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When saving a image for web and devices as JPEG, I can either choose progressive or optimized (or none). What is the difference between both and what should I use ? In my case, the image is the background image or a website (20KB).

On mouse-over, it says:

Progressive: Download in multiple passes

Optimized: Creaets smaller but less compatible files

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I know what progressive is visually. When those pictures load in all blurred up and low-res, then clear up eventually. It is sort of Layered pics in the file. This requires the file size to be a bit larger, the graphics have to re-refresh / re-drawn, and the Viewer gets to see some useless mudded up image FAST , instead of nothing . msdn.microsoft.com/en-us/library/windows/desktop/… Except in very specific instances like massive pictures , or horrible connection speeds, IMO it is usually a waste :-) of data time and effort, for little. –  Psycogeek Jan 17 '12 at 14:13
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@Psycogeek Progressive should help with browsing a large number of large(ish) files in Windows' image viewer, since you get a rough outline of the image much more quickly than having to wait for it to finish. Although the web/devices image sizes will hardly make a difference here. –  Daniel Beck Jan 17 '12 at 14:21
    
@DanielBeck Yes should, Thumbs Fast. also the different picture types use methods that dont require more data, just more work to refresh it again. I donno no mater how bad my connection, and speed of like a portable device, I have never wanted to see pixelated mud , I just want them to "get on with it" and get to the real thing. –  Psycogeek Jan 17 '12 at 14:32
    
Progressive jpegs for large images were much more useful in the dial-up modem days. –  paradroid May 24 at 12:48

1 Answer 1

Here's one answer:

progressive JPEG - Computer Definition

A JPEG image that comes into focus while it is being displayed. Instead of rendering the image a line at a time from top to bottom, the whole image is displayed as very low-quality and fuzzy, which becomes sharper as the lines fill in. It gives the illusion that the page has downloaded faster even though it takes the same time to achieve the final sharpness. The progressive JPEG is created in multiple passes (scans) of the image. See interlaced GIF and JPEG.

from: http://www.yourdictionary.com/progressive-jpeg

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Whilst this is accurate, you might find the article here can be used to explain how an optimized JPEG differs from a normal one: impulseadventure.com/photo/optimized-jpeg.html. It has a very detailed explaination but the overall idea is that Photoshop runs additional analysis on the image and takes extra steps to ensure that it generates the smallest possible file. –  James Apr 24 at 10:14

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