12 Myths & Facts about the JPEG Format
What really happens when you save your picture in JPEG
by En Chance
With the influx of blogs and how easy it is to develop one these days, I always get questions about the JPEG format. For you new web warriors don't believe everything you hear about JPEGs. Get the facts here.
Myth #1 : 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 a quality setting of 100% will usually not be detectable by the average naked eye. But keep in mind that subtle color shifting is one effect of JPEG compression—even at high quality settings, so JPEG should be avoided where precise color matching is important.
Myth #2: If I compress a JPEG at 60%, then later reopen it and compress it at 90%, the final image will be restored to a quality setting of 90%
FALSE. This is a common misconception to those new to web design. The initial save at 70% introduces a permanent loss in quality that cannot be restored. Saving the image again at 90% quality only introduces more degradation.
Myth #3: All programs produce the same exact results as the same JPEG quality setting in another program
FALSE. Quality settings are not standardized across graphics software programs. 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.
Myth #4: JPEGs lose quality every time they are opened
FALSE. Opening or displaying a JPEG image does not harm the image in any way. The only way you would lose any further image quality is unless any additional editing was performed on the image and saved again as JPEG.
Myth #5: JPEGs lose quality every time they are edited and saved
TRUE. If a JPEG image is opened, edited, and saved again it results in additional image degradation. If you must perform editing functions in several sessions or in several different programs, use an image format that is not lossy (PSD, TIFF, PNG) before saving the final version in JPEG.
Myth #6: 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 it uses a different compression scheme which does not require as much processing power to decode. This slight delay is barely noticeable, except perhaps on extremely slow connections.
Myth #7: 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 gradually become clearer until the image is fully downloaded. On a slow Internet 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.
Myth #8: 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. JPEG is not suitable for most small images under a few hundred pixels in dimension, and it is not suitable for screen shots, images with text, images with sharp lines and large blocks of color, and images that will be edited repeatedly.
Myth #9: JPEG images do not support transparency
TRUE. You may think you've seen JPEG image with transparency on the Web, but in fact the image was created with the intended background incorporated into the JPEG in such a way that it appears seamless when layered on a Web page with the same background image.
Myth #10: I can save disk space by converting my GIF images to JPEGs
FALSE. GIF images have already been reduced to 256 colors or less. GIFs are ideal for images with sharp lines and large areas of a single color while JPEG images are ideal for large photographic images with millions of colors. Converting the typical GIF image to JPEG will result in color shifting, blurring, and loss in quality, and in most instances the resulting file will be larger.
Myth #11: 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 edited and saved, they should be avoided for archival situations where the images will need to go through further processing steps. Always keep a lossless master copy of any image you expect to edit again in the future.
Myth #12: JPEGs lose quality every time they are used in a page layout program like InDesign, Illustrator or Quark Express
FALSE. Using a JPEG Image in a page layout program does not edit the source JPEG image so 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.
