                        Scaled Window Support in DMX

                    Rickard E. Faith and Kevin E. Martin

                 15 October 2003 (created 19 September 2003)

                                  Abstract

      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 scal-
     ing 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.  Copyright 2003 by Red Hat, Inc., Raleigh,
     North Carolina

1.  Introduction

1.1  DMX

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-resolu-
tion 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.

1.2  Problem Statement

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 dis-
play 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 appli-
cations.

1.3  Task

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" exten-
          sion:

             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

2.  Previous Work

This section reviews relevant previous work.

2.1  VNC

2.1.1  Scaling under VNC

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.

2.2  The X Video Extension

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 spec-
ification, 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.

3.  Possible Solutions

This section briefly discusses possible solutions, including major advantages
and disadvantages from both the implementation and the end-user programmer
standpoint.

3.1  VNC-like Scaling

3.1.1  Software Scaling

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 consid-
ered 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.)

3.1.2  Scaling with the X Video Extension

The scaling in the Windows vncviewer application makes use of a scaled blit
that is supplied by the underlying system library.  Several video cards cur-
rently 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 pro-
vide 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 pro-
vide 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 prelimi-
nary patch against vnc-3.3.7-unixsrc is available in the dmx/external direc-
tory.  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 typi-
     cally 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 com-
     plete 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 suit-
     able 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 envi-
     ronment.

3.1.2.1  Implementing the X Video Extension for DMX

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 pro-
vides 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.

3.1.2.2  Supporting RGB formats for the X Video Extension

Assuming an XFree86 driver already supports the X Video Extension, and assum-
ing the target hardware supports an RGB format, then adding support for that
format is relatively simple and straightforward.

3.1.3  Scaling with an XPutImageScaled Extension

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 over-
lay plane (unlike the X Video Extension).

This approach has many of the same advantages and disadvantages as the XCopy-
AreaScaled Extension, discussed in the next section.  Discussion of XPutIm-
ageScaled 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.

3.1.4  Scaling with an XCopyAreaScaled Extension

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, XCopyAreaS-
caled 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 sim-
     ple 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 XCopyAr-
     eaScaled extension.  However, this implementation pathway would make
     this extension somewhat more difficult to implement on certain cards.

3.1.5  Scaling with OpenGL

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 applica-
tion needs to scale a 1025x1025 image, which would require a 2048x2048 tex-
ture 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.

3.2  Application-transparent Scaling for DMX

3.2.1  Back-end Scaling Without Disconnect/Reconnect

VNC does scaling on the client side (in the vncviewer application).  Imple-
menting 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 dimen-
sions will change to the dimensions in the Modes line, but the logical dimen-
sions 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 dis-
play.

3.2.2  Back-end Scaling With Disconnect/Reconnect

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 fol-
lowing:

   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 com-
pleted, the back-end servers could be disconnected, reconfigured, and recon-
nected 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.

3.2.3  Server-side Scaling

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 solu-
tion 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.

3.3  XCreateScaledWindow API

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 applica-
tions which are already using (or can be modified to use) XPutImage.  There-
fore, 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 applica-
tion.  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.

3.3.1  XCreateWindow

The XCreateWindow call takes width and height as parameters.  An XCre-
ateScaledWindow call could take all the same parameters, with the addition of
a scaling factor.

3.3.2  XSetWindowAttributes

An X11 window has several attributes that would have to be scaled:

   o Background and border pixmaps

   o Border width

   o Cursor

3.3.3  XGetWindowAttributes, XGetGeometry

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 dec-
orate 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 Exten-
sion aware clients may also need to determine the scaled geometry.  There-
fore, at least two additional extension calls should be implemented:
XGetScaledWindowAttributes and XGetScaledGeometry.

3.3.4  Popup and Child window positions

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.

3.3.5  Events

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.

3.3.6  Implementation

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 sup-
port), or by modifying all of the various drawing operations to perform scal-
ing.  Because of the complexity involved, the frame buffer option is recom-
mended.

4.  Conclusion and Recommendations

We recommend a three phase implementation strategy, based on how an applica-
tion 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 pro-
      tocol extension, writing this up in the usual format for extension doc-
      umentation, 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 graph-
      ics 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 hard-
      ware-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 ren-
      dering engine to cache a master copy of the desktop image for all oper-
      ations.  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 not recommend implementation of the XCreateScaledWindow extension
because of the complexity involved.  We do not 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 not 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.

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