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Intraframe vs. Interframe Compression

So far, we’ve been discussing ways to reduce the file size of individual frames. Reducing the file size of our individual frames will obviously reduce the size of our movie overall, but there are qualities moving images possess allowing us to compress them in ways that we can’t compress still images.

When compressing a movie, if you simply compress each frame separately, you are performing what is known as intraframe compression. Some of you may have played intramural sports before or used an intranet. All these words share the prefix intra from the Latin meaning within or inside. Intramural sports are held within an institution and an intranet operates only within an organization. Intraframe compression only happens within each frame.

So where else would compression happen? It can happen between frames. It’s a simple idea and only works with sequential images like movies and animation. To help illustrate it, let’s look at some of the frames of our boomerang animation again.

1

Notice that the boomerang changes from frame to frame, but most of the rest of the frame doesn’t. Interframe compression uses this fact to compress moving images.

2

Interframe compression involves the analysis of the changes in the movie from frame to frame and makes note of only the parts of the image that have changed. The initial frame that is used to compare the others to is called…a keyframe! Obviously when there is a dramatic change from one frame to the next, as happens when one shot in a movie cuts to another, a new keyframe must be saved by the compressor. As it plays, a movie compressed with interframe compression only redraws the part of the frame that has changed. Instead of saving a complete image for each frame of a movie, it only saves a portion of the image. More keyframes means a smoother movie, but also means a bigger file.

More Info

  • More details about interframe compression

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