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✏️ Editing: damageproto.txt

The DAMAGE Extension
			Protocol Version 1.1
			 Document Revision 1
			     2007-01-08
			     
			    Keith Packard
			  keithp@keithp.com

Eric Anholt
			   eric@anholt.net
		     Open Source Technology Center
			  Intel Corporation
1. Introduction

Monitoring the regions affected by rendering has wide-spread use, from
VNC-like systems scraping the screen to screen magnifying applications
designed to aid users with limited visual acuity.  The DAMAGE extension is
designed to make such applications reasonably efficient in the face of
server-client latency.

2. Acknowledgements

As usual, the author had significant input from many people, in particular:

+	Havoc Pennington who designed and implemented a Damage extension
 	last year which was then lost to the mists of time.
			    
 +	Bill Haneman whose work on accessibility in the Gnome environment
 	is legendary.

+	Jim Gettys who found a way to avoid streaming damage rectangles
 	to the client in many cases.

+	Owen Taylor who suggested that streaming damage rectangles may
 	be warranted in some cases after all.

3.  Damage Model

We call changes made to pixel contents of windows and pixmaps 'damage'
throughout this extension.  Another notion of 'damage' are drawable regions
which are in need of redisplay to repair the effects of window manipulation
or other data loss.  This extension doesn't deal with this second notion at
all; suggestions on a better term which isn't easily conflated with existing
notions are eagerly solicited.

Damage accumulates as drawing occurs in the drawable.  Each drawing operation
'damages' one or more rectangular areas within the drawable.  The rectangles
are guaranteed to include the set of pixels modified by each operation, but
may include significantly more than just those pixels.  The desire is for
the damage to strike a balance between the number of rectangles reported and
the extraneous area included.  A reasonable goal is for each primitive
object drawn (line, string, rectangle) to be represented as a single
rectangle and for the damage area of the operation to be the union of these
rectangles.

The DAMAGE extension allows applications to either receive the raw
rectangles as a stream of events, or to have them partially processed within
the X server to reduce the amount of data transmitted as well as reduce the
processing latency once the repaint operation has started.

Damage to a window reflects both drawing within the window itself as well as
drawing within any inferior window that affects pixels seen by
IncludeInferiors rendering operations.  To reduce the computational
complexity of this, the DAMAGE extension allows the server to monitor all
rendering operations within the physical target pixel storage that fall
within the bounds of the window.  In a system with a single frame buffer
holding all windows, this means that damage will accumulate for all
rendering operations that lie within the visible part of the window.

The precise reason for this architecture is to enable the Composite
extension which provides multiple pixel storage areas for the screen
contents.

3.1 Additions in the 1.1 version of the protocol

Damage is automatically computed by the X Server for X rendering operations,
but direct rendering extensions have allowed clients to perform rendering
outside of the control of the X Server.  The 1.1 version of the protocol
added a request to allow direct rendering clients to report damage to a
drawable.  Some direct rendering clients, due to architectural limitations,
always perform rendering to the root window, even in when it should be
performed to the backing pixmap in the Composite case.  To provide
less-incorrect rendering in this cases, the direct rendering client should
translate its damage region to screen coordinates and report the damage against
the root window rather than the drawable.

4. Data types

The "Damage" object holds any accumulated damage region and reflects the
relationship between the drawable selected for damage notification and the
drawable for which damage is tracked.

5. Errors

Damage
	A value for a DAMAGE argument does not name a defined DAMAGE.

6. Types

DAMAGE		32-bit value (top three bits guaranteed to be zero)
	
	DamageReportLevel		{ DamageReportRawRectangles,
					  DamageReportDeltaRectangles,
					  DamageReportBoundingBox,
					  DamageReportNonEmpty }

DamageReportRawRectangles

Delivers DamageNotify events each time the screen
		is modified with rectangular bounds that circumscribe
		the damaged area.  No attempt to compress out overlapping
		rectangles is made.

DamageReportDeltaRectangles

Delivers DamageNotify events each time damage occurs
		which is not included in the damage region.  The
		reported rectangles include only the changes to that
		area, not the raw damage data.

DamageReportBoundingBox

Delivers DamageNotify events each time the bounding
		box enclosing the damage region increases in size.
		The reported rectangle encloses the entire damage region,
		not just the changes to that size.

DamageReportNonEmpty

Delivers a single DamageNotify event each time the
		damage rectangle changes from empty to non-empty, and
		also whenever the result of a DamageSubtract request
		results in a non-empty region.

7. Events

DamageNotify

level:				DamageReportLevel
	drawable:			Drawable
	damage:				DAMAGE
	more:				Bool
	timestamp:			Timestamp
	area:				Rectangle
	drawable-geometry:		Rectangle

'more' indicates whether there are subsequent damage events
	being delivered immediately as part of a larger damage region

8. 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 clients 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.

9. Enable Monitoring

DamageCreate

damage:				DAMAGE
		drawable:			Drawable
		level:				DamageReportLevel
		
	Creates a damage object to monitor changes to Drawable

DamageDestroy
		damage:				DAMAGE

Destroys damage.

DamageSubtract

damage:				DAMAGE
		repair:				Region or None
		parts:				Region or None

Synchronously modifies the regions in the following manner:

If repair is None:

1)	if parts is not None, parts = damage
		2)	damage = <empty>
		
	    Otherwise:
	
		1)	tmp = damage INTERSECT repair
		2)	damage = damage - tmp
		3)	if parts is not None, parts = tmp
		4)	Generate DamageNotify for remaining damage areas

DamageAdd

drawable:			Drawable
		region:				Region

Reports damage of the region within the given drawable.  This may be
	used by direct rendering clients to report damage that the server would
	otherwise be unaware of.  The damage region is relative to the origin
	of the drawable.

Damage posted in this way will appear in DamageNotify events as normal,
	and also in server internal damage tracking (for shadow framebuffer
	updates, pixmap damage, and other uses).

MR SKB