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JPEG Format and its History


JPEG, the acronym for Joint Photographic Experts Group, is a file format that uses lossy compression (explained in the “Image Compression” section). The JPEG file format offers 13 compression settings — the higher the quality, the less the compression, and vice versa. JPEG compression is very effective. It can squeeze your file size to practically nothing. JPEG compression works best with continuous-tone images, such as photographs. But because the compression is lossy, I don’t recommend this format for high-end printing and color separations. JPEG supports RGB, CMYK, and Grayscale image modes. It doesn’t support alpha channels. If you want to post your image on the Web, you have to save it as a JPEG, GIF, or PNG. JPEG works great with photographic images that have a wide range of colors. Check out Book IX, Chapters 1 and 2, for the lowdown on all the JPEG options and settings. JPEG 2000 . JPEG 2000 is a new member to Photoshop repertoire of supported file formats. A cousin to standard JPEG, it provides a few more bells and whistles, including better compression rates and more quality settings. In addition to the standard lossy compression algorithms, JPEG 2000 also offers lossless compression, shown in Figure 2-5, and can support 16-bit images, alpha and spot channels, and transparency (8 bit-images only). You can save this format using the following image modes: RGB, CMYK, Grayscale, and Lab Color. If you want to be able to utilize this file format, be sure to install this optional plug-in found on the Photoshop CS Install CD. One of the coolest features of this format is it supports the use of a Region of Interest (ROI). This feature allows you to selectively choose a region of an image that you can then optimize to ensure the best quality. You save an alpha channel to define that vital portion of the image where detail retention is critical. You can then compress the rest of the image more heavily and with lesser quality, resulting in a smaller file size. Although praising the qualities of JPEG 2000 is all well and good, be warned that you currently need a plug-in to be able to view these files on the Web. In the future, this format is sure to become a standard on the Web and with digital cameras.


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