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September 21, 2009

jpeg Picture Processing

Back in July I wrote this post about jpg artifacts and promised that I would give some tips on handling jpg (jpeg) files to avoid artifacting. Just as a quick refresher, jpg artifacts result from the over-compression of jpg files to the point that pixel information is so degraded that you start to see large groups of square pixel shapes where once many pixels existed.

JPEG files are an image file format that is "lossy". This means it is a compressed file type with some of the original file information "lost". Jpg file types were invented to display images online in the early days of the "dial-up Internet" when we all used to wait (im)patiently for files to appear in our browser windows.

Example of jpge artifacts
The compression of a jpg file happens at the time you save the file. Initially you won't notice any apparent difference in the image, but save the file often enough, you eventually will end up with jpg artifacts, just like my cow example to the left. Yes, that was what I did in my early days as a naive Photoshop user.

So here are my easy steps to avoid jpeg artifacts.
  • If you are shooting your pictures as jpeg files, shoot at the highest quality possible for your camera in the first place. Most cameras have settings for file quality under their menu settings. With memory cards as big and cheap as they are today, there's no reason to shoot at anything but the highest quality jpeg file if you are shooting in jpeg. This will mean you are starting out in the first place with the most image information possible.

  • When you are ready to process your file , make your very first step that you take a conversion to a "lossless" file type such as tiff (.tif), or .psd if you use Photoshop. Depending on what image editing program you use, there may be a different file type other than psd, which is Photoshop's proprietary file type. All image editing programs probably offer tiff as an option, which is a standard/generic lossless image file. Lossless means what it says: there is no compression of the file when you save it, so no matter how many times you save the file, none of the image information is going to be lost. In fact, you may find it actually gets bigger as you process it.

  • Now do all your processing such as cropping, red-eye removal, whatever you do typically, with the lossless file that you have opted for.

  • When your picture is ready for prime time and sending to the printer or emailing or uploading and you want or need it to be a .jpg file once again, do one, and one only "Save as jpg". Never save that jpg file ever again.

  • Treat your lossless psd or tif file as an archive file, and if you subsequently want to tweak your picture some more, come back to that file. You can save it with a different name so that you have a new version. Once again, only save it as a jpg file at the very end of your processing.
Follow these easy steps in processing jpeg pictures and you should never be visited by the horrors of jpeg artifacts. Happy photo editing!

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