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Digital Video for Multimedia: Considerations for Capture, Use and Delivery
Section 2: Digital video: issues and choices
Considerations and Guidelines for Digital Video Capture
Preparing to shoot the video
If you are in the position of being involved before the material is actually filmed there are a few points worth considering:
- Use plenty of light. Most cameras tend to become noisier at lower light levels
- Use a tripod (or 'steadycam'). A one pixel jump may represent a large change when taken over the whole image
- Have something simple, but not plain or detailed, as a backdrop. Noise is more easily 'seen' on a plain background. See also Hamilton et al. (1995).
- Try to avoid excessive panning and zooming
- For material which has already been shot, but not edited, ask the editor not to use the standard fade up from/down to black. The effect may represent a change in every pixel for the duration of the transition; the worst case being white to black or vice versa. This will cause difficulties for the codec.
- Consider the use of pre-compression clean-up software for old/noisy video.
Tip: Imagine the situation from the point of view of the codec. Little or no change in a scene means less interframe information and lower data rates (or higher quality for the same data rate). Anything which causes a change over a large area of the image will need to be stored in the final video file.
Video Capture and Editing
- Prepare the system:
- Keep the hard disc de-fragmented.
- Find out if your caching software slows actual transfer rates and turn it off (at least for your capture drive), if it does.
- Set up a pre-allocated capture file larger than you think you'll need (most capture applications support this).
- Put this file on your fastest hard drive
- Capture at the highest quality you can. It is easier to reduce quality at the compression stage than to re-capture the video. Archive your raw data.
- Capture at fractional frame rates. There is some debate concerning the importance of capturing/compressing with fractional frame rates. The term is really a bit of a misnomer. It arises from the difference between American video rates (30fps) and European (25fps). As most systems struggle to play back quarter screen video at 30 fps it is often recommended to cut this rate (and the resulting file size) by the simple expedient of dropping (at capture or, preferably, compression but not playback) every other frame i.e. ending up with a 15fps video file. However, do this to a 25fps European video and we should get a fractional, 12.5 fps video (or 25 frames per 2 seconds). Unfortunately not all capture/editing software (e.g. the current version Adobe Premiere, v4.2) supports this 'fractional' frame rate. Some options are:
- Try and work at 25fps. This may work on fast computers with small frame sizes, but watch data rates of finally delivering form CDROM
- Ignore 12.5fps and use 12 fps. However, at some point in each second, 2 adjacent frames need to be dropped or there is some messy interpolation to do (it may not show, depending on your material)
- Find some software that captures/compresses at fractional rates
- Join others in complaining to the software publishers about this U.S.centric view
- Editing software. Most, if not all, capture cards come with some form of video editing software in addition to capture software. Packages such as Adobe Premiere combine the two. Unless you have specialist requirements, push the supplied software to its limits before selecting an editor suitable for your needs. You will be in a stronger position to assess the strengths and weaknesses of the alternatives. All software can appear appropriate to your needs when demonstrated by an interested third party.
- Special Effects. Most editors come with a plethora of special effects. While there is no doubt about the use of these for advertising and promotional material, their use within educational software demands care. Note that special effects can demand valuable processor time during editing.
- Microsoft VidCap and VidEdit. Although no longer being sold or supported, Microsoft's VidCap and VidEdit (originally part of the Video for Windows Software Developers Kit) have a few advantages if you still have them. They are not as resource hungry as some applications and multiple copies of VidEdit can be run, facilitating the cutting and pasting of clips. VidCap supports fractional frame rates. If running Windows 95 and having difficulties running VidCap or VidEdit, contact Microsoft for the appropriate information. It can be found in their Knowledge Base, article Q131745.
- Choice of Codec.
- Editing software may not support all the features that are available in each codec.
- Define the distribution medium and playback platform: know the bit stream the storage device can sustain and the total storage. available. For CD-ROM rates refer to the table under CD-ROM issues earlier in this section.
- Check that the codec you wish to use is available on all the platforms you want your package to run on, or, there are comparable codecs that can be substituted.
- Know the codec's ability to adapt the synchronised playback speed to the available hardware without user interference.
- Check the content of the video. Codecs differ according to type of content (talking heads, fast action scenes), whether shot outdoors or indoors, etc.
- Reduce the colour depth to 256 colours if the codec supports this unless there is a good reason not to. This will also significantly reduce the file size and increase playback performance. An important consideration if using CD-ROM.
- Consider developer issues. Is there enough time to use an asymmetrical codec, that is, one which takes longer to compress than to decompress? If you have not been involved with obtaining the video source, know its source and determine whether it has been previously compressed. Do not recompress already compressed video as this may cause artefacts.
- Data and key frame rates. Choose carefully. Experiment with the default settings provided for each codec. If a frame is dropped, the sequence may not pick up properly until the next key frame.
- Editing sound. Some of the video editing applications are able to edit sound as well. Most sound cards come with some form of editing software, but, depending on your requirements these may not be sufficiently powerful/flexible and you may have to budget (and evaluate) more specialist applications.
Authoring and Playback
- Artefacts. Any codec using lossy compression may introduce artefacts. The greater the compression the worse the effect will be. If the video is likely to be used in an analytical/diagnostic manner consider linking to an associated high quality still image or a shorter, less compressed, sequence. For further information on alternatives to digital video refer to section 1.
Tip: Do you need motion video?. Would the material be better served by other techniques e.g. still image sequences? Leave the video to do what it is good at. That is showing motion.
- If the authoring software permits, place the top left corner of video windows on horizontal pixel co-ordinates that are divisible by 4. Some software will automatically align the video window. According to Microsoft's guidelines performance can be up to 50% worse for unaligned video.
- Codec support. As with capture/editing software, not all authoring tools will necessarily support every feature of every codec.
- Backgrounds. Nothing on the screen can be brighter than white. Using a white, or very light, background can make the video appear dim. Try a mid grey, or similar, background. Similarly a screen running at a resolution of 1024 x 768 pixels or higher is going to make even a 320 x 240 pixel video look comparatively small. Consider changing to to 800 x 600.
- Play back only at the resolution of the original file frame size. Scaling the video window is heavy on processing power at a time when there is not much to spare.
- Colour depth. Check your display card's colour depth against playback performance. An 8 bit setting is usually best, but increasingly, with the newer display cards, performance is better at 16 bit (64K colours). Obtain the most recent drivers for the cards.
- Other software. Try not to have other software running at the same time as running the application. Low resources/memory can reduce playback performance.
- Consider hardware assisted playback. However, this will mean equipping all users' machines with extra hardware.
- VIDTEST: a video and sound testing program from Microsoft is extremely informative on CD-ROM and hard disc transfer rates, CPU usage, etc. Do try and obtain a copy. Although is is not longer available from Microsoft and unsupported, we are attempting to seek permission to distribute the software via our World Wide Web site.
Virtual Environments Visualisation