A GIF image that gradually comes into focus. Instead of displaying a line at a time from top to bottom, lines at various intervals throughout the frame are displayed until all the lines are filled in. Most popular for Web graphics with slow dial-up service, the subject of the image can be recognized faster than with a regular GIF. In addition, it is also entertaining to watch the image come into focus. With high-speed Internet access, the interlaced GIF offers no advantage as both interlaced and regular GIFs display instantaneously.
It Takes Four Passes
Lines come on screen in four passes as follows: Pass 1 - starting at row 0, display every eighth row; Pass 2 - starting at row 4, display every eighth row; Pass 3 - starting at row 2, display every fourth row; Pass 4 - starting at row 1, display every other row. See progressive JPEG