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Image Optimization > JPEG—Joint Photographic Experts Group

JPEG—Joint Photographic Experts Group

JPEG is currently the single most important format for photos and images requiring more than 256 colors. In the future, we may see JPEG losing ground to PNG, which offers some advantages, such as lossless compression and Alpha channel transparency, but for now, and certainly for the next few years, JPEG will remain an extremely popular and important format.

DCT Compression

JPEG uses a compression algorithm called the Discrete Cosine Transform, or DCT. DCT compression is based on the fact that the human eye is less sensitive to changes in color than to variations in brightness. While most image formats for the Web save the RGB color value for every pixel, JPEG actually splits color and brightness information and compresses each individually. The JPEG algorithm works on a block of 8 by 8 pixels at a time: First it calculates the DCT, which is then quantified, and finally, a variable-length code compression scheme is used on it.


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