Field 1 Field 2
Field rendering computer animations results in much smoother video sequences once they have been recorded to video.
The reason is that in video, each frame is composed of two fields, which "interlace" to create a full frame. While we usually think of the video frame rate as 30 FPS in NTSC or 25 FPS in PAL, the reality is that video signals are drawn at 60 fields per second in NTSC, and 50 fields per second in PAL. The following diagram illustrates how a full frame is "drawn" on screen:
Field 1 Field 2
In field 1, the odd-numbered lines are drawn, and in field 2, the even-numbered lines are drawn. The two fields combine to form a single frame.
Since there is a time difference (1/60th of a second in NTSC or 1/50th of a second in PAL) between the two fields, objects in motion will appear in different positions in subsequent fields.
But in a QuickTime animation created at 30 FPS (25 PAL), moving objects remain in the same position for both fields of each frame. And virtually all video tape and disc recorders record a full frame at a time. This often results in visible "jumping" or "jitter" of moving objects when an animation is recorded to video. MovieTools works by turning adjacent images (frames) of a QuickTime movie into interlaced fields, which it then combines into a single frame.
Viewed on the Mac, each frame will look very odd, with moving objects offset on alternating lines (as in the graphic at the top of this file, which was field rendered in MovieTools from an animation movie). When such a file is recorded to video, however, the animation will appear much smoother than with non-field-rendered movies.
Most of the 3-D animation and rendering programs available for the Macintosh do not perform field rendering. Even if they can create movies with a frame rate of 60 FPS (or 50 FPS in PAL), each image is rendered complete and intact, with no field interlace.
Until now, in order to field interlace a QuickTime animation file, one of the few options available was to use Adobe AfterEffects as a utility to perform field rendering. Now let us not be misunderstood; AfterEffects is a great program that can do a whole lot more than simply field render. If you're looking for extremely sophisticated QuickTime editing and effect capabilities, we highly recommend it.
On the other hand, if all you need to do is field render your animations before recording them to video, then MovieTools can do it for a lot less money than AfterEffects.
Here's the bad news: Rendering a file in your animation program for field rendering in MovieTools will take twice as long as creating a file for standard frame rendering. Achieving smooth video output requires that twice as much information be created.
If your rendering program can create files at 60 FPS (50 PAL), you should set that as your frame rate. In such instances, you can design your animation at the correct time values. In other words, a sequence that will have a final on-tape duration of five seconds, for example, can be designed with a five-second duration in your rendering program at 60 FPS NTSC/50 FPS PAL. It will consist of 300 (NTSC) or 250 (PAL) images. The final file after field rendering in MovieTools will consist of 150 (NTSC) or 125 (PAL) field-rendered frames.
If your rendering program tops out at 30 FPS (or 25 for PAL), then you should design your animation at twice the duration in your rendering program. For example, if the final animation should have a five-second duration, you would design it with a ten-second duration in your renderer, again consisting of 300 (NTSC) or 250 (PAL) frames. After field rendering in MovieTools, the final duration will be a file half of that, or five seconds, comprised of 150 or 125 field-rendered frames.
MovieTools doubles the original frame rate for a movie. Also, each frame of the output movie is created as a "Key Frame". This results in lousy performance for live, real-time playback on the Mac, but excellent performance for recording to video media.
Because MovieTools compresses time as a basic aspect of its operation, it provides no support for audio. As its use was envisioned, this should present no problem, since 3-D rendering software doesn't create an audio track in the first place. If you need to add sound channels to a movie, please do so after you've run it through MovieTools.
Finally, since MovieTools works with pairs of frames, if the input movie has an odd number of frames, the final frame will not be included in the output movie. For correct results, create your original movies with an even number of frames.
Field rendering a movie will require free disk space equal to slightly more than 50% of the original movie size. For example, if you're field rendering a movie with an original size of 100 MB, you'll need at least 50 - 60 MB free disk space.
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