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Unformatted text preview: are positioned near the
upper left corner, and the higher frequencies (i.e., sharp edges) are positioned near
the lower right corner. The coefficients of the output DCT matrix are divided by
the corresponding entries of an 8x8 quantization table (Q-table). Each entry of the
Q-table describes the quality factor of the corresponding DCT output coefficient.
The lower the quality factor, the higher the magnitude of value. For example, for
DCT compression, the lower frequencies are more important than the higher ones.
Therefore, a Q-table with smaller numbers near the upper-left corner and larger
ones near the lower-right corner is used. The division of the DCT output matrix by
the Q-table causes the lower-right corner portion of the resultant matrix to the
resultant matrix to become mostly zeros. As a result, the greater portion of the
high-frequency information is discarded. The modification of the Q-table is one
way to change the degree of compression. The entries in the Q-table are based on eye masking experiments and are essentially an approximation to the best estimate
of levels that provide just-noticeable distortion in the image.
The DCT decoder performs the reverse process and converts the 64 coefficients
back to pixel values. Since quantization is a lossy process, it may not reproduce
the originals exactly.
Some Standard Multimedia Compression Techniques
Over the years, several standards have evolved for compression and
decompression of multimedia objects. Some of the most commonly used standards
are described below. In selecting the standards to be described here, we have taken
care to select one each for still image, video and audio media.
JPEG (Joint Photographic Experts Group)
JPEG is the standard compression technique for still images. It is of reasonably
low computational complexity, is capable of producing compressed images of
high quality, and can provide both lossless and lossy compression of arbitrarily
sized grayscale and color images. As will be clear from the explanation below, it is
a hybrid of several basic compression methods. It supports the following four
modes of encoding:
Sequential. In this mode, the image...
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This document was uploaded on 04/07/2014.
- Spring '14