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Creating an MPEG Movie

MPEG movies can be made in a similar fashion (by selecting Make MPEG in the Action menu). The MPEG parameters entry in the Options menu controls the quality and compression settings (see the documentation for mpeg_encode for more details on their meanings). One thing to keep in mind when making MPEG movies, however, is that this format is designed to produce 30 frames per second, but the files get very large if you try to include that many frames in your movies. In the days when StageManager was first written, this was not so much of a problem, since computers were not generally fast enough to display frames at that rate. Today, however, this is readily done, so we need to generate movies that accomplish this.

It is possible to make movie files that are small, but still play at a reasonable rate, by repeating each frame of the movie several times. To get a movie that plays at 7.5 frames per second (or one quarter the usual speed), you would repeat each frame four times. This can be accomplished in StageManager by using the Frame Parameters item in the Options menu. This opens a dialog box that lets you set some global values that tell StageManager how fast the movie should play and how many times to repeat each frame. For example, setting Pause to 4 and Speed to 1/4 will cause each frame to be repeated four times, and the movie to play at one quarter its natural speed, namely 7.5 frames per second. To make this more effective, the MPEG parameters in the Options menu should be adjusted so that the frame pattern is a multiple of four letters in length, for example, IPPP or IPPPPPPP or IPBBPBBP, depending on the compression and quality that you want. The first of these will cause every fourth frame (the ones where the movie changes) to be "I" frames, the highest quality ones, and the repeated frames to be "P" frames, which have more compression. Because the additional frames are simply duplicates of the existing ones, the MPEG file should not be much larger than one that has only one fourth the number of frames and runs four times as fast.

The examples that appear in my Math 53 web site were constructed in this way, as were the movies in the virtual art gallery, "Surfaces Beyond the Third Dimension". The other movies linked to this web site were not. The StageManager web site contains several additional example movie scripts that illustrate some of the more sophisticated features of StageManager, including the ability to manipulate several objects at the same time, and the ability to communicate with CenterStage to create more dynamic geometry.

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Created: 08 Sep 2001
Last modified: 07 Jan 2001 06:23:49
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