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deterioration with multiple saves
.JPG / JPEG is a format standardized in 1992 and stands for Joint Photographic Experts Group. It is normally pronounced "J-peg."
The JPG format is excellent for saving images. It saves a tremendous amount of hard drive space by compressing images. You can control the amount of compression in order to balance file size with overall appearance. JPGs save more space than any other image format.
Although compression is the single feature that makes JPGs so popular, that very feature can cause probles: JPGs lose information every time you save.
You can open JPG images an infinite number of times with no loss in quality. However, each time you save JPG images, you lose quality. For that reason, the JPG format is known as a "lossy" format.
As explained later, the loss is most severe if you change pixels in any way before you save.
I don't want you to be afraid of saving images as JPGs, but I want you to understand that the amount of loss depends on two factors: the program you use and the amount of compression you use (more compression = more loss in quality.)
The one sure fix
The easiest way to overcome the loss of quality problem is to save your master images as TIFs. TIFs are 'lossless,' meaning they do not lose detail when saved. Unfortunately, TIFs have the opposite problem of JPGs; they consume a huge amount of space for no gain in quality. I recommend saving irreplaceable images as TIFs and replaceable images as JPGs. Please understand that all formats have their assets, shortcomings and primary intended uses. (See my review of popular File Formats.)
How severe is the quality loss?
The loss is not bad IF you simply open and re-save a file a few times. The better the program, the less severe the loss. I have purposely opened and re-saved JPGs in Photoshop as many as 25 times with very little visible loss in quality. Photoshop is the expensive gold standard of image manipulation programs. I have tested several other top notch programs using the same methods and they all fall short of Photoshop. Sorry friends, I have never encountered any program that even comes close to the quality and features of Photoshop.
If you open a JPG, re-size or rotate the image and then save it, you will acquire visible problems in as few as three or four passes. Again, the deterioration is less evident when using Photoshop compared to any other program I've tested.
Examples of multiple saves without changing pixels
Here is an original image.
Using three popular image programs, I opened the original image and re-saved it as a new JPG. Then, I opened that new file, re-saved it again as a new JPG. I repeated the process ten times with the intent of discovering whether all programs performed similarly. They DO NOT!
The resulting examples are enlarged about 150% in order to make the problems more visible. In order to accelerate image deterioration each time I saved, I set the programs to compress at 75 out of a 100 (or 25% "quality" depending on the program.)
This image was saved with the #1 image manipulation program. There is less image deterioration, although the eyes and hair still show definite quality loss after 10 saves at 75% compression.
Photopaint® is part of the Corel Draw suite. In this example, image deterioration is more pronounced after 10 saves. JPG "artifacts" (sparkly disortion) are most noticeable above the child's helmet and under his arm. As this example shows, the image is still completely usable.
(now owned by Corel)
This is a low-cost competitor to the above two programs. Simple open-saves do not deteriorate the JPG quality noticeably more than the higher-priced programs. Still, having said that, you can notice more image deterioration in the arm at left and the engraved 'swooshes' at right.
If you open a JPG file, change the pixels in some manner, and then re-save the image at less than top quality, you can get into trouble very quickly. You can change pixels by rotating, re-sizing, changing colors, or changing brightness.
If you save JPGs at the highest possible quality, deterioration stays manageable for longer than many writers would have you believe.
I wanted an extreme example, so I took the original image, rotated it one degree using Photoshop®, re-saved the file at 25% quality and closed it. Then, I re-opened the file, rotated it another degree, and saved again. After 9 rotations, I rotated the image nine degrees back to its original orientation.
Original image saved at only 25% quality and very much appropriate was images delivered via the web.
10 simple open/saves. Although the image is still very much usable for the web, you can notice some odd coloration at left. Deterioration in quality is minimal, but most evident in the softened details around the eyes and at the ends of the swooshes.
10 open/rotate/saves. Even with Photoshop, you can notice significant deterioration in details. Much JPG artifact is also evident. Probably no one would open, adjust and re-save a file ten successive times, but now you know what to expect.
This example illustrates the problems with multiple saves very clearly. When you change pixels, and then re-save, you radically deteriorate JPG quality, even with Photoshop. (The versions of Photopaint and Paintshop I tested lacked precise rotational controls, so I did not bother to create potentially misleading examples.)
What if you need to change an existing JPG image?
This is kind of a tricky question. If you need to make a change, and don't think you will need to make more changes at a later time, then bite the bullet. Simply open the JPG, change it, and save it back.
However, if you think you might want to make several changes at different times, take a different approach. I'd recommend opening the JPG in your image program, then saving it immediately as a TIF. Do all your alterations and experimental work on the TIF, and then, when satisfied, save back as a JPG.
See Understanding JPGs: JPGs are not created equal for more examples of problems with JPG compression.
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