What's the difference between the 'Normal' and 'Interlaced' when saving a .gif in Adobe Photoshop?
And which one is better for an image for the web?
when you're on a slower connection, you got the full effect of waiting for the image to come in. It can be torture sometimes. That's where the brilliant Interlaced GIF89a idea came from.
Interlacing is the concept of filling in every other line of data, then going back to the top and doing it all again, filling in the lines you skipped. Your television works that way. The effect on a computer monitor is that the graphic appears blurry at first and then sharpens up as the other lines fill in. That allows your viewer to at least get an idea of what's coming up rather than waiting for the entire image, line by line......Interlaced vs. Non-Interlaced GIF
People don’t like waiting. Today, even 5 seconds is a long time! The advantage of an interlaced image is that a viewer can start seeing the image as a whole (although not clear) from the very start of the page load.
An interlaced image renders by drawing every x number of horizontal lines first, then going back to the top and filling in the next set of lines, and repeats. It gives the effect of an image slowly becoming higher-res as it loads.
It was useful back in the day of dial-up connections. It's not used much these days.
Here's a good visual explanation of what happens as you load an interlaced gif. Unless you've got a really slow connection, the actual linked examples won't show as any different.
The page makes a good point that interlaced gifs are not smaller and don't actually complete downloading any faster - they just look as if they do, because they start showing content sooner.
Note that the image is not usually rendered so as to show the missing lines (Venetian blind style) but actually is rendered as if initially from very large squares and then decreasingly smaller squares by powers of two until single pixel lines finally reveal the intended resolution.
The interlaced gif perhaps is not used so much these days, but interlacing is still important where slow connections are prevalent - such as the much overlooked aid charity websites providing information in war torn or disaster struck areas, where images admittedly are used sparingly anyway, but as one example, the provision of updated maps (for secure or passable routes) in these situations is an important service. GIF is not the only interlaced format - for example PNG and JPEG have interlaced options.