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Digital Artifacts…

Compression

artGirl
The file before compression.

Data file compression can be divided into two obvious camps. "Non lossy" compression implies that there is no loss of image quality in the process, but usually doesn't afford much decrease in data size. "Lossy", as the name suggests, involves data shedding and therefore implies image quality loss particularly when using highly compressed settings. The most common, ubiquitous even, lossy file format is JPEG, so called because it was proposed by and is maintained by the Joint Photographic Experts Group. Just about every digital camera on the market can save to this format. Note that there are a variety of JPEG format variations, but those originating from cameras are all readable by common imaging software.

artGirlCom
A radical amount of compression produces a very small file but the cost is in clarity. Mushiness, blocking and color jumps are typical problems with over-compression.

What varies most obviously, is the amount of compression applied to the image data. This can vary from say 1:4 (great quality) to 1:28 (rather poor), with each camera manufacturer deciding on what compression options to offer and what mathematical formulas will be used to achieve them. The worst results come from high compression of small data sets, such as you would get from cameras with small sensor arrays. So what are JPEG compression artifacts likely to look like? It depends to some extent on what algorithms are used, but generally speaking, more compression is likely to produce "mushy" areas that lack sharpness, especially obvious in the flat areas of an image. Overemphasized edges and unnatural color distribution are other common artifacts. Random pixels that are quite different from those that surround them are also likely to be seen. Note also that because compression is done last, image artifacts such as sharpening and color saturating are likely to be compounded.

For some applications, such as displaying thumbnail images on a web page, high ratios can be quite acceptable, but for best results use compression sparingly. Are you wondering just how much compression to use? There is no set rule apart from: try it and see. It's important to see the results as your viewer would in final form such as on a print or on the computer screen. Be aware also, that most image manipulation software will show you the image at the original quality setting BEFORE compression was done. You have to close the file and reopen it in its new compressed form to see exactly what it looks like.

 

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