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Jpeg Quality Loss

Tuesday, November 9th, 2004 by | Filed Under: Digital Photography
 
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Watch Those JPEGs!

JPEG is a great format, ya know? They can take a huge 30 meg file and make it look great at 3 meg – if only my diet worked so well :-)

However, there is a hidden danger behind all that digital dexterity. JPEGs are a “lossy” compression scheme. What they do is “dump” unnecessary information when they are created. The higher the compression rate, the more unnecessary information they tend to find. Before you lose too much sleep over that little fact, let’s take a closer look.

First off, a JPEG really does an incredible job at getting rid of information we either can’t see, or that’s easy to re-create when it’s opened.

For instance, let’s say you’re hangin’ out in Montana and happen to catch some of that big sky they have with your digital camera. You end up with a photo that’s just chock full of blue pixels. Since most of these blue pixels are exactly the same, JPEG compression dumps the look-alikes and recreates them when the photo is reopened.

It’s not a perfect process though, and if you zoom in with your imaging program you may notice that some areas seem “jaggy”. (Hey, something had to give with all that pixel dumping.) So here’s the rule:

Avoid re-saving JPEGs at all costs (OK, not “all costs”. If someone with a gun is demanding you re-save a JPEG, go ahead and do it – but only once! :-)

See, each time you resave the photo (not open and close, but resave it) you sacrifice some image quality on the alter of high compression. Every time a JPEG is re-compressed, some of the “imperfect” information that was created when the photo was previously compressed is made even more “imperfect”. Bottom line, the more you re-save a JPEG image, the more it’s going to deteriorate.

Here’s a sample:

Let’ start with this eagle:

Here’s a close-up of a wing from the original:

Now, after some repeated re-saving:

I know what you’re thinking – you have a gazillion JPEGS and now the idea of doing a little retouching scares the you-know-what outta ya. Well, that was the whole point :-) (kidding)

Actually, here’s how you work with JPEGs – save them in another format.

Open the JPEG you want to retouch in your imaging software, then Save As a TIFF file. Since TIFFs don’t lose info when they are saved (it’s a lossless format), you can edit, open, and save them all you want with no ill effects. Go ahead, spend an afternoon doing it and you’ll see I’m not just makin’ it up.

Sure, you’ll still have all of the “compression artifacts” from the original JPEG file, but they aren’t going to get any worse. To minimize the effect, be sure you have your digital camera set to save the highest quality JPEG possible.

Anyhow, once you’re done with your retouching, keep the TIF as your “original”. Then if you need a smaller file, you can always re-save the TIF as a JPEG and you’re all set.

Sneaky huh?

~ Steve

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