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July 8, 2009

What is a JPEG Artifact?

Example of jpeg artifactsArtifacts aren't just things that archaeologists dig up; they also show up in jpeg files.

The jpeg file format was developed as a compressed image format in the "bad-old-days" to save space on expensive hard drives and for display on the web. Just think back to the early days of the Internet when we all had slow dial-up connections that delivered email not much faster than snail mail, to the sound of "You've got mail". In those pre-broadband days, the emphasis was on small file size. Ta dah! JPEG and file compression to the rescue.

The JPEG format is called a "lossy" file format. The opposite of "lossy" is "lossless". Lossy is one of those geeky words that get made up to make long expressions short. Basically it refers to the fact that you will lose data when the lossy file-type is compressed.

Just when does a "lossy" file get compressed? Answer: When you save it.

So if you take photos in jpeg, or convert another image format, such as tiff, to jpeg by doing a "Save As jpeg", you are compressing the file each time you save it. Just what does compression do? Well it samples the pixels that are next to each other and if the surrounding pixels are the same or close to the same in their image information, the small amounts of data that make up the differences are thrown away and substituted with one piece of information for all of the pixels that formerly each had slightly different information. The outcome is a smaller or "compressed" file.

None of this is noticeable to the eye when you have saved the file only once, but when you compress the file multiple times by saving it over and over as you work on it, you are on the way to turning your beautiful photo into a Roman ruin or a broken Grecian urn. Each time you save the file, a little more image information is discarded until you get to the point where you end up with mere "artifacts". All that is left is for you to read some Keats and dream of beauty that once was.

Just like in archaeology, an artifact is a remnant of a larger whole that once existed intact. So jpeg artifacts are what remains of your original pixel information before you threw away much of that information each time you saved the file.

The example of jgeg artifacts that I have posted with this topic was once a photo of a cow I took on a trip to Mexico almost 10 years and three cameras ago. At the time I had just gotten my first version of Photoshop and was completely clueless. This could have been a great photo before I ruined it in my ignorance.

So stop saving your jpeg files over and over and keep an eye out for my next post which will go over some steps you can take to avoid jpeg artifacts in your images.

Gallery of examples for the "What is...?" series

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