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About Codecs > About Codecs - Pg. 290

Understanding QuickTime This simplifed example illustrates the power of compression software, whose job it is to make graphics fles smaller by recording their pixel colors more effciently. This kind of compression explains why a JPEG fle always takes up far less space on your hard drive (and less time to download by email) than, for example, the Photoshop or AppleWorks document that created it; the JPEG fle has been compressed. This form of fle-size reduction is called spatial or intraframe compression. iMovie analyzes the picture on each individual frame and reduces the amount of information needed to describe it. Temporal compression But there's another way to reduce the size of a QuickTime fle, too. Not only is there a lot of redundant color information from pixel to pixel on a single frame, but also from frame to frame. Suppose, for example, that you've captured some footage of a man sitting behind a desk, talking about roofng materials. Picture the frst pixel of the back wall in that piece of footage. Chances are good that this pixel's color remains absolutely consistent, frame after frame, for several seconds at least, especially if the footage was shot using a tripod. Same thing with the rug, the color of the desk, the fern in the pot beside it, and so on. These elements of the picture don't change at all from one frame to the next.