Non-interlaced versus interlaced
It takes some time to load a picture over the Internet - especially if the picture is big or you are loading in us prime time. To get the smoothest result on the screen, GIF pictures can be in non-interlaced or interlaced mode.
Non-interlaced: The first n lines is read and displayed then the next n lines etc.
Interlaced: Line 1, n+1, 2n+1 ,... is read and displayed then line 2, n+2, 2n+2, ... etc.
Some browsers will smooth interlaced pictures by stretching the loaded lines to fill the rest. It usually looks very nice and creates the illusion, that the image is loaded faster.
You should generally choose interlaced mode for big pictures.
The images below are animated - but they show what it is all about.
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