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I think I just noticed that IE5.5 doesn't render a progressive jpeg until the whole image is there, and then it drops the graphic onto the page in one piece, fully rendered. Netscape, however, does the multiple scans, progressively sharpening the image as more data downloads.
Is this always the way it is?
I've already verified the difference between MSIE 5.5 and Netscape 4.7 on Windows. Netscape quickly shows the first scan and progressively renders the sharpening (similar to an interlaced GIF). But Explorer 5.5 show a blank page until the whole file is downloaded. What do you know -- a place where Netscape is superior!
This has important implications for some of my clients, who sell high end art and photography, and often offer enlargements in the 100-200 kb area.
If I make the jpg file standard, and not progressive, then the viewer can at least see the scan line progress down the page in either browser.
If I make the file a progressive jpg, the file is almost always smaller in total size, but nothing at all shows on the MSIE page until everything is downloaded. That would be very unfriendly to visitors on dial-up modems.
I read yesterday that IE has always had a problem with correctly rendering progressive JPEG files... but apparently they figured out a fix for it in the Mac version.
Considering IE/Win is far and away the most common platform/browser combo though, that could definitely become a problem for image-oriented sites.
From The FireWorks Bible by Joseph W. Lowery page 498
Internet Explorer doesn't fully support Progressive JPEGs. They are displayed just as if they were not Progressive, though, so there's no harm in using them.
Although Internet Explorer displays Progressive JPEG images, it displays them without using the progressive look of developing as they download. They seem to the user to be regular JPEG images.
Well, I'd say Mr. Lowry got it wrong. MSIE definitely waits and displays a progressive JPEG all at once, instead of displaying the image gradually from top to bottom, as it does with a standard JPEG.
On these art-oriented sites, I've been accustomed to using a pop-up window to display enlargements. Wherever possible, I will now avoid progressive JPEG. I was trying to be user-friendly, and ended up being unfriendly to the visitor.
I feel it's often a good thing to give the pop-up window a status bar. It's a courtesy, so the viewer can tell if the page is hanging, or just rendering slowly. Unfortunately, Explorer doesn't give the kind of detailed status info that Netscape does, so this is little help with a progressive JPEG. In fact, Netscape 6 also went to less informative status messages. The "AOL-factor" is at work I fear.
My aggravation is that Progressives are the smaller file size, and with a large image, the difference can be considerable. Still, it seems to me that the best choice is to at least show the visitor something, even if the entire download takes a bit longer.
That's wierd. When I look at it, it does do it from top to bottom .. it *doesn't* wait until the whole thing is loaded. It does a little of the "fade in" like a progressive is supposed to do, but not as much.
Using IE 5.5 on a Mac.
However, for Brett's sake, I will not quote Mr. Lowery again...
<edit_later>Just read Teresa's post...sigh...</edit_ later> What do you know, a place where Mac is superior to PC ;)