vic(1)

NAME

vic - video conference

SYNOPSIS

vic [ -A proto ] [ -B kbps ] [ -C conference ] [ -c dither ] [ -D
device ] [ -d display ] [ -f format ] [ -F fps ] [ -H ] [ -I channel ]
[ -K key ] [ -M colorfile ] [ -m mtu ] [ -N sessionname ] [ -n network
] [ -o clipfile ] [ -P ] [ -s ] [ -t ttl ] [ -U interval ] [ -u script
] [ -V visual ] [ -X resource=value ] dest/port[/format/ttl]

DESCRIPTION

Vic is an experimental video conferencing tool that allows groups of users to transmit video to each other over an IP Multicast network (``vic'' is a contraction of VIdeo Conference). A host must be equipped with a camera and frame digitizer to send video, but no special hardware is required to receive and display it. Audio is handled by a separate application.

The conference is carried out on the address specified by dest. Pointto-point conferences are initiated by supplying a standard IP address, while multiparty conferences use a Class D group address. Port specifies the UDP ports to use, and ttl specifies the IP time-to-live (see below). For RTPv2, port specifies two consecutive ports, one for data and one for control. The data port is set to the greatest even integer less than or equal to port. The control port is one greater than the data port.

Video is coded in a variety of formats. The default format depends on the host video capture hardware, but can be overridden. Vic will take advantage of certain hardware compression and decompression units, if present, but since decompression hardware is not always available, all supported coding formats can be decoded in software.

OPTIONS

-A Use the transport protocol specified by proto, which may be rtp,: for RTPv2 (version 2 of the real-time transport protocol), nv for the variant of RTPv1 used by Xerox PARC's network video tool, or ivs for the variant of RTPv1 used by the INRIA Videoconference System.
-B Set the maximum value of the bandwidth slider to kbps kilobits: per second. If the conference address is a multicast address and -B is not specified, the maximum bandwidth is based on the session scope, using the well-known, agreed-upon MBone design parameters (i.e., that define permissible bit-rates for a given transmission scope).
-C Use conference, as the title of this vic window. If the -C flag: is ommitted, the destination address and port are used as the window title.
-c On a color-mapped display, use the algorithm indicated by dither: to convert video (typically represented in 24-bit YUV color space) to the available color palette. For monochrome and truecolor displays, this argument is ignored. dither may be one of the following:
ed a simple, error-diffusion dither
(i.e., Floyd-Steinberg dither)

gray 32-levels of gray
od a simple 4x4 ordered dither
quantize straight quantization; The ``od'' algorithms uses the standard 5x5x5 color cube. Since this color palette is used by several other applications (wb, nv, and gs), colors can be shared which avoids the use of a private color map.
-D Use device as the default choice for video capture. On systems: with multiple capture devices, this option may be used to specify the selected device at start-up. The argument may be one of the following:
cosmo SGI Cosmo (JPEG) adaptor
galileo SGI Galileo Video
indigo SGI Indigo Video
j300 DEC J300
parallax Parallax Xvideo card (jpeg board for sparcs)
rtvc Sun /dev/rtvc*
tx DEC tx/pip frame grabber
videopix Sun VideoPix card
vigrapix Vigra card (for sparcs)
vino SGI VINO (Indy Video)
xil Sun XIL library; The -D command option overrides the Vic.device resource.
-d Connect to the X server indicated by the display argument.
-f Use the video coding indicated by the format argument for trans: mission. Format may be one of
jpeg Motion JPEG
mpeg MPEG
h261 CCITT H.261
nv Xerox PARC Network Video
cellb Sun CellB
bvc Berkeley Video Codec; Not all encodings are compatible with all frame grabbers. For instance, you need JPEG compression hardware in order to source a JPEG stream (e.g., a DEC J300, SunVideo, etc.).
-F Set the maximum value of the frame rate slider to fps frames per: second.
-H Set the initial disposition of the Use-Hardware-Decode button to: ``on''. Vic uses software decoding by default.
-I Use the ``LBL Conference Bus'' to interact with other multimedia: conferencing tools. The small integer channel, which must be non-zero, is used as the channel identifier for group interprocess communication on the local host. This value should be consistent across each group of applications that belong to a single conference, and should be unique across conferences. The session directory tool (sd) will allocate appropriate values. (Vic and vat currently use this mechanism to coordinate voiceactivated video switching. Vat version 2.17, or later, is required.)
-K Use key as the encryption key for this vic session. (This only: works if you have a binary with encryption support. Because of U.S. export controls, the standard distribution from LBL does include this support.)
-M Use colorfile as the base lookup table for the error-diffusion: or quantizing color rendering algorithms. This file is generated from a colormap using ppmtolut(1). The input to ppmtolut is a ppm(5) file, which contains a single pixel of each color in the colormap (the geometry of the pixmap is irrelevant).; The error-diffusion code can utilize any colormap in which the chrominance level of each color falls on the lattice 0, 16, 32, ... 240. mkcube(1) is a simple utility for generating such colormaps with varying color densities.; Note that this option can also be used in conjunction with the ordered dither, but doing so is not advisable. The reason is that an ordered dither relies on colors uniformly spaced throughout the (5x5x5) RGB color cube, so overriding this default colormap probably will not produce improved results.
-m Set the packet transmission size to mtu bytes, but limited to: 1024 bytes (per the application protocol). The default is 1024.
-N Use session, in lieu of your user name and local host, to iden: tify you to other sites. If -N is ommitted, the X resource Vic.sessionName is used.
-n Use network as the communications protocol underlying the RTP: transport, which may be ip, for IP or IP Multicast, atm for the Fore SPANS ATM API, or rtip for the U.C. Berkeley Tenet group's Real-time Internet Protocol (see http://tenet.berkeley.edu for more information.) In the case of ATM and RTIP, only point-topoint communication is allowed.; RTIP is a simplex protocol requiring connection establishment in both directions. The vic with the lower valued RTIP address will block, waiting for a connection from the other vic. Once the first connection is set up, the two vic's exchange roles to setup the second connection.
-o Dump the RTP video stream sourced from the local host to a file.
-P Use a private X colormap.
-s Don't use shared buffers with the X server.
-t Set the multicast ttl (time-to-live) to ttl. (The ttl is: ignored if the destination address is not an IP multicast address.) If no -t flag is given, the value of the X resource Vic.defaultTTL is used. A ttl of 1 restricts traffic to the local net; a ttl of 0 restricts the traffic to the local host (e.g., only loopback works, which is useful for testing).
-U Use interval as the update period, in seconds, for the thumbnail: sized images of each video source.
-u Source script, in addition to the compiled-in script, to build: the user interface. This is only useful during development.
-X Override the X resource Vic.resource with value.

OPERATION

The main vic window provides an abbreviated summary of all sources that are actively transmitting video to the conference address. If no sources are active, the text ``Waiting for video...'' is displayed in the window. Otherwise, each source has a panel composed of a thumbnail image, identification text, some bit and frame rate statistics, a ``mute'' button, a ``color'' button, and an ``info'' button.

The first line of the identification text contains the RTP NAME attribute of the corresponding source, which for vic, is set using -N, Vic.sessionName, or manually entered in the control menu (see below). The second line of text contains the RTP CNAME attribute and format of transmitted video. If the NAME and CNAME are identical (or very similar), or if the CNAME does not contain a numeric IP address, the second line will instead list the source's IP address (along with the video format). The third line contains filtered frame and bit rate statistics, and a loss indicator. These rates may differ from the actual sender's rate because of network packet drops or loss due to local socket buffer overflows because of CPU saturation. The gain of the low-pass filter used to smooth the statistics is controlled by the Vic.filterGain resource. Note that smoothing can be effectively disabled by setting Vic.filterGain to 1.

Loss Computation

The number of missing packets is computed as the difference between the total number of packets received and the total number of packets sent (which is inferred from sequence numbers). At each sampling interval, a loss percentage is computed by dividing the number of packets missing into the number of packets received during that interval. This percentage is then low-passed filtered (again using the Vic.filterGain constant), which is what finally appears as the parenthesized loss indicator.

Mute & Color

The ``mute'' button, when selected, causes vic to ignore video from the corresponding host. In general, you want to disable any site your not interested in to shed load. Also, it is a good idea to mute your own looped-back transmission to make the encoding process run faster.

The toggle button labeled ``color'' controls the color disposition of the output. When enabled (by default), video is displayed in color;
otherwise, it is displayed in grayscale. Using grayscale reduces the CPU load (for machines without TrueColor displays) because color dithering is costly. The ``color'' button does not affect your transmitted video.

XXX Statistics

Clicking on the ``stats'' button brings up a top level window containing network and video coding statistics for the corresponding source. These statistics are updated in real-time once per second.

The window consists of three panels. The top panel lists the RTP NAME attribute, coding format and geometry, and times of reception of the most recent control and data packets.

The middle panel lists the actual statistics, which depend on the underlying coding format. (For example, only H.261 streams have a BadGOB stat.) The statistics are displayed in three columns. The first column is the change since the last sampling period (i.e., change over the last second); the middle column is a smoothed version of the first column (smoothing controlled by Vic.statsFilter); and the last column is the accumulated value since startup.

The bottom panel contains a stripchart that displays the statistic from the middle panel that is selected with the radio button. The stripchart plots one point per sampling interval.

Viewing Windows

The thumbnail image is not updated in real-time, but rather every few seconds (the default update interval can be overridden with the X resource Vic.stampInterval or -U). Left-clicking on the image will open a larger viewing window of the corresponding source. Along the bottom of the window are some additional controls and the corresponding site name. The ``Dismiss'' button will close the window, as will typing a 'q' into the window. The popup menu labeled ``Size'' is used to set the window size, while the menu labeled ``Mode'' changes the switching mode of the window. By default, the switching mode is ``locked'', which means that the window is locked onto the indicated video source. In ``browse'' mode, vic cycles through the set of active video sources, switching participants every Vic.switchInterval seconds. Additionally, when in ``browse mode'', you can cycle through the participants by hand using the '>' and '<' keys. The last mode is ``voice-activated''. When running in tandem with vat(1), voice-activated switching causes the video window to switch to whoever is talking (see -I). You can run multiple voice-activated windows simultaneously, which will cause the remote participants who have spoken recently to be displayed.

The Control Menu

Clicking on the ``Menu'' button in the lower righthand corner of the main vic window will bring up a control panel, which is composed of three subpanels: transmission controls, encoder controls, and session controls. The transmission controls include a toggle button label ``Transmit'', which opens the video capture device and begins transmission. The ``Lock'' toggle button prevents any external agents from automatically initiating or terminating transmission. (For example, a ``video silence suppression'' algorithm might remotely turn off transmission if there are no interested receivers.) The ``Release'' button terminates the transmission if active, and explicitly closes the capture device (so it may be opened by another application if the device is exclusive access). If the device cannot be opened (e.g., because no capture device is present or the device server isn't running), then a dialog box containing an error message will appear in response to invoking the Transmit button.

Adjacent to the transmission buttons are rate control sliders. The bit rate is limited with the top slider while the frame rate is limited with the bottom slider. Vic uses the more constraining control to limit the output transmission rate. The frame rate slider ranges from 1 to 30 frames/sec, while the bit-rate slider ranges from 10 to Vic.maxbw kilobits/sec. The actual capture (and encode) rates are displayed above the two sliders.

The ``Encoder'' panel contains controls for selecting the coding format, video image size, coding quality level, device ports, signal type, and device. Not all options are supported by all devices. The upper lefthand panel contains a list of supported coding formats, which may be changed at any time. Formats that are not supported by the underlying device (or by software compression) are grayed out and disabled.

The video image size is controlled by selecting generic ``small'', ``normal'', and ``large'' formats. The actual size depends on the coding format and underlying signal type. In general, NTSC images are 640x480 (lg), 320x240 (norm), or 160x120 (sm); PAL images are 768x576 (lg), 384x288 (norm), or 192x144 (sm); and H.261 images are converted from their native signal size to CIF size 352x288 (norm) or QCIF size 176x144 (sm). If a size is not supported by the underlying hardware, the corresponding button will be disabled.

To the right of the size selector is the device selector. Typically, a single binary contains support for only one device type, but eventually there will be support for multiple types (for example, VideoPix, SunVideo, and Parallax on a Sparcstation).

If the selected coding format supports a quality adjustment, then the quality slider will be enabled and the corresponding quality ``value'' displayed next to the slider. The semantics of the quality setting depend on the particular coding format, but in general, higher quality settings are obtained by moving the slider to the right. For nv format, the setting controls the size of the dead-zone region of the Haar transform coefficient quantizer. For motion JPEG, the setting corresponds to the Independent JPEG group's 1-100 compression value. Finally, for H.261, the value corresponding to the GQUANT and MQUANT quantizers from the CCITT standard (this is the nominal quantizer -- if the quantizer is too small to adequately represent the dynamic range of a block, then a larger quantizer is used for that block).

Adjacent to the quality slider are two pull-down menu buttons. The ``Port'' button selects among the analog input jacks to the capture device (for example, a VideoPix has two composite inputs and an S-Video input). The ``Type'' button selects the analog video types, which is one of auto, NTSC, PAL, or SECAM. The ``auto'' setting attempts to determine the signal type from the actual input (provided the hardware supports this).

The ``Session'' panel controls conference address information, some type-in boxes, and other session controls. The first line of the panel lists the numeric IP address UDP port of the conference, the RTP source identifier of the local instance, and the multicast TTL. There are two type-in boxes labeled ``Name'' and ``Key''. The ``Name'' box can be used to change the RTP session name announced to other sites. The ``Key'' box contains a session key for encryption described below. Below the type in boxes are toggle switches for controlling session behavior. The ``Mute New Sources'' button, when selected, causes sources that transmit video to come up ``muted''.

Encryption

(N.B.: Because of U.S. export controls, the standard distribution of vic from LBL does not support encryption. In this case, the ``Key'' type-in box will be disabled.)

Since vic conversations are typically conducted over open IP networks, there is no way to prevent eavesdropping, particularly for multicast conferences. To add some measure of privacy, vic allows the video streams to be DES encrypted. Presumably only sites sharing the same key will be able to decrypt and listen to the encrypted video.

Encryption is enabled by entering an arbitrary string in the key box (this string is the previously agreed upon encryption key for the conference - note that key distribution should be done by mechanisms totally separate from vic). Encryption can be turned off by entering a null string (just a carriage return or any string starting with a blank) in the key box.

Tiling

Along the bottom of the control menu are several buttons. The button labeled ``Tile'' is a pull-down menu which allows you to specify the number of columns to use for displaying the thumbnail summaries of each active source. The default is single column. The number of columns can also be specified by typing a single digit into the main window.

Session Member Listing

Clicking on the ``Members'' button brings up a top level window with a scrollable list of all the participants in the session. This list includes participants that are not actively sending video.

Colormap Optimization

The ``Colors'' button invokes a dynamic optimization of the color map used by the error diffusion or ordered dither algorithms. The distribution of colors for all ``unmuted'' sources is collected and handed off to a separate process to compute an improved colormap. Vic forks off histtolut(1), which must be in your execution path, to perform the computation. Because this optimization is computationally intensive, it may take a non-negligible time to complete. During this time, the ``Colors'' button is disabled and grayed out.

CODING FORMATS

Vic supports a variety of video coding formats and it's a good idea to be familiar with the tradeoffs among formats before deciding which to use for a transmission. All of the formats (except Motion JPEG) utilize a block-based conditional replenishment algorithm, where the video image is divided up into 8x8 blocks and only those blocks that change are transmitted. By coding each block independently of the past, the decoding process is made robust to packet loss. Because block updates are driven by scene activity, receivers might accumulate many stale blocks from packet loss or from joining an in-progress session. This is circumvented by running a background refresh process which cycles through all the blocks continuously transmitting them at some low rate. The efficacy of this approach has been nicely demonstrated by Ron Frederick in nv.

Once the conditional replenishment step determines that a block is to be transmitted, the block must be coded. How it is coded depends upon the selected format. For the nv format, the block is transformed to a frequency domain representation via the 8x8 Haar transform. The Haar coefficients are quantized with a simple dead-zone only quantizer (i.e., coefficients that fall below some threshold are truncated to zero; otherwise, they are unchanged). The coefficients are then runlength encoded. Unlike traditional transform coders, there is no Huffman or arithmetic coding step (which typically yields a factor of two in compression gain -- but because of the dead-zone only quantizer, entropy coding would be less effective here).

For H.261, the blocks are coded as intra-mode macroblock updates using an H.261 compliant syntax. Note that vic never uses motion-compensated macroblock types since this type of coding is very susceptible to packet loss. H.261 codecs typically do not have provisions for producing this type of bit stream, which we call ``Intra-H.261'', but decoders have no problem decoding it since the syntax is fully compliant. (Most H.261 codecs have an ``intra'' operating mode, but this is typically very inefficient because every block of every frame is coded.) The Intra-H.261 and nv encoders are both transform coders and are in fact quite similar. The differences are: (1) H.261 uses a discrete cosine transform (DCT) instead of a Haar transform; (2) H.261 uses a linear quantizer instead of dead-zone only quantizer; and (3) H.261 applies Huffman coding to the run-length encoded symbols.

For the ``simple conditional replenishment'' (scr) format, the block update is sent uncompressed. This approach has very high image quality but works very poorly over low bandwidth networks. Even on high bandwidth networks, slower end-systems have a hard time keeping up with the data rates associated with processing uncompressed video.

For the CellB format, blocks are encoded according to Sun Microsystems CellB syntax. CellB is a block truncation coding technique that gives a 16:1 compression gain with relatively low image quality. The simplicity of the CellB codec results in a fast software implementation.

Finally, for Motion JPEG format, entire frames are coded via the JPEG still image standard. Motion JPEG is suitable only in high bandwidth environments and is supported only with capture devices that support hardware JPEG compression. Vic can, however, decode Motion JPEG in software.

Coding Format Tradeoffs

As in nv, vic limits its transmission bandwidth by using a variable frame rate. When scene activity is high, the video becomes harder to code and the frame rate slows. Under this scheme, higher compression gain turns into higher frame rates.

Because overall perceived quality depends very much on scene content, it's not always clear which coding scheme is best. For example, it's better to view slides at a low frame rate and high image quality, whereas most people prefer viewing a human speaker at a higher frame rate at the expense of lower image quality. The Haar transform in the nv algorithm tends to code edges, and hence text, better than the DCT in H.261. On the other hand, for typical scenes, the Intra-H.261 encoder tends to outperform the nv encoder by a factor of two to four (Frederick has reported a similar factor of two by replacing the Haar transform by the DCT in the nv coding algorithm).

MONITOR GAMMA

Because computer monitors are not designed to display generic composite video and because analog video standards bias source signals with a display gamma correction, most computer monitors are not properly calibrated for displaying digital video signals. In other words, cameras adjust for a gamma response that is not typically present in computer monitors. For color mapped rendering (i.e., error diffusion or ordered dither), vic allows you to specify a gamma correction factor that is tailored to your monitor. You can choose a proper gamma using the test image, gamma.gif, included in the vic distribution. View the image from several feet away and choose the bar which appears to have a uniform gray level. The number printed below this bar is the gamma of your display. Take this number, divide it by 2.2 (the gamma correction built into an analog video signal), and use this result for vic's gamma correction (i.e., Vic.gamma).

This gamma calibration procedure is due to Robert Berger (rwb@J.GP.CS.CMU.EDU), who provides an excellent discussion of monitor gamma in
http://www.cs.cmu.edu:8001/afs/cs/user/rwb/www/gamma.html. The gamma.gif calibration image is redistributed with the permission of Robert Berger.

X RESOURCES

The following are the names and default values of X resources used by vic. Any of these resources can be overridden by the -X command switch, which may be used multiple times on the command line. For example, "-Xmtu=800" overrides Vic.mtu with 800.

Vic.mtu (1024): the maximum transmission unit for vic, with respect to the RTP layer
Vic.framerate (2): the default initial setting of the frame rate slider
Vic.defaultTTL (16): the default IP multicast time-to-live
Vic.maxbw (-1): the maximum allowable transmission rate; -1 causes vic to automatically choose the maximum based on the MBONE heuristics that relate ttl scopes to maximum transmission rate
Vic.bandwidth (128): the default initial setting of the bandwidth slider in kb/s
Vic.iconPrefix (vic:): a string that is prefixed to the vic icon names
Vic.priority (10): a scheduling priority that is set using the nice(3) system call;
typically, video is run at a lower priority to prevent computationally expensive decoding from interfering with vat(1) to avoid audio breakups
Vic.format (none): the default coding format, which may be nv, cellb, bvc, jpeg, or h261.
Vic.stampInterval (1000): the time interval (in milliseconds) between updates of the thumbnail image; the thumbnail is not rendered in real-time to avoid decoding overhead when the stream is not being actively viewed
Vic.switchInterval (5): the time interval (in seconds) to wait before switching to the next video source in timer-switched mode
Vic.dither (od): the default mode for dithering for 8-bit displays; see the -c command line option for more information.
Vic.tile (1): the default number of columns used for displaying thumbnails in the main vic window
Vic.filterGain (0.25): the low pass filter constant used for smoothing the frame rate and bit rate statistics
Vic.statsFilter (0.0625): the low pass filter constant used for smoothing the decoder and network statistics (in the ``stats'' popup window)
Vic.medianCutColors (150): the number of colors to use in the dynamic colormap optimization, run when the ``Colors'' button is invoked
Vic.gamma (0.7): the default gamma correction factor to use in the color mapped rendering algorithms
Vic.rtipXmin (655): the RTIP ``xmin'' traffic spec parameter
Vic.rtipXave (655): the RTIP ``xave'' traffic spec parameter
Vic.rtipI (6553): the RTIP ``I'' traffic spec parameter
Vic.rtipSmax (1200): the RTIP ``Smax'' traffic spec parameter
Vic.rtipD (1200): the RTIP ``D'' QOS spec parameter
Vic.rtipJ (3279): the RTIP ``J'' QOS spec parameter
Vic.rtipZ (10000): the RTIP ``Z'' QOS spec parameter
Vic.rtipW (1000): the RTIP ``W'' QOS spec parameter
Vic.rtipU (1000): the RTIP ``U'' QOS spec parameter
Vic.rtipType (1): the RTIP type parameter

ACKNOWLEDGMENTS

Vic was inspired by nv, the pioneering Internet video tool developed by Ron Frederick at Xerox PARC (frederick@parc.xerox.com). Portions of vic (the ordered dither, the nv-format codec, and some of the video capture code) were derived from the nv source code.

Lance Berc (berc@src.dec.com) provided the j300/jvideo video server;
his model for video capture and decompression shaped vic's hardware codec support architecture. Lance has been tremendously helpful in the development process. He has helped to diagnose and fix several particularly nasty bugs and provided many excellent suggestions for the user interface and overall functionality.

The CellB codec is based on an implementation from Michael Speer (speer@eng.sun.com).

Amit Gupta (amit@cs.berkeley.edu) originally suggested the abstraction that evolved into the voice-activated switching mechanism.

Elan Amir (elan@cs.berkeley.edu) implemented the error diffusion dithering code and dynamic color allocation (median cut) algorithms. Chris Goodman (goodman@sgi.com) provided valuable advice on the error diffusion algorithm and helped debug the implementation.

Martin Vetterli (martin@diva.eecs.berkeley.edu) provided input on fast DCT implementations. He pointed out that Arai, Agui, and Nakajmia's 8pt 1D DCT can be used to compute scaled row and column DCTs leading to a 80 multiply 8x8 2D DCT.

Thanks to Robert Berger (rwb@J.GP.CS.CMU.EDU) for his excellent web page on monitor gamma and for his permission to redistribute the gamma calibration test image (gamma.gif).

Many thanks to the early alpha testers who invested tremendous effort fielding version after version of bug ridden binaries. Their feedback, patience, and willingness to cope with our source code distribution policies are very much appreciated. The cast includes Lance Berc (berc@pa.dec.com), Toerless Eckert <Toerless.Eckert@Informatik.UniErlangen.de> Atanu Ghosh (A.Ghosh@cs.ucl.ac.uk), Mark Handley (M.Handley@cs.ucl.ac.uk), Don Hoffman (hoffman@eng.sun.com), George Michaelson (G.Michaelson@cc.uq.oz.au), Bob Olson (olson@mcs.anl.gov), Joe Pallas (Pallas@Apple.COM), Hoofar Razavi (hoofar@sgi.com), Michael Speer (speer@eng.sun.com), Craig Votava (Craig.M.Votava@att.com). and Ian Wakeman (I.Wakeman@cs.ucl.ac.uk),

The extension for compositing graphical overlays in the capture path was suggested by Lance Berc (berc@pa.dec.com).

Thanks to the Xunet research community for using an early version of vic to conduct research meetings over the Xunet backbone during Fall 1993. This experiment led to an important design change in vic: the separation of viewing window from the underlying video source. With this separation, a window could be ``switched'' among the many active sources present in the relatively large Xunet conferences.

This software is based in part on the work of the Independent JPEG Group and the Portable Video Research Group.

This work was co-sponsored by the the Lawrence Berkeley National Laboratory and the Tenet Group of the University of California Berkeley and of the International Computer Science Institute. Support was provided by (1) an AT&T Graduate Fellowship; (2) for Lawrence Berkeley National Laboratory: (i) the Director, Office of Energy Research, Scientific Computing Staff, of the U.S. Department of Energy, Contract No. DEAC03-76SF00098, (ii) Sun Microsystems, (iii) Digital Equipment Corporation, and (iv) Silicon Graphics Inc.; and (3) for the Tenet Research Group: (i) the National Science Foundation and the Advanced Research Projects Agency (ARPA) under Cooperative Agreement NCR-8919038 with the Corporation for National Research Initiatives, (ii) Digital Equipment Corporation, and (iii) Silicon Graphics Inc.

AUTHOR

Steven McCanne (mccanne@ee.lbl.gov), University of California, Berkeley and Lawrence Berkeley National Laboratory, Berkeley, CA, and Van Jacobson (van@ee.lbl.gov), Lawrence Berkeley National Laboratory, Berkeley, CA.

BUGS

MPEG is not yet supported. We plan to implement an ``Intra-MPEG'' encoder using the same principle underlying vic's ``Intra-H.261'' encoder.

The (software) JPEG decoder makes no attempt to interpolate unnatural aspect ratios and does not have deinterlace support (i.e., it will display 640x240 fields as is).

There are no contrast or brightness controls.

The error-diffsuion dithering code needs more work. At low luminosities, strange pastel colors appear. Blue skies are often rendered green.

Monochrome displays are not supported.

Vic cannot operate on the loopback interface because it gets confused by it's own stream. Similarly, routing loops due to application level gateways are not yet dealt with gracefully.

The J300 only produces 8-bit dithered output, so you must run vic with an 8-bit visual if you want to use the J300 to decode JPEG to a window.

If you invoke the colormap optimization and then change the dithering algorithm, the optimized colormap is lost.

Quarter-sized NTSC input video is truncated from 160x120 to 160x112 due to limitations in the way vic performs conditional replenishment (i.e., it uses 16x16 blocks and 120 is not an integral multiple of 16).

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