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#1
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JPEG Questions: Loss In Quality When "Saving"
I'm confused about quality deterioration when repeatedly opening and
"saving" JPEG images. I've read that every time a JPEG image is "saved," some quality is lost due to the lossy compression format. My questions are.... 1) What is meant by "saving" a JPEG image? 2) If I simply "open" a JPEG image in Microsoft Photo Editor, Adobe Photoshop CS or any other image editor, View the JPEG image and then simply exit out of viewing the JPEG, am I actually "saving" it again and therefore loosing quality? 3) If in Windows I right click on a JPEG and make a duplicate copy of the JPEG, is the copy (with a different name) "saved" and therefore the quality of the copy diminished. 4) If I open a JPEG in a photo editor and "save as" (instead of simply exiting) and use the same name as the JPEG I just opened, is the quality diminished? When I do all of the above things, I don't notice any decrease in quality and the JPEG seems to be exactly the same size so I'm guessing I haven't degraded the original. If I'm wrong, I'm guessing I should make each JPEG file "read only" or burn them to a CD or DVD so they can't be changed when I view them. If I open up a JPEG in a photo editing program and make any changes to the JPEG at all, I can understand that the quality degrades. Any advice on my questions are appreciated. |
#2
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JPEG Questions: Loss In Quality When "Saving"
Xtx99 wrote:
I'm confused about quality deterioration when repeatedly opening and "saving" JPEG images. I've read that every time a JPEG image is "saved," some quality is lost due to the lossy compression format. My questions are.... 1) What is meant by "saving" a JPEG image? File- Save or Save As. Or in some software, File- Export, although that generally exports a *copy* and won't overwrite the original unless you tell it to by giving it the same file name in the same directory. 2) If I simply "open" a JPEG image in Microsoft Photo Editor, Adobe Photoshop CS or any other image editor, View the JPEG image and then simply exit out of viewing the JPEG, am I actually "saving" it again and therefore loosing quality? No. You didn't save. 3) If in Windows I right click on a JPEG and make a duplicate copy of the JPEG, is the copy (with a different name) "saved" and therefore the quality of the copy diminished. No. You haven't saved it, you've copied it. 4) If I open a JPEG in a photo editor and "save as" (instead of simply exiting) and use the same name as the JPEG I just opened, is the quality diminished? Yes. You've overwritten the original file with a copy that is additionally compressed. Whether you will be able to immediately see new artifacts would depend, I guess, on what your compression is set to. If it's compressing by only 1% each time, you probably wouldn't see it until after you'd done so several times. When I do all of the above things, I don't notice any decrease in quality and the JPEG seems to be exactly the same size so I'm guessing I haven't degraded the original. If I'm wrong, I'm guessing I should make each JPEG file "read only" or burn them to a CD or DVD so they can't be changed when I view them. Well, you could quit using .jpg as an archival file format. Use .tif, ..bmp, or your image editor's native file format. These should all be lossless. -- Angela M. Cable Neocognition, digital scrapbooking source: http://www.neocognition.com/ PSP Tutorial Links: http://www.psplinks.com/ 5th Street Studio, free graphics, websets and mo http://www.fortunecity.com/westwood/alaia/354/ |
#3
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JPEG Questions: Loss In Quality When "Saving"
Xtx99 wrote:
I'm confused about quality deterioration when repeatedly opening and "saving" JPEG images. I've read that every time a JPEG image is "saved," some quality is lost due to the lossy compression format. My questions are.... 1) What is meant by "saving" a JPEG image? Most software has a File/Save or File/Save As feature, this causes it to re-compress the jpeg file. 2) If I simply "open" a JPEG image in Microsoft Photo Editor, Adobe Photoshop CS or any other image editor, View the JPEG image and then simply exit out of viewing the JPEG, am I actually "saving" it again and therefore loosing quality? No, your just viewing it. 3) If in Windows I right click on a JPEG and make a duplicate copy of the JPEG, is the copy (with a different name) "saved" and therefore the quality of the copy diminished. No, your copying rather then saving. 4) If I open a JPEG in a photo editor and "save as" (instead of simply exiting) and use the same name as the JPEG I just opened, is the quality diminished? Yes. What happens is that when a Jpeg is saved, the compressor throws away information it doesn't think it needs, in order to achieve it's high compression ratios. You probably don't notice unless you load both the before and after images and use high magnification. When I do all of the above things, I don't notice any decrease in quality and the JPEG seems to be exactly the same size so I'm guessing I haven't degraded the original. If I'm wrong, I'm guessing I should make each JPEG file "read only" or burn them to a CD or DVD so they can't be changed when I view them. If I open up a JPEG in a photo editing program and make any changes to the JPEG at all, I can understand that the quality degrades. Any advice on my questions are appreciated. Jpeg is good when the ultimate goal is tiny files, where small details are probably lost anyway. For example Jpeg is good on the web, where tiny file size is most important. Other compression systems, that are lossless don't compress as well. Saving as TIFF or PNG is better, when your manipulating the files potentially several times over. My preference is to work with TIFF and then compress using Bzip2, as it's one of the better compressors, for cold storage on CD-R. Paul |
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