compression programs

This info comes from an article at About.com by Sue Chastain. I thought it was very useful info for all out there storing their photos in the JPEG format. I have edited the content a bit to shorten it.

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With the explosion of scanners, digital cameras and the web, the JPEG image format has quickly become the most widely used digital image format. It’s also the most misunderstood. Here’s a collection of some common misconceptions and facts about JPEG images.

JPEG is the Proper Spelling
True. Although the files often end in the three-letter extension JPG, when referring to the file format it is spelled JPEG. JPEG is an acronym for Joint Photographic Experts Group, the organization that developed the JPEG format.
JPEGs Lose Quality Every Time They are Opened and/or Saved
False. Simply opening or displaying a JPEG image does not harm the image in any way. Saving a JPEG repeatedly during the same editing session (without ever closing the image) will not accumulate a loss in quality. Copying and renaming a JPEG will not introduce any loss, but some image editors do recompress JPEGs when the “Save As” command is used. To avoid more loss, you should duplicate and rename JPEGs in a file manager, rather than using “Save As JPEG” in an editing program.
JPEGs Lose Quality Every Time They are Opened, Edited and Saved
True. If a JPEG image is opened, edited, and saved again it results in additional image degradation. It is very important to minimize the number of editing sessions between the initial and final version of a JPEG image. If you must perform editing functions in several sessions or in several different programs, you should use an image format that is not lossy (TIFF, BMP, PNG) for the intermediate editing sessions before saving the final version. Repeated saving within the same editing session won’t introduce additional damage. It is only when the image is closed, re-opened, edited, and saved again.
JPEGs Lose Quality Every Time They are Used in a Page Layout Program
False. Using a JPEG Image in a page layout program does not edit the source JPEG image, therefore no quality is lost. However, because each page layout software uses different types of compression on their native document files, you may find your layout documents are considerably larger than the sum of the embedded JPEG files.
If I Compress a JPEG at 70%, Then Later Reopen It and Compress It at 90%, the Final Image Will be Restored to a Quality Setting of 90%
False. The initial save at 70% introduces a permanent loss in quality that can’t be restored. Saving again at 90% quality only introduces additional degradation to an image that has already had considerable loss in quality. If you must decompress and recompress a JPEG image, using the exact same quality setting each time seems to introduce little or no degradation to the unedited areas of the image. However, the same setting rule just explained doesn’t apply when cropping a JPEG. JPEG compression is applied in small blocks, typically 8 or 16 pixel increments. When you crop a JPEG, the entire image is shifted so that the blocks are not aligned in the same places.
Choosing the Same Numeric Quality Setting for JPEGs Saved in One Program will Give the Exact Same Results as the Same Numeric Quality Setting in another Program
False. Quality settings are not standard across graphics software programs. In other words, a quality setting of 75 in one program may result in a much poorer image than the same original image saved with a quality setting of 75 in another program. It’s also important to know what your software is asking for when you set the quality. Some programs have a numeric scale with quality at the top of the scale so that a rating of 100 is the highest quality with little compression. Other programs base the scale on compression where a setting of 100 is the lowest quality and the highest compression. Some software and digital cameras use terminology like low, medium, and high for the quality settings.
A Quality Setting of 100 Does not Degrade an Image at All
False. Saving an image to JPEG format, always introduces some loss in quality, though the loss at a quality setting of 100 is barely detectable by the average naked eye. In addition, using a quality setting of 100 compared to a quality setting of 90-95 or so will result in a considerably higher file size relative to the degree of image loss. If your software doesn’t provide a JPEG preview, try saving several copies of an image at 90, 95, and 100 quality and compare file size with image quality. Chances are, there will be no distinguishable difference between the 90 and 100 image, but the difference in size could be significant. Keep in mind, though, that subtle color shifting is one effect of JPEG compression–even at high quality settings–so JPEG should be avoided in situations where precise color matching is important.
Progressive JPEGs download faster than ordinary JPEGs
False. Progressive JPEGs display gradually as they download, so they will appear initially at a very low quality and slowly become clearer until the image is fully downloaded. On a slow dial-up connection, this may give the illusion of a faster download, but usually a progressive JPEG is larger in file size and requires more processing power to decode and display. Also, some software is incapable of displaying progressive JPEGs, most notably, the free Imaging program bundled with many versions of Windows.
JPEGs Require More Processing Power to Display
True. JPEGs not only need to be downloaded, but decoded as well. If you were to compare display time for a GIF and a JPEG with the exact same file size, the GIF would display marginally faster than the JPEG because its compression scheme does not require as much processing power to decode. This slight delay is barely noticeable, except perhaps on extremely slow systems.
JPEG is an All-purpose Format Suitable for Just about any Image
False. JPEG is best suited for large photographic images where file size is the most important consideration, such as images that will be posted on the web or transmitted via email and FTP. JPEG is not suitable for most small images under a few hundred pixels in dimension, and it is not suitable for images with text, images with sharp lines and large blocks of color, and images that will be edited repeatedly.
JPEG is Ideal For Long-term Image Archival
False. JPEG should only be used for archival when disk space is the primary consideration. Because JPEG images lose quality each time they are opened, edited, and saved, it should be avoided for archival situations where the images require further processing. Always keep a lossless master copy of any image you expect to edit again in the future.
JPEG2000 is On the Way and Will Solve all the Problems with JPEG
Let’s wait and see. JPEG2000 was initially proposed in 1996. More than a decade later, JPEG2000 is still not a fully published standard. It can take years before graphics software and web browsers are able to support a new format and that can’t happen until the format is finalized and accepted as a standard. Although most photo editors now support JPEG2000 (JP2) files, the major web browsers still don’t, and few (if any) digital cameras can write to this format.
I can Save Disk Space by Converting my GIF Images to JPEGs
False. GIF images have already been reduced to 256 colors or less. JPEG images are ideal for large photographic images with millions of colors. GIFs are ideal for images with sharp lines and large areas of a single color. Converting a typical GIF image to JPEG will result in color shifting, blurring, and loss in quality, and often the resulting file will be larger. It’s generally not of any benefit to convert GIF to JPEG if the original GIF image is more than 100 Kb. PNG is a better choice.
All JPEG Images are High Resolution, Print-quality Photos
False. Print quality is determined by the pixel dimensions of the image. To print a 4 by 6 inch photo, the image must have at least 480×720 pixels for an average quality print, and 960×1440 pixels or more for a medium to high quality print. Because JPEG is often used for images to be transmitted and displayed via the web, these images are typically reduced to screen resolution and do not contain enough pixel data to get a high-quality print. When saving JPEGs from your digital camera, you may wish to use your camera’s higher quality compression setting to reduce the damage done by JPEG compression. I’m referring here to the quality setting of your camera, not resolution (which effects pixel dimensions). Not all digital cameras offer this option.
Looking for a good alternative to the JPEG format? PNG, or portable network graphic, was developed to combat the inherent flaws in both the GIF and JPEG formats.

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