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+                            Generic Rootless Layer
+                                 Version 1.0
+                                July 13, 2004
+
+                               Torrey T. Lyons
+                              torrey@xfree86.org
+
+
+Introduction
+
+        The generic rootless layer allows an X server to be implemented 
+on top of another window server in a cooperative manner. This allows the 
+X11 windows and native windows of the underlying window server to 
+coexist on the same screen. The layer is called "rootless" because the root 
+window of the X server is generally not drawn. Instead, each top-level 
+child of the root window is represented as a separate on-screen window by 
+the underlying window server. The layer is referred to as "generic" 
+because it abstracts away the details of the underlying window system and 
+contains code that is useful for any rootless X server. The code for the 
+generic rootless layer is located in xc/programs/Xserver/miext/rootless. To 
+build a complete rootless X server requires a specific rootless 
+implementation, which provides functions that allow the generic rootless 
+layer to interact with the underlying window system.
+
+
+Concepts
+
+        In the context of a rootless X server the term window is used to 
+mean many fundamentally different things. For X11 a window is a DDX 
+resource that describes a visible, or potentially visible, rectangle on the 
+screen. A top-level window is a direct child of the root window. To avoid 
+confusion, an on-screen native window of the underlying window system 
+is referred to as a "frame". The generic rootless layer associates each 
+mapped top-level X11 window with a frame. An X11 window may be said 
+to be "framed" if it or its top-level parent is represented by a frame.
+
+        The generic rootless layer models each frame as being backed at 
+all times by a backing buffer, which is periodically flushed to the screen. 
+If the underlying window system does not provide a backing buffer for 
+frames, this must be done by the rootless implementation. The generic 
+rootless layer model does not assume it always has access to the frames' 
+backing buffers. Any drawing to the buffer will be proceeded by a call to 
+the rootless implementation's StartDrawing() function and StopDrawing() 
+will be called when the drawing is concluded. The address of the frame's 
+backing buffer is returned by the StartDrawing() function and it can 
+change between successive calls.
+
+        Because each frame is assumed to have a backing buffer, the 
+generic rootless layer will stop Expose events being generated when the 
+regions of visibility of a frame change on screen. This is similar to backing 
+store, but backing buffers are different in that they always store a copy of 
+the entire window contents, not just the obscured portions. The price paid 
+in increased memory consumption is made up by the greatly decreased 
+complexity in not having to track and record regions as they are obscured.
+
+
+Rootless Implementation
+
+        The specifics of the underlying window system are provided to the 
+generic rootless layer through rootless implementation functions, compile-
+time options, and runtime parameters. The rootless implementation 
+functions are a list of functions that allow the generic rootless layer to 
+perform operations such as creating, destroying, moving, and resizing 
+frames. Some of the implementation functions are optional. A detailed 
+description of the rootless implementation functions is provided in 
+Appendix A.
+
+        By design, a rootless implementation should only have to include 
+the rootless.h header file. The rootlessCommon.h file contains definitions 
+internal to the generic rootless layer. (If you find you need to use 
+rootlessCommon.h in your implementation, let the generic rootless layer 
+maintainers know. This could be an area where the generic rootless layer 
+should be generalized.) A rootless implementation should also modify 
+rootlessConfig.h to specify compile time options for its platform.
+
+        The following compile-time options are defined in 
+rootlessConfig.h:
+
+      o ROOTLESS_ACCEL: If true, use the optional rootless acceleration
+        functions where possible to a accelerate X11 drawing primitives.
+        If false, all drawing will be done with fb.
+
+      o ROOTLESS_GLOBAL_COORDS: This option controls the way that frame
+        coordinates are passed to the rootless implementation. If false,
+        the coordinates are passed per screen relative to the origin of 
+        the screen the frame is currently on. Some implementations may 
+        prefer to work in a single global coordinate space that spans all 
+        screens. If this option is true, the coordinates are passed after 
+        adding the coordinates of the screen origin and an overall offset of 
+        (rootlessGlobalOffsetX, rootlessGlobalOffsetY).
+
+      o ROOTLESS_PROTECT_ALPHA: By default for a color bit depth of 24 and
+        32 bits per pixel, fb will overwrite the "unused" 8 bits to optimize
+        drawing speed. If this is true, the alpha channel of frames is
+        protected and is not modified when drawing to them. The bits 
+        containing the alpha channel are defined by the macro 
+        RootlessAlphaMask(bpp), which should return a bit mask for 
+        various bits per pixel.
+
+      o ROOTLESS_REDISPLAY_DELAY: Time in milliseconds between updates to
+        the underlying window server. Most operations will be buffered until
+        this time has expired.
+
+      o ROOTLESS_RESIZE_GRAVITY: If the underlying window system supports it,
+        some frame resizes can be optimized by relying on the frame contents
+        maintaining a particular gravity during the resize. In this way less
+        of the frame contents need to be preserved by the generic rootless
+        layer. If true, the generic rootless layer will pass gravity hints
+        during resizing and rely on the frame contents being preserved
+        accordingly.
+
+      o ROOTLESS_TRACK_DAMAGE: The generic rootless layer draws to the
+        frames' backing buffers and periodically flushes the modified
+        regions to the underlying window server. If this option is true,
+        the generic rootless layer will track these damaged regions. 
+        Currently it uses the miRegion code and will not simplify damaged 
+        regions even when updating a bounding region would be more 
+        efficient. Some window systems provide a more efficient way to 
+        track damaged regions. If this option is false, the rootless 
+        implementation function DamageRects() is called whenever a 
+        backing buffer is modified and the rootless implementation is 
+        expected to track the damaged regions itself.
+
+        The following runtime options are defined in rootless.h:
+
+      o rootlessGlobalOffsetX, rootlessGlobalOffsetY: These are only 
+        used if ROOTLESS_GLOBAL_COORDS is true. They specify the global
+        offset that is applied to all screens when converting from
+        screen-local to global coordinates.
+
+      o rootless_CopyBytes_threshold, rootless_FillBytes_threshold, 
+        rootless_CompositePixels_threshold, rootless_CopyWindow_threshold:
+        The minimum number of bytes or pixels for which to use the rootless
+        implementation's respective acceleration function. The rootless
+        acceleration functions are all optional so these will only be used
+        if the respective acceleration function pointer is not NULL.
+
+
+Accelerated Drawing
+
+	The rootless implementation typically does not have direct access 
+to the hardware. Its access to the graphics hardware is generally through 
+the API of the underlying window system. This underlying API may not 
+overlap well with the X11 drawing primitives. The generic rootless layer 
+falls back to using fb for all its 2-D drawing. Providing optional rootless 
+implementation acceleration functions can accelerate some graphics 
+primitives and some window functions. Typically calling through to the 
+underlying window systems API will not speed up these operations for 
+small enough areas. The rootless_*_threshold runtime options allow the 
+rootless implementation to provide hints for when the acceleration 
+functions should be used instead of fb.
+
+
+Alpha Channel Protection
+
+	If the bits per pixel is greater then the color bit depth, the contents 
+of the extra bits are undefined by the X11 protocol. Some window systems 
+will use these extra bits as an alpha channel. The generic rootless layer can 
+be configured to protect these bits and make sure they are not modified by 
+other parts of the X server. To protect the alpha channel 
+ROOTLESS_PROTECT_ALPHA and RootlessAlphaMask(bpp) must be 
+set appropriately as described under the compile time options. This 
+ensures that the X11 graphics primitives do not overwrite the alpha 
+channel in an attempt to optimize drawing. In addition, the window 
+functions PaintWindow() and Composite() must be replaced by alpha 
+channel safe variants. These are provided in rootless/safeAlpha.
+
+
+Credits
+
+	The generic rootless layer was originally conceived and developed 
+by Greg Parker as part of the XDarwin X server on Mac OS X. John 
+Harper made later optimizations to this code but removed its generic 
+independence of the underlying window system. Torrey T. Lyons 
+reintroduced the generic abstractions and made the rootless code suitable 
+for use by other X servers.
+
+
+Appendix A: Rootless Implementation Functions
+
+	The rootless implementation functions are defined in rootless.h. It 
+is intended that rootless.h contains the complete interface that is needed by 
+rootless implementations. The definitions contained in rootlessCommon.h 
+are intended for internal use by the generic rootless layer and are more 
+likely to change.
+
+	Most of these functions take a RootlessFrameID as a parameter. 
+The RootlessFrameID is an opaque object that is returned by the 
+implementation's CreateFrame() function. The generic rootless layer does 
+not use this frame id other than to pass it back to the rootless 
+implementation to indicate the frame to operate on.
+
+/*
+ * Create a new frame.
+ *  The frame is created unmapped.
+ *
+ *  pFrame      RootlessWindowPtr for this frame should be completely
+ *              initialized before calling except for pFrame->wid, which
+ *              is set by this function.
+ *  pScreen     Screen on which to place the new frame
+ *  newX, newY  Position of the frame. These will be identical to pFrame-x,
+ *              pFrame->y unless ROOTLESS_GLOBAL_COORDS is set.
+ *  pNewShape   Shape for the frame (in frame-local coordinates). NULL for
+ *              unshaped frames.
+ */
+typedef Bool (*RootlessCreateFrameProc)
+    (RootlessWindowPtr pFrame, ScreenPtr pScreen, int newX, int newY,
+     RegionPtr pNewShape);
+
+/*
+ * Destroy a frame.
+ *  Drawing is stopped and all updates are flushed before this is called.
+ *
+ *  wid         Frame id
+ */
+typedef void (*RootlessDestroyFrameProc)
+    (RootlessFrameID wid);
+
+/*
+ * Move a frame on screen.
+ *  Drawing is stopped and all updates are flushed before this is called.
+ *
+ *  wid         Frame id
+ *  pScreen     Screen to move the new frame to
+ *  newX, newY  New position of the frame
+ */
+typedef void (*RootlessMoveFrameProc) 
+    (RootlessFrameID wid, ScreenPtr pScreen, int newX, int newY);
+
+/*
+ * Resize and move a frame.
+ *  Drawing is stopped and all updates are flushed before this is called.
+ *
+ *  wid         Frame id
+ *  pScreen     Screen to move the new frame to
+ *  newX, newY  New position of the frame
+ *  newW, newH  New size of the frame
+ *  gravity     Gravity for window contents (rl_gravity_enum). This is always
+ *              RL_GRAVITY_NONE unless ROOTLESS_RESIZE_GRAVITY is set.
+ */
+typedef void (*RootlessResizeFrameProc)
+    (RootlessFrameID wid, ScreenPtr pScreen,
+     int newX, int newY, unsigned int newW, unsigned int newH,
+     unsigned int gravity);
+
+/*
+ * Change frame ordering (AKA stacking, layering).
+ *  Drawing is stopped before this is called. Unmapped frames are mapped by
+ *  setting their ordering.
+ *
+ *  wid         Frame id
+ *  nextWid     Frame id of frame that is now above this one or NULL if this
+ *              frame is at the top.
+ */
+typedef void (*RootlessRestackFrameProc)
+    (RootlessFrameID wid, RootlessFrameID nextWid);
+
+/*
+ * Change frame's shape.
+ *  Drawing is stopped before this is called.
+ *
+ *  wid         Frame id
+ *  pNewShape   New shape for the frame (in frame-local coordinates)
+ *              or NULL if now unshaped.
+ */
+typedef void (*RootlessReshapeFrameProc)
+    (RootlessFrameID wid, RegionPtr pNewShape);
+
+/*
+ * Unmap a frame.
+ *
+ *  wid         Frame id
+ */
+typedef void (*RootlessUnmapFrameProc)
+    (RootlessFrameID wid);
+
+/*
+ * Start drawing to a frame.
+ *  Prepare a frame for direct access to its backing buffer.
+ *
+ *  wid         Frame id
+ *  pixelData   Address of the backing buffer (returned)
+ *  bytesPerRow Width in bytes of the backing buffer (returned)
+ */
+typedef void (*RootlessStartDrawingProc)
+    (RootlessFrameID wid, char **pixelData, int *bytesPerRow);
+
+/*
+ * Stop drawing to a frame.
+ *  No drawing to the frame's backing buffer will occur until drawing
+ *  is started again.
+ *
+ *  wid         Frame id
+ *  flush       Flush drawing updates for this frame to the screen. This
+ *              will always be FALSE if ROOTLESS_TRACK_DAMAGE is set.
+ */
+typedef void (*RootlessStopDrawingProc)
+    (RootlessFrameID wid, Bool flush);
+
+/*
+ * Flush drawing updates to the screen.
+ *  Drawing is stopped before this is called.
+ *
+ *  wid         Frame id
+ *  pDamage     Region containing all the changed pixels in frame-local
+ *              coordinates. This is clipped to the window's clip. This
+ *              will be NULL if ROOTLESS_TRACK_DAMAGE is not set.
+ */
+typedef void (*RootlessUpdateRegionProc)
+    (RootlessFrameID wid, RegionPtr pDamage);
+
+/*
+ * Mark damaged rectangles as requiring redisplay to screen.
+ *  This will only be called if ROOTLESS_TRACK_DAMAGE is not set.
+ *
+ *  wid         Frame id
+ *  nrects      Number of damaged rectangles
+ *  rects       Array of damaged rectangles in frame-local coordinates
+ *  shift_x,    Vector to shift rectangles by
+ *   shift_y
+ */
+typedef void (*RootlessDamageRectsProc)
+    (RootlessFrameID wid, int nrects, const BoxRec *rects,
+     int shift_x, int shift_y);
+
+/*
+ * Switch the window associated with a frame. (Optional)
+ *  When a framed window is reparented, the frame is resized and set to
+ *  use the new top-level parent. If defined this function will be called
+ *  afterwards for implementation specific bookkeeping.
+ *
+ *  pFrame      Frame whose window has switched
+ *  oldWin      Previous window wrapped by this frame
+ */
+typedef void (*RootlessSwitchWindowProc)
+    (RootlessWindowPtr pFrame, WindowPtr oldWin);
+
+/*
+ * Copy bytes. (Optional)
+ *  Source and destinate may overlap and the right thing should happen.
+ *
+ *  width       Bytes to copy per row
+ *  height      Number of rows
+ *  src         Source data
+ *  srcRowBytes Width of source in bytes
+ *  dst         Destination data
+ *  dstRowBytes Width of destination in bytes
+ */
+typedef void (*RootlessCopyBytesProc)
+    (unsigned int width, unsigned int height,
+     const void *src, unsigned int srcRowBytes,
+     void *dst, unsigned int dstRowBytes);
+
+/*
+ * Fill memory with 32-bit pattern. (Optional)
+ *
+ *  width       Bytes to fill per row
+ *  height      Number of rows
+ *  value       32-bit pattern to fill with
+ *  dst         Destination data
+ *  dstRowBytes Width of destination in bytes
+ */
+typedef void (*RootlessFillBytesProc)
+    (unsigned int width, unsigned int height, unsigned int value,
+     void *dst, unsigned int dstRowBytes);
+
+/*
+ * Composite pixels from source and mask to destination. (Optional)
+ *
+ *  width, height   Size of area to composite to in pizels
+ *  function        Composite function built with RL_COMPOSITE_FUNCTION
+ *  src             Source data
+ *  srcRowBytes     Width of source in bytes (Passing NULL means source
+ *                  is a single pixel.
+ *  mask            Mask data
+ *  maskRowBytes    Width of mask in bytes
+ *  dst             Destination data
+ *  dstRowBytes     Width of destination in bytes
+ *
+ *  For src and dst, the first element of the array is the color data. If
+ *  the second element is non-null it implies there is alpha data (which
+ *  may be meshed or planar). Data without alpha is assumed to be opaque.
+ *
+ *  An X11 error code is returned.
+ */
+typedef int (*RootlessCompositePixelsProc)
+    (unsigned int width, unsigned int height, unsigned int function,
+     void *src[2], unsigned int srcRowBytes[2],
+     void *mask, unsigned int maskRowBytes,
+     void *dst[2], unsigned int dstRowBytes[2]);
+
+/*
+ * Copy area in frame to another part of frame. (Optional)
+ *
+ *  wid         Frame id
+ *  dstNrects   Number of rectangles to copy
+ *  dstRects    Array of rectangles to copy
+ *  dx, dy      Number of pixels away to copy area
+ */
+typedef void (*RootlessCopyWindowProc)
+    (RootlessFrameID wid, int dstNrects, const BoxRec *dstRects,
+     int dx, int dy);
+