Why Do JPEGs Degrade?

Okay, so basically…

JPEGs have an aggressive data compression scheme that discards some image data as a result of continuous editing/saving of that image. This is known as lossy data compression. What this means is that, in order to save space, a little bit of the information is lost. For things like photographs to be displayed on computer monitors from a Web Page, you won’t notice it most of the time. That’s why it’s the preferred image technology for showing images on the Web.

The compression algorithm used to produce the JPGs is based on 8×8 pixel squares. There is a correlation between these 8×8 blocks and the file’s compression. Even if you were to use the highest possible Quantization Coefficient, (100 in Photoshop’s Save for Web command), there will be the loss of information as the image’s compression algorythm attempts to reduce the image size—which consists of higher frequency colors being discarded and lower frequency colors being kept. If you look closely at some overly-saved JPGs you will notice 8×8 tiles of pixels across the surface that result in a blurry image.

So,

If a master copy is lost for an image, the original quality is gone forever and successive editing of the JPG will ruin the original image in time because of lost information.

As best practice for the future, try getting in the habit of making your JPG’s from an archived TIFF and you’ll have great success at creating beautiful images from the original for years to come. Original images are often stored as TIFF image format because this format retains all of the original image’s information.

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After editing and re-saving an image multiple times.

 

 

 

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A photo of a cat with the compression rate decreasing, and hence quality increasing left to right.

 

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After rotating an image and re-saving 2000 times.

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Sources:

https://answers.yahoo.com/question/index?qid=20101224113706AAgIV0v

http://www.reddit.com/r/explainlikeimfive/comments/27222k/eli5_how_do_digital_images_degrade_over_time_from/

http://en.wikipedia.org/wiki/JPEG

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Love, Kellie.

 

 

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