What does the interlaced option in Photoshop do?

Interlaced image loads an early degraded version of the whole image as soon as possible and then progressively renders the image to clear state. Interlaced will almost always be a bit bigger in filesize.

Non-interlaced image will load up in tiles showing clear image in each tile as it progresses to load in the image.

• .gif format follows the same idea.
• In .jpgformat
  • progressiveis the same as interlaced
  • baseline is the same as not interlaced

GIF Simulation of loading an interlaced png and a non interlaced png using Firefox.

Click to view full size »

The point of the simulation is to show how these two methods look visually when loading the image and not to compare their load times. Interlaced almost always adds a little to the filesize and therefore loads a little slower. There's also the perceived speed that is somewhat subjective. In this simulation, I used GPRS speeds (~7KB/s) and interlaced loaded 3 seconds later. Some people might say it looked like the Interlaced was faster. Some might say it's true, but it looked terrible when the image first started loading in. My personal preference is to not use interlacing.