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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 errors: Window, Value Redirection of the specified window will be terminated. If the specified window was not selected for redirection by the current client, a 'Value' error results. UnredirectWindows: window: Window 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, 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.