Do jpg images degrade every time they're stored and retrieved?

[lemastre]

I recently acquired my first digital camera. It produces only jpg images. I understand that when jpg

images are saved in a computer, something is "lost" via some compression process. This

suggests to me that each time I open a jpg image and work on it, e.g., fiddling with the color or

sharpening the image, etc., it loses something when it's saved again in its jpg form. The result of

repeated savings would thus seem to be repeated losses, resulting in degradation of the image. If my

understanding is correct, then would it be better to convert the jpg images to tiff immediately and do all

work on these, which I understand don't suffer such losses? If so, I suppose the final version of the

images could be archived as jpg to save disk space.

[fanta]

It makes sense to me :) Only thing, I would store the final version of the images as TIFF anyway: later on, even years from now, you may want to do some further changes to your immage, and then you want to resume from the TIFF file. With single-layer DVDs storing 4.7 DB of data, and hard disk that are cheaper and cheaper, I see no convenience in storing the final image as Jpeg.

[rainer_t]

Keith, yes, I think your understanding is correct.

 

Only one minor adjustment ... JPG files do NOT loose any information

when being 'saved' ... you can copy a jpg from one disk to another 100 times without any further loesses. JPG files DO loose information when the jpg-compression is applied on an image (this happenes the first time in your camera ... here's where you loose some information).

 

Later, if you modify the image, you have to apply the jpg-compression again. Again you loose information. Therefore, when you want to edit an image, it's a good idea to use tiff. Tiff also has options for compression, but usually tiff is uncompressed, and tiff knows some lossless (but therefore not as effective) compressions like lzw and zip.

 

Rainer

[eric_boutilier_brown1]

Your understanding is totally correct, though given the low cost of DVD burners and disks, I would not recommend returning to JPG format from a TIFF format unless you were posting the image to the internet, using it in an e-mail, or in some other context where JPG is the only format usable.

[Craig_Cooper11664875449]

<i>JPG files do NOT loose any information when being 'saved' ... JPG files DO loose information when the jpg-compression is applied on an image (this happenes the first time in your camera ... here's where you loose some information)</i>

<p>

Make sure you understand something before you commit it to an informational thread. JPEGs lose information <b>every</b> time you (re)save them. How much depends on the quality settings.

[esteve]

One thing is copying a file byte-by-byte. This is of course lossless.<br/>Another thing is loading an image in a photo-editing software, and saving it later. If you save it in JPEG, there is a loss, always, nothing to do about it.<br/><br/>

So, load the image, process it and save in TIFF (or any other lossless format). Saving in JPEG again has to be the last step in your workflow, if you require to deliver files in JPEG.<br/><br/>

Esteve

[Richard Williams]

Rainer is making the distinction between byte-by-byte copying (e.g. 'saving' to a different location using your computer's operating system) and 'saving' using your image editor (which usually re-runs the jpeg compression algorithm). Note also that SOME image editors (e.g. IrfanView) can perform SOME very limited and specific jpeg image manipulations losslessly (like 90 degree rotation) - see section 10 of the jpeg FAQ:

 

http://www.faqs.org/faqs/jpeg-faq/part1

 

Dedicated Exif editors like ExifTool also make and save lossless changes to the file's metadata (this may not be true of image editors with built-in Exif editing). But it's generally safest to assume the jpeg algorithm will be re-run by an image editor (and hence degrade the quality) even if you've made no changes to the image (unless the documentation says otherwise). It's therefore a very good idea to use a lossless format like tiff (or Adobe's PSD) as your 'working copy'. If you don't want to keep this indefinitely, at least archive the original jpeg that came out of the camera so you can go back to it if necessary.

[lemastre]

I appreciate your prompt responses to my question. I'll henceforth tiff most of my images.

If I decide to pursue digital photography, I 'll get a camera that offers a choice of output

formats. Also, I expect I'll need a lot more storage. As computer users go on amassing

more and more files, I envision a time when home computers are lashed to terrabyte-size

memory units. When a person retires, the first thing the spouse wants done may still be

cleaning the garage, but number two may be cleaning out the computer files.

[bobatkins]

Actually JPEGs don't lose appreciable quality when opened and stored at the same compression settings. It's a bit like compressing a compressed file. The compression has already been done and the compression algorithm finds little to compress.

 

If it's opened, changed and resaved, again most of the image is already compressed and so those areas don't get significantly recompressed

 

It's not a great idea and you may lose something, but it's not the absolute disaster some people make it out to be.

 

Here's an example of an image that was opened, changed and saved as a new file 10 times, so the final file had been through the same JPEG compression algorithm (15:1 compression) 10 times. There's a small difference, but not much. If the file was just opened and saved 10 times (with no changes), the JPEG recompression would have no visible effect on the image at all<div></div>

[blueviews]

Another alternative is jpeg2000 which can be lossless. We did the open, save thing 500

times and the file size stopped getting smaller after 8 or 9 repeats with JPEG

 

Bill