Degradation of quality in Jpeg format

HS
Posted By
Hania_Stadtler
Dec 11, 2003
Views
238
Replies
4
Status
Closed
I understand that Jpeg compresses by throwing out pixels when you save a photo, and replacing them when you open. It replaces the pixels by guessing their color based on adjoining pixels. My questions are:

If you just open a pic to view without editing, will it always throw and replace the same pixel (lets say every 5th pixel) and therefore not add to the damage every time, or does it pic and choose the pixels randomly every time?

If it does replace the same pixel every time, would the degradation stop if all one does is adjust color, and only be and issue if one messes around with the placement of pixels, as in cropping etc?

I am new at this, so please forgive if I am an a totally wrong track.

Thank you in advance, you guys/gals have been great at clearing things up, and the existing posts have been a lot of help.

Hania

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CS
Chuck_Snyder
Dec 11, 2003
Hania, if you open and close the same JPEG repeatedly without editing, it will do the same thing every time. The rest of your question regarding what has the most effect on degradation requires expertise I don’t have, but directionally the less editing you do, the less degradation takes place. Also, saving at the highest quality and least compression is advisable for those images that you want to keep at full size for printing. Many forum residents who have tried to find the effects of editing, saving and reopening JPEG’s have had a difficult time finding the problem areas; it can be subtle unless you make major changes during multiple editing sessions and save at a higher compression/lower quality. Hope that helps directionally….

Chuck
BG
Byron_Gale
Dec 11, 2003
Hania,

I concur with Chuck’s comments.

As far as the affect done my merely adjusting color… If you zoom into your image close enough, you will see that it’s all just a bunch of colored pixels. PSE (or any other raster image editor) does not know that two neighboring pixels may both be part of someone’s shoe, or the sky. They’re just pixels with assigned color and brightness values.

If you do a color adjustment, you’re changing pixels.

Whenever you save a JPEG, the algorithm is applied anew, and compression done as permitted by the current settings, and the current image pixels.

Byron
BH
Beth_Haney
Dec 11, 2003
The simple act of opening a JPEG, viewing it, and closing it again will not cause the photo to degrade. The gradual reduction in quality comes from opening the JPEG, making any edits – cropping or color, and then saving the images so the edits will be retained.

Someone else may be able to explain the way in which pixels are chosen for destruction, but not me.

You can actually do quite a bit of editing and resaving to a JPEG before you begin to notice any loss of quality. However, as a general rule, working on images in JPEG is discouraged for a couple of reasons. First, JPEGs cannot support layers, and editing through the use of layers can be very useful. Second, that issue of quality loss is real, but there’s no good way to accurately predict the point at which you’ve placed that last straw on the camel’s back and lost some image detaill you really wish you still had.

For people who are concerned about hard drive space, it has been suggested that an image be converted to a non-lossy format like PSD or TIFF for the editing process and then resaved as a JPEG for long term storage. I happen to prefer the method where I archive one original JPEG and then keep either PSDs or TIFFs as my working files – moving them to CDs as I need to in order to free up space. In my case space isn’t a big issue, but CDs also provide good backup in case of system failure.

Edit: Duh. I type too slow! (Talk too much?!) There wasn’t an answer when I first got here! 🙂
HS
Hania_Stadtler
Dec 12, 2003
Chuck, Byron, and Beth,

Thank you so much for responding so quickly and in such detail. I am in a very juvenile state of my Adobe venture but looking forward to enjoying the creative journey. Knowing that you all are there when I get stuck is very comforting and encouraging. Thank you again.

Hania

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