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2.3.2 Video conversion

Video conversion is the process of converting a computer animation, possibly in individual image files, obtained from one of the processes in Section 2.3.1, into a format suitable for use with video systems. The video systems refers to all manner of video equipment, but mainly storage devices (such as VHS or Beta video cassettes), display devices (such as CRTs and projectors) and processing systems (such as those for adding effects, dubbing, etc).

The first step is to process any individual image files into a playable animation, such as Quicktime or MPEG. This is usually acheived with some small utilities to ensure that each image of the visualisation is held on the display for longer than a single frame.

The resolution of the video system must be considered. Animations larger than the video resolution must be scaled down to fit, and animations smaller than the video resolution must be scaled up fit. If the aspect ratio of the animation is different to the aspect ratio of the video system, then a process called letterboxing is used. This adds black bars to the horizontal or vertical edges of the animation as needed. A typical video resolution is 768x576, used by PAL. This has an aspect ratio of 4:3, which is fortunately the same as most computer displays.

Video systems have what is termed a safe area, regions outside this area cannot be guaranteed to be displayed on all video displays. When preparing videos care must be taken to not go to the very edge of the display. Refer to Section 5.5.1 for more details.

The frame rate (or fps, for frames-per-second) of the video system is important. Most video systems have a frame rate of about 25 (PAL) or 30 (NTSC) fps. As detailed above, it is impractical to generate 25 images of the visualisation for each second of animation, hence each frame (of the animation) must be held for potentially several frames of video.

Video systems are usually interlaced, in an effort to reduce movie flicker. Interlacing is where first the odd (or even) scanlines are displayed, followed by the even (or odd) scanlines. This reduces flicker, but means that thin lines on the screen, which are one or two pixels wide (especially horizontal lines) often flicker or seem to ``wobble'' on the screen. As such, video preparation involves making thin lines slightly thicker to avoid this problem.

Video systems generally have a smaller dynamic colour range than computer systems, and are not normalised in the same way, either. As such, the colours must be adjusted to compensate for this. This usually involves a low-pass filter to remove small changes in colours (ie. similar colours become the same) and also increases contrast. In addition, the gamma of the image is also increased to compensate for the loss in colour range.

next up previous
Next: 2.3.3 Animation presentation Up: 2.3 Issues involved in Previous: 2.3.1 Image generation
Kevin Pulo