Introductory video documentation
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Jack Jansen
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CMIF video tools
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This document gives a quick introduction to some of the tools useful
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for recording, editing and playing back video data in CMIF video
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format. All the tools mentioned currently live in /ufs/guido/bin/sgi.
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A description of the CMIF video file format can be found in
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/ufs/jack/cmif-film.ms.
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Recording video
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There are two tools to record video, Vrec and and Vrecb. Vrec does
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continuous capture, but can capture at most 15 frames per second.
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Vrecb uses burst capture, enabling it to capture all frames. It
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captures to main memory, however, so it cannot handle long video
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fragments. The following options are common to both programs:
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-r rate Capture one out of 'rate' frames. Default (and
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minimum) is 1 for Vrecb, 2 for Vrec. There are some
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caveats for recording PAL, see below.
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-w width Set initial window width (and height, implicitly).
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-g bits Create greyscale film in stead of 8-bit dithered color
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film. Allowable values for 'bits' are 2, 4 or 8.
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-m Create monochrome dithered film. These look horrible.
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-M threshold Create monochrome thresholded film with specified
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threshold (in range 0..255).
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-G Create 2-bit dithered greyscale film.
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-f Capture fields in stead of frames. The film is created
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in such a way that the picture will be suitably
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enlarged on playback, so aspect ratio, etc. are
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maintained.
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-d Drop fields if they would cause data from two video
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fields to be combined. See the section on PAL for more
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details.
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Options for Vrec:
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-a Record audio as well. You will have to twiddle audio
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and video later if you want lipsync playback.
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-q queuesize Set size of the video board circular buffer. A little
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experimentation with this may lead to more video being
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captured, but not always.
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-r rate Set video rate. A rate of 2 tries to capture every
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second frame, etc.
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-P frames Preallocate diskspace for 'frames' images. This may
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enable you to capture more frames.
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Options for Vrecb:
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-n number Capture 'number' frames. Default is 60 (2 seconds).
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Both programs accept a filename to store the video on (default
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film.video) and Vrec also accepts an audio file name (default
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film.aiff). When you want to record you press the left mouse button.
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Vrec stops recording when you release the mouse button and Vrecb stops
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after the predetermined number of frames have been recorded. During
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recording the picture may look funny, but do not let this worry you,
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the film will be ok.
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After recording and saving, Vrec will print the deltas of the field
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numbers recorded. A list of '4 4 4 4'... means that all possible
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frames (one out of two) have been recorded. Vrecb will tell you how
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many duplicate fields have been skipped. See below on PAL useage
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again.
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PAL caveats
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The IndigoVideo board converts the incoming video signal to the 60Hz
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(59.something, actually) display rate. All further operations,
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including capture, are done with respect to display rate. This is
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especially bothersome in the case of PAL video, since it means one out
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of 5 frames will be duplicated. Together with the fact that, in
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continuous capture, you can only capture every second frame this leads
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to strange and wondrous effects. The frame numbers returned by the
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video board (and displayed by Vrec) are display frame numbers, and so
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bear only a very complicated (and probably non-deterministic) relation
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to PAL frame numbers. For recording simple videos this is probably no
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problem. You can use Vrec and try using -d to see which gives the best
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result. On the other hand, if you need every frame and no duplicates
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either you have to use Vrecb and supply the -d and -f option. This
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will give you exactly the fields as they appeared on the PAL tape.
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Video playback
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The easiest way to play back a video is to use Vplay. Calling it with
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an argument of -? will make it list all its options. A few options may
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need a bit of explanation:
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-M magnify Magnify the images by the given factor. This still
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takes any magnification specified in the film (if it
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is a field film, for instance) in account.
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-w width and
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-h height Normally the window will be the correct size for the
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film. You can set height and width, though, if you
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want the window to be bigger. This is especially
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useful if you want to record the film back to
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videotape, since you can playback in a window that is
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bigger than PAL size with a black background and you
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will have no distracting window-manager thingies on
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your videotape.
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Video Editing
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Vedit is a (very simple) video editor that allows you to select images
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from the input movie and copy them to an output movie. Both input and
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output films have a modest buffer that you can move around in.
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Vaddcache takes a movie and adds a frame location cache to it. This
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will make some other programs run faster.
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rgb2video converts a set of SGI .rgb files to a movie.
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There are a few programs that can do image format conversion on movies
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(i.e. convert an 8-bit dithered RGB movie to a greyscale movie), but
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nothing very complete yet. Look at Vcopy.py for more information.
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