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-  Scaled Window Support in DMX
-  Rickard E. Faith and Kevin E. Martin
-  15 October 2003 (created 19 September 2003)
-
-  This document investigates the possibility of adding scaled window
-  support to the DMX X server, thereby allowing a window or some
-  selected part of the logical DMX area to be displayed using a scaling
-  factor.  For example, this might allow the contents of a window to be
-  magnified for easier viewing.  In particular, scaling for the VNC
-  client is explored.  _C_o_p_y_r_i_g_h_t _2_0_0_3 _b_y _R_e_d _H_a_t_, _I_n_c_._, _R_a_l_e_i_g_h_, _N_o_r_t_h
-  _C_a_r_o_l_i_n_a
-
-  ______________________________________________________________________
-
-  Table of Contents
-
-
-  1. Introduction
-     1.1 DMX
-     1.2 Problem Statement
-     1.3 Task
-
-  2. Previous Work
-     2.1 VNC
-        2.1.1 Scaling under VNC
-     2.2 The X Video Extension
-
-  3. Possible Solutions
-     3.1 VNC-like Scaling
-        3.1.1 Software Scaling
-        3.1.2 Scaling with the X Video Extension
-           3.1.2.1 Implementing the X Video Extension for DMX
-           3.1.2.2 Supporting RGB formats for the X Video Extension
-        3.1.3 Scaling with an XPutImageScaled Extension
-        3.1.4 Scaling with an XCopyAreaScaled Extension
-        3.1.5 Scaling with OpenGL
-     3.2 Application-transparent Scaling for DMX
-        3.2.1 Back-end Scaling Without Disconnect/Reconnect
-        3.2.2 Back-end Scaling With Disconnect/Reconnect
-        3.2.3 Server-side Scaling
-     3.3 XCreateScaledWindow API
-        3.3.1 XCreateWindow
-        3.3.2 XSetWindowAttributes
-        3.3.3 XGetWindowAttributes, XGetGeometry
-        3.3.4 Popup and Child window positions
-        3.3.5 Events
-        3.3.6 Implementation
-
-  4. Conclusion and Recommendations
-
-
-  ______________________________________________________________________
-
-  11..  IInnttrroodduuccttiioonn
-
-  11..11..  DDMMXX
-
-  The DMX X server (Xdmx) is a proxy server that is designed to allow X
-  servers on multiple machines to be combined into a single multi-headed
-  X server.  Combined with Xinerama, these heads can appear as a single
-  very high-resolution screen.  Typical applications include the
-  creation of a video wall with 16 1280x1024 displays arranged in a
-  rectangle, for a total resolution of of 5120x4096.
-
-
-
-  11..22..  PPrroobblleemm SSttaatteemmeenntt
-
-  Applications displayed on a physically large video wall that provides
-  high pixel-resolution may be difficult to see, especially if the
-  application is designed for use on a typical desktop computer with a
-  relatively small display located close to the human operator.  The
-  goal of this paper is to describe and discuss solutions to this
-  problem.
-
-  The original driving problem for this work is to provide scaling for
-  the vncviewer application when displayed using DMX (VNC scaling is
-  currently available only with the Windows client, and there is no plan
-  to extend that capability to other clients).  While this specific
-  problem will be addressed in this paper, the general solution space
-  will also be explored, since this may lead to a good solution not only
-  for vncviewer but also for other applications.
-
-  11..33..  TTaasskk
-
-  For reference, here is the original description of the task this paper
-  addresses:
-
-  +o  Scaled window support (for VNC)
-
-     +o  Investigate possibility of implementing a "scaled window"
-        extension:
-
-        +o  Add XCreateScaledWindow call that could be used in place of
-           XCreateWindow
-
-        +o  All primitives drawn to scaled window would be scaled by
-           appropriate (integral?) scaling factor
-
-     +o  Alternate approach: special case VNC support
-
-  22..  PPrreevviioouuss WWoorrkk
-
-  This section reviews relevant previous work.
-
-  22..11..  VVNNCC
-
-  22..11..11..  SSccaalliinngg uunnddeerr VVNNCC
-
-  When using the vncviewer program for Windows, it is possible to
-  specify a scaling factor (as numerator and denominator).  When scaling
-  is in effect, the viewer software uses StretchBlt (instead of BitBlt)
-  to display the pixels for the user.  When this call is made, the
-  viewer already has received all of the pixel information (at full
-  unscaled resolution).
-
-  The scaling in VNC is primitive.  It does not conserve bandwidth, it
-  does not treat textual information differently (i.e., by using a
-  suitably scaled font), and it does not provide any anti-aliasing other
-  than that provided by the underlying (Windows-only) system library.
-
-  22..22..  TThhee XX VViiddeeoo EExxtteennssiioonn
-
-  The X Video Extension is a widely-available extension to the X11
-  protocol that provides support for streaming video.  Integral to this
-  support is the ability to arbitrarily scale the output.  In version
-  2.2 of the X Video specification, support for scaled still images was
-  provided, using both shared memory and traditional transport.  The API
-  for this support uses calls that are quite similar to XCreateWindow,
-  XPutImage, and XShmPutImage.  Currently, most of the drivers
-  implemented in XFree86 only support data in various YUV formats.
-  However, several modern video adaptors support RGB as well.
-  Note, though, that the target output for this scaling is an overlay
-  plane -- so X Video provides functionality that is fundamentally
-  different from that provided by the Windows StrechBlt call.
-
-  33..  PPoossssiibbllee SSoolluuttiioonnss
-
-  This section briefly discusses possible solutions, including major
-  advantages and disadvantages from both the implementation and the end-
-  user programmer standpoint.
-
-  33..11..  VVNNCC--lliikkee SSccaalliinngg
-
-  33..11..11..  SSooffttwwaarree SSccaalliinngg
-
-  The vncviewer application could be modified to provide software
-  scaling.  This is not a general solution, but it does solve one of the
-  goals of this work.
-
-  A prototype of this solution was implemented and a patch against
-  vnc-3.3.7-unixsrc is available in the dmx/external directory.  Because
-  of limited time available for this work, all of the edge cases were
-  not considered and the solution works well mainly for integer scaling.
-
-  Currently, vncviewer writes to the X display with XPutImage,
-  XCopyArea, and XFillRectangle.  All instances of these calls have to
-  be aware of scaling and must round correctly.  In the prototype
-  solution, rounding is incorrect and can cause artifacts.
-
-  A better solution would be to cache all updates to the desktop image
-  in vncviewer and only send the damaged area to the X display with
-  XPutImage.  This would allow the damaged area to be computed so that
-  rounding errors do not create artifacts.  This method is probably
-  similar to what is used in the Window client.  (The whole VNC suite is
-  being re-written in C++ and the forthcoming version 4 has not been
-  evaluated.)
-
-  33..11..22..  SSccaalliinngg wwiitthh tthhee XX VViiddeeoo EExxtteennssiioonn
-
-  The scaling in the Windows vncviewer application makes use of a scaled
-  blit that is supplied by the underlying system library.  Several video
-  cards currently provide support for a scaled blit, and some X servers
-  (including XFree86) expose this capability to applications via the
-  XvPutImage interface of the X Video Extension.  The capability exposed
-  by XvPutImage results in the scaled image being drawn to an overlay
-  plane.  Most video cards also provide support for a scaled blit into
-  the normal output planes, but this is not exposed via XvPutImage.
-
-  The vncviewer program could be modified to use the X Video Extension
-  to provide scaling under X11 that is similar to the scaling currently
-  provided under Windows.  Unfortunately, Xdmx does not currently export
-  the X Video Extension, so this would not provide an immediate solution
-  usable with DMX.
-
-  A very early-stage proof-of-concept prototype was implemented and a
-  preliminary patch against vnc-3.3.7-unixsrc is available in the
-  dmx/external directory.  This prototype was implemented to better
-  understand the problems that must be solved to make this solution
-  viable:
-
-  +o  As noted under the software scaling section above, vncviewer writes
-     to the X display with several different calls.  These calls write
-     to the normal output planes and are compatible with XvPutImage,
-     which writes to an overlay plane.  To eliminate artifacts caused by
-     this problem, vncviewer should be modified so that a cached copy of
-     the desktop is available, either as a client-side image or a
-     server-side off-screen pixmap, so that XvPutImage would be the only
-     method for writing to the X display.
-
-  +o
-
-     Although several modern graphics adaptors support hardware scaling
-     using an RGB format (e.g., ATI Radeon, nVidia, etc.), XFree86
-     drivers typically only implement YUV formats.  YUV generally
-     compress the pixel information in some way.  For example, two
-     commonly implemented formats, YUY2 and UYVY provide intensity
-     information for every RGB pixel, but only provide chroma and
-     luminance information for pairs of horizontal pixels.  Since VNC
-     uses pixel-resolution for communicating updates on the wire,
-     additional artifacts are introduced (because there may not be
-     enough information from the wire to update a pair of pixels).
-
-     Further, the well-known problem with YUV encoding is even more
-     evident when the image is a desktop instead of a movie.  For
-     example, consider a 1-pixel-wide vertical window border.  If the
-     border changes in color but not intensity (e.g., because a window
-     manager uses color to indicate focus), there may or may not be a
-     change in the YUY2 image, depending on the algorithm used for RGB
-     to YUV conversion and on how the border pixel is ordered in the
-     pair of pixels used by the algorithm.
-
-     Many of these artifacts could be eliminated if vncviewer cached a
-     complete RGB image of the desktop, and only did the conversion to
-     YUV for properly aligned areas of damage.  The remaining artifacts
-     could be eliminated if an RGB format was used with X Video (which
-     may require the extension of existing XFree86 drivers to support
-     RGB).
-
-  +o  Most modern video cards support exactly one overlay plane that is
-     suitable for use with X Video.  Therefore, only one application can
-     use X Video at any given time.  This is a severe limitation in a
-     desktop environment.
-
-  33..11..22..11..  IImmpplleemmeennttiinngg tthhee XX VViiddeeoo EExxtteennssiioonn ffoorr DDMMXX
-
-  The user-level API for X Video is fairly simple, but the underlying
-  support required for the full specification is large.  However, since
-  the API provides a method to query supported capabilities, a usable
-  subset of X Video can be implemented that would support XvPutImage and
-  little else.  This would require support for the following:
-
-  +o  X Video Extension API calls, including the following:
-
-     +o  XvQueryExtension
-
-     +o  XvQueryAdaptors
-
-     +o  XvQueryPortAttributes
-
-     +o  XvFreeAdaptorInfo
-
-     +o  XvListImageFormats
-
-     +o  XvGrabPort
-
-     +o  XvCreateImage
-
-     +o  XvPutImage
-
-     +o  XvShmCreateImage
-
-     +o  XvShmPutImage
-
-  +o  Support for querying back-end X Video Extension capabilities.
-
-  +o  Support for sending the image to the back-ends.  Because X Video
-     requires sending full images, there may be a trade-off between
-     bandwidth limitations and additional complexity to divide the image
-     up such that is scales properly.
-
-  +o  Possible support for a software fall-back.  For example, if all of
-     the back-ends do not support the X Video Extension, software
-     scaling can be implemented such that the image is sent to the back-
-     end with XPutImage.  This pathway would have poor performance.
-
-  33..11..22..22..  SSuuppppoorrttiinngg RRGGBB ffoorrmmaattss ffoorr tthhee XX VViiddeeoo EExxtteennssiioonn
-
-  Assuming an XFree86 driver already supports the X Video Extension, and
-  assuming the target hardware supports an RGB format, then adding
-  support for that format is relatively simple and straightforward.
-
-  33..11..33..  SSccaalliinngg wwiitthh aann XXPPuuttIImmaaggeeSSccaalleedd EExxtteennssiioonn
-
-  Instead of (or in addition to) implementing the X Video Extension in
-  DMX, one obvious solution would be to implement a new extension that
-  provides access to hardware-assisted scaled blits, similar to the
-  StretchBlt call available under Windows.  This call would scale RGB
-  images and would not use the overlay plane (unlike the X Video
-  Extension).
-
-  This approach has many of the same advantages and disadvantages as the
-  XCopyAreaScaled Extension, discussed in the next section.  Discussion
-  of XPutImageScaled is deferred in favor of XCopyAreaScaled for the
-  following reasons:
-
-  +o  XPutImageScaled can be emulated with XCopyAreaScaled by first using
-     XPutImage to copy the image to an off-screen pixmap, and then
-     calling XCopyAreaScaled between that off-screen pixmap and the
-     target drawable.
-
-  +o  Since XCopyAreaScaled would copy between two areas of on-screen or
-     off-screen memory, it has additional uses and can be viewed as
-     efficiently providing a superset of XPutImageScaled functionality.
-
-  33..11..44..  SSccaalliinngg wwiitthh aann XXCCooppyyAArreeaaSSccaalleedd EExxtteennssiioonn
-
-  As noted in the previous section, because XCopyAreaScaled provides a
-  superset of the functionality provided by XPutImageScaled, we will
-  consider this extension instead.
-
-  First, XCopyAreaScaled would provide for RGB scaling between pixmaps
-  (i.e., on-screen or off-screen areas of memory that reside on the
-  video card).  Unlike the X Video Extension, which writes into an
-  overlay plane, XCopyAreaScaled would write into the non-overlay areas
-  of the screen.  Key points to consider are as follows:
-
-  +o  Because different planes are involved, the two scaling operations
-     are usually implemented in hardware differently, so an
-     XCopyAreaScaled extension could be added in a manner that would
-     neither conflict with nor interact with the X Video extension in
-     any way.
-
-  +o  The XCopyAreaScaled extension provides new functionality that the X
-     Video Extension does not provide.  Based on anecdotal feedback, we
-     believe that many people outside the DMX and VNC communities would
-     be excited about this extension.
-
-  +o  The main drawback to this extension is that it is new and needs to
-     be implemented at the driver level in XFree86 for each video card
-     to be supported.  At the present time, it is more likely that the X
-     Video Extension will be implemented for a particular piece hardware
-     because the X Video extension has multimedia uses.  However, over
-     time, we would expect the XCopyAreaScaled extension to be
-     implemented along with the X Video extension, especially if it
-     becomes popular.
-
-  +o  Another drawback is that not all modern cards provide support for a
-     simple scaled blit operation.  However, these cards usually do
-     provide a 3D pipeline which could be used to provide this
-     functionality in a manner that is transparent to the client
-     application that is using the XCopyAreaScaled extension.  However,
-     this implementation pathway would make this extension somewhat more
-     difficult to implement on certain cards.
-
-  33..11..55..  SSccaalliinngg wwiitthh OOppeennGGLL
-
-  Another general solution to the scaling problem is to use the texture
-  scaling found in all 3D hardware.  This ability is already exposed
-  through OpenGL and can be exploited by clients without X server
-  modification (i.e., other than the ability to support OpenGL).  An
-  application using OpenGL would transmit the non-scaled image to the X
-  server as a texture, and would then display a single non-transformed
-  rect using that texture.  This also works around the single overlay
-  problem with the X Video Extension as well as the need to implement
-  additional scaled primitive extensions.
-
-  The downside is that most OpenGL implementations require power of 2
-  texture sizes and this can be very wasteful of memory if, for example,
-  the application needs to scale a 1025x1025 image, which would require
-  a 2048x2048 texture area (even a 640x480 image would require a
-  1024x512 texture).  Another downside is that some OpenGL
-  implementations have a limited about of texture memory and cannot
-  handle textures that are very large.  For example, they might limit
-  the texture size to 1024x1024.
-
-  33..22..  AApppplliiccaattiioonn--ttrraannssppaarreenntt SSccaalliinngg ffoorr DDMMXX
-
-  33..22..11..  BBaacckk--eenndd SSccaalliinngg WWiitthhoouutt DDiissccoonnnneecctt//RReeccoonnnneecctt
-
-  VNC does scaling on the client side (in the vncviewer application).
-  Implementing a similar solution for DMX would require support in the
-  back-end X servers and, therefore, is not a general solution.
-
-  XFree86 already implements some support for "scaling" that could be
-  used with DMX: if, in the XF86Config file, multiple Modes are listed
-  in the Display Subsection of the Screen Section, then pressing Ctrl-
-  Alt-Plus and Ctrl-Alt-Minus can be used to iterate through the listed
-  modes.  The display dimensions will change to the dimensions in the
-  Modes line, but the logical dimensions of the X server (i.e., the
-  dimensions that Xdmx knows about) will not change.
-
-  Further, the dimensions of the XFree86 display are under software
-  control (via the XFree86-VidModeExtension), so the Xdmx server could
-  change the screen dimensions on a per-display basis, thereby scaling
-  the information on part of that display.
-
-  However, this scaling appears to have limited use.  For example,
-  assume a 4 by 4 display wall consisting of 16 1280x1024 displays.  If
-  all of the back-end servers were simultaneously configured to display
-  640x480, the left hand corner of each display would be magnified, but
-  the composite result would be unreadable.  Magnifying one display at a
-  time could be usable, but could have limited utility, since the result
-  would still be no larger than a single display.
-
-
-  33..22..22..  BBaacckk--eenndd SSccaalliinngg WWiitthh DDiissccoonnnneecctt//RReeccoonnnneecctt
-
-  Disconnect and reconnect features are not currently supported in DMX,
-  but are scheduled to be implemented in the future.  These features,
-  combined with the XFree86-VidModeExtension Extension, would allow an
-  application to do the following:
-
-  +o  Disconnect a specific back-end server (via the DMX Extension),
-
-  +o  reconfigure the XFree86 back-end server resolution, and
-
-  +o  reconnect the back-end server to DMX -- at a new origin with the
-     new screen resolution.
-
-  For example, consider a display wall consisting of 16 1280x1024
-  displays with a total resolution of 5120x4096.  All of the screens
-  could be disconnected, repositioned, and reconnected each at a
-  resolution of 640x480.  The total resolution of the display wall would
-  be 2560x1920, allowing a view of a selected area approximately one-
-  fourth of the size of the DMX display.  This change would be
-  completely application independent (except, perhaps, for a DMX-aware
-  window manager).  When work at the increased resolution was completed,
-  the back-end servers could be disconnected, reconfigured, and
-  reconnected for the original 5120x4096 view.
-
-  Support for this type of scaling can be implemented in a DMX-aware X11
-  client assuming the DMX server support arbitrary disconnect and
-  reconnect semantics.  Because this application cannot be written
-  before disconnect/reconnect is implemented, this solution will not be
-  discussed further in this paper.
-
-  33..22..33..  SSeerrvveerr--ssiiddee SSccaalliinngg
-
-  In earlier versions of DMX, a frame buffer was maintained on the
-  server side, and XPutImage was used to move the information from the
-  server to the client (similar to some early VNC implementations).  The
-  use of a server-side frame buffer would allow the server to do
-  scaling, but is not a recommended solution because of overall
-  performance issues and server-side memory issues (i.e., the frame
-  buffer would be very large for large display walls).
-
-  Exploration of this path is not recommended.
-
-  33..33..  XXCCrreeaatteeSSccaalleeddWWiinnddooww AAPPII
-
-  The implementation of X Video Extension in DMX, and the use of
-  XvPutImage by applications requiring scaling requires significant
-  changes in DMX Further, XvPutImage is, essentially a scaled blit, and
-  it is only useful for applications which are already using (or can be
-  modified to use) XPutImage.  Therefore, a more general API will be
-  discussed as another possibility.
-
-  X applications typically create windows with the XCreateWindow call.
-  A new extension could provide an XCreateScaledWindow call that could
-  be used in place of the XCreateWindow call and be otherwise
-  transparent to the application.  This would allow applications, even
-  those that do not depend on XPutImage, to take advantage of window
-  scaling.  In this section we describe how the call would work, what
-  transparency it provides, and how to solve the potential problems that
-  transparency creates.
-
-  33..33..11..  XXCCrreeaatteeWWiinnddooww
-
-  The XCreateWindow call takes width and height as parameters.  An
-  XCreateScaledWindow call could take all the same parameters, with the
-  addition of a scaling factor.
-  33..33..22..  XXSSeettWWiinnddoowwAAttttrriibbuutteess
-
-  An X11 window has several attributes that would have to be scaled:
-
-  +o  Background and border pixmaps
-
-  +o  Border width
-
-  +o  Cursor
-
-  33..33..33..  XXGGeettWWiinnddoowwAAttttrriibbuutteess,, XXGGeettGGeeoommeettrryy
-
-  For transparency, calls that query the window attributes should return
-  unscaled information.  This suggests that all unscaled pixmaps and
-  window attributes should be cached.
-
-  Unfortunately, a window manager requires the scaled geometry to
-  properly decorate the window.  The X server can probably determine
-  which client is acting as the window manager (e.g., because that
-  client will select events that are used exclusively by the window
-  manager).  However, other Scaled Window Extension aware clients may
-  also need to determine the scaled geometry.  Therefore, at least two
-  additional extension calls should be implemented:
-  XGetScaledWindowAttributes and XGetScaledGeometry.
-
-  33..33..44..  PPooppuupp aanndd CChhiilldd wwiinnddooww ppoossiittiioonnss
-
-  Some applications may position popup and child windows based on an
-  unscaled notion of the main window geometry.  In this case, additional
-  modifications to the client would be required.
-
-  33..33..55..  EEvveennttss
-
-  Most events (e.g., for mouse motion) return information about the
-  coordinates at which the even occurred.  These coordinates would have
-  to be modified so that unscaled values were presented to the client.
-
-  33..33..66..  IImmpplleemmeennttaattiioonn
-
-  There are many implementation issues, some of which are similar to the
-  issues involved in implementing the X Video Extension for DMX.  The
-  window contents must be scaled, either by performing all operations to
-  a frame buffer and then writing the image to the display (perhaps
-  using hardware scaling support), or by modifying all of the various
-  drawing operations to perform scaling.  Because of the complexity
-  involved, the frame buffer option is recommended.
-
-  44..  CCoonncclluussiioonn aanndd RReeccoommmmeennddaattiioonnss
-
-  We recommend a three phase implementation strategy, based on how an
-  application could be written to take advantage of scaling:
-
-  1.
-
-     The XCopyAreaScaled extension should be implemented, since this is
-     the ideal solution for applications like VNC, and since making use
-     of this extension will require minimal changes to applications that
-     already use XPutImage or XCopyArea.
-
-     The initial implementation work would include the design of the X
-     protocol extension, writing this up in the usual format for
-     extension documentation, implementation of the protocol transport
-     pieces in XFree86, implementation of a software fall-back in
-     XFree86 and DMX, one example hardware implementation for XFree86,
-     and implementation of support for this extension in DMX.
-
-     We suggest implementing the extension first on the ATI Radeon
-     cards.  However, since these cards do not provide a 2D scaled blit
-     primitive, the implementation would have to make use of the 3D
-     texture engine to emulate a scaled blit.  This is recommended,
-     since other modern graphics cards also do not provide a simple 2D
-     scaled blit operation and an example of the more difficult
-     implementation pathway would be helpful to others.
-
-  2.
-
-     Until XCopyAreaScaled is widely supported, applications that
-     require scaling will have to fall back to another scaling method.
-     We suggest OpenGL as the first fall-back method because it is
-     widely available and supported by DMX.
-
-     A project centered around OpenGL-based scaling would implement this
-     scaling in VNC as an example.  This work would include re-writing
-     the vncviewer rendering engine to cache a master copy of the
-     desktop image for all operations.
-
-  3.
-
-     Since OpenGL is not implemented everywhere, and may not provide
-     hardware-assisted performance in every implementation, an
-     application that requires scaling should also fall back to using
-     the X Video Extension.
-
-     This project would add support for the X Video Extension to DMX and
-     would add support to VNC to take advantage of this extension
-     without introducing artifacts.  This would require modifying the
-     vncviewer rendering engine to cache a master copy of the desktop
-     image for all operations.  This project should also add support for
-     the RGB format to at least one XFree86 driver (e.g., ATI Radeon).
-
-     The X Video Extension is one of the few popular extensions that DMX
-     does not support.  We recommend implementing the X Video Extension
-     even if scaling is the specific goal of that work.
-
-  We do nnoott recommend implementation of the XCreateScaledWindow
-  extension because of the complexity involved.  We do nnoott recommend
-  implementation of the XPutImageScaled extension because it requires
-  the same amount of work as the XCopyAreaScaled extension, but provides
-  less functionality.  Further, server-side scaling with a large frame
-  buffer is nnoott recommended because of the performance implications.
-
-  The back-end scaling, especially with disconnect/reconnect support
-  should be explored in the future after disconnect/reconnect is
-  implemented, but not at the present time.
-
-
-