3 of the protocol adds the Composite Overlay Window, which
provides compositing managers with a surface on which to draw without
interference. This window is always above normal windows and is always
below the screen saver window. It is an InputOutput window whose width
and height are the screen dimensions. Its visual is the root visual
and its border width is zero.  Attempts to redirect it using the
composite extension are ignored.  This window does not appear in the
reply of the QueryTree request. It is also an override redirect window.
These last two features make it invisible to window managers and other X11
clients. The only way to access the XID of this window is via the
CompositeGetOverlayWindow request. Initially, the Composite Overlay
Window is unmapped.

CompositeGetOverlayWindow returns the XID of the Composite Overlay
Window. If the window has not yet been mapped, it is mapped by this
request. When all clients who have called this request have terminated
their X11 connections the window is unmapped.

Composite managers may render directly to the Composite Overlay
Window, or they may reparent other windows to be children of this
window and render to these. Multiple clients may render to the
Composite Overlay Window, create child windows of it, reshape it, and
redefine its input region, but the specific arbitration rules followed
by these clients is not defined by this specification; these policies
should be defined by the clients themselves.

3.3 Clipping semantics redefined

Version 0.4 of the protocol changes the semantics of clipping in the
presence of manual redirect children. In version 0.3, a parent was always
clipped to child windows, independent of the kind of redirection going on.
With version 0.4, the parent is no longer clipped to child windows which are
manually redirected. This means the parent can draw in the child region without using
IncludeInferiors mode, and (perhaps more importantly), it will receive
expose events in those regions caused by other actions. This new behaviour
is not selectable.

4. Errors

The composite extension does not define any new errors.

5. Types

UPDATETYPE	{ Automatic, Manual }

CompositeCoordinate
		child:			Window
		x, y:			CARD16

7. Extension Initialization

The client must negotiate the version of the extension before executing
extension requests. Otherwise, the server will return BadRequest for any
operations other than QueryVersion.

QueryVersion

client-major-version:		CARD32
		client-minor-version:		CARD32

major-version:			CARD32
		minor-version:			CARD32

The client sends the highest supported version to the server and
	the server sends the highest version it supports, but no higher than
	the requested version. Major versions changes can introduce
	incompatibilities in existing functionality, minor version
	changes introduce only backward compatible changes. It is
	the client's responsibility to ensure that the server supports
	a version which is compatible with its expectations. Servers
	are encouraged to support multiple versions of the extension.

8. Hierarchy Redirection

RedirectWindow

window:				Window
		update:				UPDATETYPE

errors: Window, Access, Match

The hierarchy starting at 'window' is directed to off-screen
	storage. 'update' specifies whether the contents are mirrored to 
	the parent window automatically or not. Only one client may specify 
	an update type of Manual, another attempt will result in an
	Access error. When all clients enabling redirection terminate,
	the redirection will automatically be disabled.

The root window may not be redirected. Doing so results in a Match
	error.

RedirectSubwindows

window:				Window
		update				UPDATETYPE

errors: Window, Access

Hierarchies starting at all current and future children of window
	will be redirected as in RedirectWindow. If update is Manual,
	then painting of the window background during window manipulation
	and ClearArea requests is inhibited.

UnredirectWindow:

window:				Window
		update:				UPDATETYPE

errors: Window, Value

Redirection of the specified window will be terminated. If
	the specified window was not selected for redirection by the
	current client, or if the update type does not match the
	current client's previously requested update type, a 'Value'
	error results.

UnredirectSubwindows:

window:				Window
		update:				UPDATETYPE

errors: Window, Value

Redirection of all children of window will be terminated. If
	the specified window was not selected for sub-redirection by the
	current client, or if the update type does not match the
	current client's previously requested update type, a 'Value'
	error results.

9. Clip lists

CreateRegionFromBorderClip

region:				Region
		window:				Window

errors: Window, IDChoice

This request creates a region containing the "usual" border clip
	value; that is the area of the window clipped against siblings and
	the parent. This region can be used to restrict rendering to
	suitable areas while updating only a single window. The region
	is copied at the moment the request is executed; future changes
	to the window hierarchy will not be reflected in this region.

10. Associating a Pixmap ID with the off-screen storage (0.2 and later)

NameWindowPixmap

window:				Window
		pixmap:				Pixmap

errors: Window, Match, IDChoice

This request makes 'pixmap' a reference to the off-screen storage
	for 'window'. This pixmap will remain allocated until freed, even
	if 'window' is unmapped, reconfigured or destroyed. However,
	'window' will get a new pixmap allocated each time it is
	mapped or resized, so this request will need to be reinvoked for
	the client to continue to refer to the storage holding the current
	window contents. Generates a 'Match' error if 'window' is not
	redirected or is not visible.

11. Composite Overlay Window (0.3 and later)

CompositeGetOverlayWindow

window:				Window

overlayWin:			Window

This request returns the XID of the Composite Overlay Window for 
    the screen specified by the argument 'window'. This request 
    indicates that the client wishes to use the Composite Overlay 
    Window of this screen. If this Composite Overlay Window has not 
    yet been mapped, it is mapped by this request.

The Composite Overlay Window for a particular screen will be 
    unmapped when all clients who have invoked this request have 
    also invoked CompositeReleaseOverlayWindow for that screen. Also,
    CompositeReleaseOverlayWindow for a screen will be implicitly 
    called when a client using the Composite Overlay Window on that 
    screen terminates its X11 connection.

CompositeReleaseOverlayWindow

window:				Window

This request specifies that the client is no longer using the 
    Composite Overlay Window on the screen specified by the 
    argument 'window'. A screen's Composite Overlay Window is 
    unmapped when there are no longer any clients using it.

MR SKB