If the image is grayscale this step is not needed 2

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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: 1. Sequential. In this mode, the image...
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This document was uploaded on 04/07/2014.

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