Scaled Window Support in DMX

Kevin E. Martin

Rickard E. Faith

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. Copyright 2003 by Red Hat,
Inc., Raleigh, North Carolina

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Table of Contents

Introduction

    DMX
    Problem Statement
    Task

Previous Work

    VNC
    The X Video Extension

Possible Solutions

    VNC-like Scaling
    Application-transparent Scaling for DMX
    XCreateScaledWindow API

Conclusion and Recommendations

Introduction

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-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.

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 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.

Task

For reference, here is the original description of the task this paper
addresses:

  ● Scaled window support (for VNC)

      ○ Investigate possibility of implementing a "scaled window" extension:

          □ Add XCreateScaledWindow call that could be used in place of
            XCreateWindow

          □ All primitives drawn to scaled window would be scaled by
            appropriate (integral?) scaling factor

      ○ Alternate approach: special case VNC support

Previous Work

This section reviews relevant previous work.

VNC

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.

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 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.

Possible Solutions

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

VNC-like Scaling

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 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.)

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
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:

  ● 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.

  ● 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).

  ● 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.

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 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:

  ● X Video Extension API calls, including the following:

      ○ XvQueryExtension

      ○ XvQueryAdaptors

      ○ XvQueryPortAttributes

      ○ XvFreeAdaptorInfo

      ○ XvListImageFormats

      ○ XvGrabPort

      ○ XvCreateImage

      ○ XvPutImage

      ○ XvShmCreateImage

      ○ XvShmPutImage

  ● Support for querying back-end X Video Extension capabilities.

  ● 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.

  ● 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.

Supporting RGB formats for the X Video Extension

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.

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 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:

  ● 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.

  ● 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.

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, XCopyAreaScaled
would write into the non-overlay areas of the screen. Key points to consider
are as follows:

  ● 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.

  ● 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.

  ● 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.

  ● 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.

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 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.

Application-transparent Scaling for DMX

Back-end Scaling Without Disconnect/Reconnect

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.

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
following:

  ● Disconnect a specific back-end server (via the DMX Extension),

  ● reconfigure the XFree86 back-end server resolution, and

  ● 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.

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 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.

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
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.

XCreateWindow

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.

XSetWindowAttributes

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

  ● Background and border pixmaps

  ● Border width

  ● Cursor

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
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.

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.

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.

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 support), or
by modifying all of the various drawing operations to perform scaling. Because
of the complexity involved, the frame buffer option is recommended.

Conclusion and Recommendations

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 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.

