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Video Processing with IMPACT Compression

Last Change: 05/Jan/2004

(this page is still under construction)
All the information on this page can be found in the various man pages and other information sources available for IMPACT Compression (IMPCOM for short); however, it's always good to be able to start using an option board straight away, so this page contains what I would regard as the most useful items of information which will allow one to exploit an IMPCOM board without delay. You should still read the various man pages later though.

The most important fact about using IMPCOM is to forget about using MediaRecorder. Like MovieMaker, MediaRecorder has not been written to properly support IMPCOM. Besides, MediaRecorder is just a GUI interface to dmrecord anyway, so using dmrecord directly will be more reliable as there is less overhead processing going on. Plus, if you get used to use command line tools like dmrecord and dmplay, then creating custom scripts for more complex tasks (such as batch processing or scheduled video capture) is alot easier. Similarly, I don't use MediaPlayer when dealing with IMPCOM-generated data. I always playback files with dmplay directly. It seems that most of the later work on the media tools was designed to support O2's ICE more than the old Cosmo boards, so in general the command line tools are better for using IMPCOM.


I assume here that you're using an IMPACT Indigo2 with an IMPCOM board installed. None of the examples listed require anything better than SolidIMPACT graphics, and - at least for the moment - it is assumed you do not also have an IMPACT Video board (IMPVID). The implications of having just an IMPCOM board are explained along the way.

I'm also assuming that your system is using IRIX 6.5, and preferably 6.5.15. This is because many versions after 6.5.15 have bugs in the video libraries which certainly create problems with using an O2 or Cosmo1 Indigo2 for video tasks and so very like adversely affects Cosmo2 systems too (that includes IMPCOM Indigo2 and Octane with Octane Compression). All of my video systems have 6.5.15 installed.

I don't know if more recent IRIX releases (6.5.20 or later) have had the bugs fixed; the latest update I have is 6.5.17 and so far I haven't had a need to upgrade beyond that.

IMPCOM is great for video capture and playback. It is capable of quality levels better than that supported by O2 and is generally more reliable with the supplied video tools. IMPCOM also seems to be more resistant to signal noise and so is better than O2 at capturing from standard VHS sources. It still helps to have a clean signal source though, if possible.

IMPCOM is less flexible than O2 though. It can only handle MJPEG (or uncompressed) data, whereas O2 can deal with MPEG and other formats. However, for my purposes (archiving my vast VHS library of documentaries), it is ideal.

The examples given here use only the supplied programs which are part of the IRIS Digital Media Tools. Even with 6.5.15, these do have bugs. They're not professional applications. However, used in the appropriate way they can be most effective and are perfectly adequate for many tasks. The key point about the video tools is that although dmrecord and dmplay have been coded to use IMPCOM, the MovieMaker application has not. Thus, one can capture and playback using IMPCOM, but editing would be done in software. My advice is to edit using an O2 if you can (even an R5K/180 would do fine) and then either use the O2 or the IMPCOM for final playback. Editing on the Indigo2 will still work, but it will be slow.

An interesting point: movies captured with O2 always seem to give a one-time non-fatal error when loaded into MovieMaker and played for the first time (ie. it doesn't prevent one editing the movie), whereas movies recorded with IMPCOM do not give any such error. The error doesn't seem to harm how MovieMaker works, but it does suggest the code which uses IMPCOM within dmrecord is more robust than the equivalent code for the O2 version.

With respect to main CPU speed, I would expect an R4K/200 or better would be just fine, but please note that all examples given here have only been tested with an R10K/195 Indigo2. As for RAM, 256MB should be ok. I don't think the capture process uses that much. My own Indigo2 has 1GB but that's overkill. Lastly, I'm assuming you have plenty of disk space available; my system has an 18GB system disk and a 36GB option disk. I do have a disk array, but the examples described here are designed to work fine for those who do not have any fast disk array. Here is the hinv of my system:

  FPU: MIPS R10010 Floating Point Chip Revision: 0.0
  CPU: MIPS R10000 Processor Chip Revision: 2.6
  1 195 MHZ IP28 Processor
  Main memory size: 1024 Mbytes
  Secondary unified instruction/data cache size: 1 Mbyte
  Instruction cache size: 32 Kbytes
  Data cache size: 32 Kbytes
  Integral SCSI controller 0: Version WD33C93B, revision D
    Disk drive: unit 1 on SCSI controller 0
    Disk drive: unit 2 on SCSI controller 0
    CDROM: unit 3 on SCSI controller 0
  Integral SCSI controller 1: Version WD33C93B, revision D
  On-board serial ports: 2
  On-board bi-directional parallel port
  Graphics board: Solid Impact
  Integral Ethernet: ec0, version 1
  Iris Audio Processor: version A2 revision 1.1.0
  EISA bus: adapter 0
  IMPACT Compression: unit 0, revision 0:0


After installing the IMPCOM drivers, make sure the video daemon is active. Enter:

    chkconfig videod on

and either reboot or restart videod:

    /etc/init.d/videod start

Next, because I use an O2 for video editing, I try and get used to using a screen refresh of 50Hz, so before anything else I use xsetmon to set the display to 1280x1024 at 50Hz. This is more useful when using an O2, and for those who do have an IMPVID board installed. However, it's wise to become accustomed to using 50Hz if you can. Since I don't use IMPVID, this probably doesn't make any difference in practice. Or one could use 75Hz instead. The important point is to encourage the use of a screen refresh which is a fixed multiple of the video source rate, ie. some multiple of 25 for PAL, or some multiple of 30 for NTSC. Users of PAL have more options here, ie. 50 or 75. NTSC users are pretty much stuck with 60Hz. I've not yested tested the example commands on this page with a 75Hz refresh; they'll probably work fine though I expect.

When processing data to or from a video device, it helps if there is a minimum of interuptions from other processes, especially background daemons. Thus, make sure there isn't anything else running which is obvious, eg. any desktop clock programs or other utilities which cause constant CPU activity. Much more importantly, I find it definitely helps to deactive the web server and the mediad processes. To aid this task, I create two extra programs in /usr/local/bin, called editon and editoff. These are simple scripts to save typing. editon should contain the following:

   /etc/init.d/sgi_apache stop
   /etc/init.d/mediad stop

while editoff should contain:

   /etc/init.d/sgi_apache start
   /etc/init.d/mediad start

Remember to use chmod to make these scripts executable (and any other scripts described on this page), ie.:

   chmod ugo+rx scriptname

If you have any other non-essential background daemons which can be turned off, then add them to the list of actions in the appropriate way. Use the 'top' command to see what processes are running. If all is well, there ought to be just 3 or 4 processes regularly doing anything, ie. the X server, fam and the shell one is using, and of course top itself. Check before and after using editon - you will see a noticable difference in CPU activity.

Lastly by way of preparation, you should have some kind of TV device connected to the video output for playback checking. As mentioned earlier, without an IMPVID board installed one cannot play video to the screen with sound at the same time using the supplied tools (because there is no direct link from IMPCOM to the graphics subsystem). Note that this likely only applies to the supplied tools, in particular the version of dmplay which is installed. I highly doubt it is a general restriction; indeed, one could probably write a custom program which sets up a video to memory and then a memory to graphics path which would allow playback with only an IMPCOM board just fine. Unfortunately, dmplay always tries to connect as video to graphics. dmplay can play a movie to the screen without sound ok, but if you try to play a movie with sound aswell then the system suddenly becomes very unresponsive and frames are dropped, and it's difficult to quit the dmplay. Thus, after capturing a video sequence, you can check the basic visual quality by playing back to the screen, but it's best to have a TV or other device connected so that a proper check can be made by playing the movie to video out.

My own setup is a Panasonic NV-FJ780 VCR with a Watson 28" TV attached; the VCR has the very useful bonus of having three AV inputs aswell as a separate dedicated AV output. Most VCRs only have two SCART AV inputs and perhaps a separate phono AV output. The FJ780 has two SCART sockets for general AV I/O (AV1 and AV2), one set of phono sockets for dedicated AV input (AV3) and an extra set for AV output (it is this which is connected to my TV). I use AV3 for monitoring playback checks and recording from the IMPCOM board. AV1 is connected to my cable and AV2 is connected to a switch box which has inputs from my PS2 and other devices.

Note that the source code for dmrecord, dmplay and lots of other example programs is included under /usr/share/src; for someone with any C/C++ programming competence, there should easily be enough material present to write a version of dmplay that can do screen playback with sound.

Video Capture

Assuming you have all video devices connected and are ready to go, here is a typical example capture command using dmrecord, in this case grabbing approximately six minutes of PAL video from a camcorder with mono sound:

   dmrecord -p video,device=impact,comp=jpeg,engine=impact,brate=30000000 -p audio,channels=1 test.mv

and here is the output:

  Hit the Enter key to begin recording... 
  Hit -c to stop recording...
  Recording was done in real time successfully.
  Post-processing output file 'test.mv' ... 
  Timing information:
  9090 image frames = 363.60 seconds of video captured.

  Compression information:
  Average compressed frame size:   149769.9 bytes
  Average compression ratio:       5.9 : 1
  Bit rate achieved:               29953989.4 bits/sec

Not a single dropped frame! The file size was 1.4GB, or 1396606575 bytes to be exact. When I did this test, the system disk was a 9GB IBM 7200rpm 50pin DNES 309170, so it is clear than an older disk will work just fine. Since then, to have more space, I've replaced the disk with an 18GB 10000rpm IBM drive and added a 36GB option disk.

In the command sequence, 'device' selects which video device to use (in this case 'impact'), comp selects which compression scheme to use (jpeg here), engine selects which device to use for the compression processing (the IMPCOM board; one could use software mode instead) and brate determines the capture bitrate (and thus the overall picture quality). The example uses a 30Mbit bitrate, but other tests I've done show a bitrate as low as 20 to 25Mbit still gives good quality. IMPCOM is capable of capturing as high as 87Mbit, but that is a quality level far beyond what one could conceivably ever need for general video tasks. If you try to use bitrates over 40 to 45Mbit then you will probably start to see dropped frames since that will start to get too close the physical hard sustained limit of the FastSCSI2 channel.

The audio setting selects mono sound (typical of many camcorders). When dealing with VHS material one would usually use channels=2.

Using dmrecord in the above way does not begin recording until one presses the Enter key. Hold down CTRL and press C at any time to stop recording.

Alternatively, one can specify a certain number of frames to capture and then stop recording. This is done with the -t option, eg. one could capture exactly 5 minutes of video with:

   dmrecord -t 300 -p video,device=impact,comp=jpeg,engine=impact,brate=30000000 -p audio,channels=1 test.mv

Again, recording would not begin until Enter is pressed. It is also possible to make recording begin immediately using a different option (I'll include an example of this later).


To playback the movie to the screen without sound, a typical use of dmplay would be thus:

  dmplay -p graphics,engine=impact -E auto test.mv

The -E option with 'auto' specifies that the dmplay application will exit automatically when the movie has finished playing - otherwise the dmplay window will remain until one does CTRL+C in the relevant xterm.

To playback to the video output with sound (and thus to a TV, VCR, etc.) this would be the usual method:

   dmplay -p video,engine=impact -p audio -E auto test.mv 

Again, no frame drops at all with the example I used.

As a side note, if you try to play to the screen with audio, what happens is the audio is played ok, but frames are dropped and the system becomes unresponsive. A typical output in the xterm is:

  # dmplay -p graphics,engine=impact -p audio test.mv
  dmplay: frame 3 skipped
  dmplay: frame 7 skipped
  dmplay: frame 14 skipped
  dmplay: frame 18 skipped
  (and so on...)

One has to CTRL-C the application, or enter killall dmplay in a different xterm, or rlogin from elsewhere and kill off dmplay remotely. This doesn't really matter though since, for proper production work, I'd rather playback a sequence to video output for display on a TV and with the audio going through a proper hifi. The example command shown earlier which plays to video output does exactly that. Thus, as the above shows, grabbing from one source such as a video camera and then recording to another, eg. VCR, is very easy indeed.

The source code for serveral example programs are included with the IMPCOM drivers; I haven't tried them out yet because my Indigo2 doesn't have the compilers installed at the moment. The impactcomp man page says:

  * blendicomp: Real-time decompression of a JPEG compressed movie file,
    alpha blended with graphics (Requires an
    IMPACT Video option).

  * compdecomp: Real-time compression of video input and
    decompression to video output to demonstrate the effect of
    a selected compression ratio on the video source images.

  * comptodsk: Low compression ratio data capture to disk.

  * contcapt: Continueous real-time capture of uncompressed video to disk.

The man page lists the location incorrectly though; the examples can be found in:


and there are lots of other example programs and source code resources under /usr/share too, eg.:

   # ls /usr/share/src/dmedia
   audio/               midi/
   cd+dat/              movie/
   compression/         pluginmgr/
   dmnet/               plugins/
   dmplay/              scsiplayer/
   dmrecord/            soundcommands/
   dvaudiotest/         soundplayer/
   libTransportButton/  timecode/
   libmidifile/         tools/
   libmidiplayer/       video/

Alias Commands and Scripts

One can use aliases and scripts to simplify the use of dmrecord and dmplay. For example, if one is always going to use a bitrate of, say, 30Mbit, then one could create a script in /usr/local/bin called, for example, 'dmr' which could contain the following:

   dmrecord -p video,device=impact,comp=jpeg,engine=impact,brate=30000000 -p audio,channels=2 $1

Thus, to record some data, one could just enter:

   dmr filename.mv

One can take this another step by, for example, having an environment variable to set the bitrate, eg. have the following in .cshrc:

   setenv BITRATE 30000000

and changing /usr/local/bin/dmr to contain:

   dmrecord -p video,device=impact,comp=jpeg,engine=impact,brate=$BITRATE -p audio,channels=2 $1

Similarly, one could create a script called dmp (for dmplay) in /usr/local/bin containing:

   dmplay -p video,engine=impact -p audio -E auto $1

and perhaps a second script for playing to the graphics screen, called dmg:

   dmplay -p graphics,engine=impact -E auto $1

Thus, a full capture and test playback would consist simply of:

   dmr filename.mv
   (various output shown onscreen)
   dmp filename.mv

One could have a separate script for recording to a set number of frames, and/or any other differences required.

I'll add more to this page later. Comments and suggestions welcome!

Ian's SGI Depot: FOR SALE! SGI Systems, Parts, Spares and Upgrades

(check my current auctions!)
[WhatsNew] [P.I.] [Indigo] [Indy] [O2] [Indigo2] [Crimson] [Challenge] [Onyx] [Octane] [Origin] [Onyx2]
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