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Chapter 14. GIF Format > Interlacing

14.6. Interlacing

Normal GIFs are either displayed one row of pixels at a time, from top to bottom, or they wait until the entire file has downloaded before the image appears. On slow connections, this can mean potentially long waits with empty space and generic graphic icons on the screen.

As an alternative, you can save a GIF87a or 89a with interlacing. An interlaced GIF is displayed in a series of four passes, with the first hint of the upcoming image appearing after only 1/8th (12.5%) of the file has downloaded. The first pass has the appearance of a blurry mosaic; as more data flows in, the blurred areas are filled in with real image information and the image becomes more defined. The three subsequent passes fill in 25%, 50%, and 100% of the image information, respectively.


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