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The Joint Photographic Experts Group (JPEG) image format uses discrete cosine transforms rather than a
pix-by-pix encoding. In JPEG, the image is divided into 8 X 8 blocks
for each separate color component. The goal is to find blocks where the
amount of change in the pixel values (the energy) is low. If the energy
level is too high, the block is subdivided into 8 X 8 subblocks until
the energy level is low enough. Each 8 X 8 block (or subblock) is
transformed into 64 discrete cosine transforms coefficients that
approximate the luminance (brightness, darkness, and contrast) and
chrominance (color) of that portion of the image. JPEG is generally
considered to be lossy compression because the image recovered from the
compressed JPEG file is a close approximation of, but not identical to,
the original (Johnson and Jajodia 1998A; Monash University 2004; Provos
and Honeyman 2003).
Most digital image applications today support 24-bit true color, where each picture element (pixel) is encoded in 24 bits, comprising the three RGB bytes as described above. Other applications encode color using eight bits/pix. These schemes also use 24-bit true color but employ a palette that specifies which colors are used in the image. Each pix is encoded in eight bits, where the value points to a 24-bit color entry in the palette. This method limits the unique number of colors in a given image to 256 (28).
The Joint Photographic Experts Group (JPEG) image format uses discrete cosine transforms rather than a pix-by-pix encoding. In JPEG, the image is divided into 8 X 8 blocks for each separate color component. The goal is to find blocks where the amount of change in the pixel values (the energy) is low. If the energy level is too high, the block is subdivided into 8 X 8 subblocks until the energy level is low enough. Each 8 X 8 block (or subblock) is transformed into 64 discrete cosine transforms coefficients that approximate the luminance (brightness, darkness, and contrast) and chrominance (color) of that portion of the image. JPEG is generally considered to be lossy compression because the image recovered from the compressed JPEG file is a close approximation of, but not identical to, the original (Johnson and Jajodia 1998A; Monash University 2004; Provos and Honeyman 2003).
Most digital image applications today support 24-bit true color, where each picture element (pixel) is encoded in 24 bits, comprising the three RGB bytes as described above. Other applications encode color using eight bits/pix. These schemes also use 24-bit true color but employ a palette that specifies which colors are used in the image. Each pix is encoded in eight bits, where the value points to a 24-bit color entry in the palette. This method limits the unique number of colors in a given image to 256 (28).