Digital Video Compression Part 5
General description
The basic idea behind vector quantization is to divide the image into blocks (4×4 pixels in the YUV color scheme for Indeo and Cinepak compressors). As a general rule, some blocks are similar to each other. In this case, the compressor identifies a class of similar blocks and replaces them with a common block. Furthermore, a binary table (map) of said common blocks is generated from the shorter code words. The VQ decoder then uses a table to assemble the image block by block from common blocks. It is clear that this encoding method is lossy, since, strictly speaking, the similarity of blocks is very relative. Here the approximation of blocks of real images to the common that joins them is allowed. The encoding process is long and laborious, since the encoder needs to identify the membership of each block of the image to some common block. However, the decoding task in this case is reduced to the task of constructing an image from a given map from common blocks and does not require much hardware and time resources. A table or map is also called a codebook, and the binary codes included in it are codewords, respectively. The highest compression using the VQ algorithm is achieved by reducing the number of common block classes, that is, by assuming the similarity of a relatively large number of image blocks and consequently reducing the codebook. As the size of the codebook decreases, the quality of the played video deteriorates. As a result, an artificial “lock” appears on the image. (Vector quantization and signal compression, A. Gersho and R. Gray, Boston, MA: Kluwer, 1992). A table or map is also called a codebook, and the binary codes included in it are codewords, respectively. The highest compression using the VQ algorithm is achieved by reducing the number of common block classes, that is, by assuming the similarity of a relatively large number of image blocks and consequently reducing the codebook. As the size of the codebook decreases, the quality of the played video deteriorates. As a result, an artificial “lock” appears on the image. (Vector quantization and signal compression, A. Gersho and R. Gray, Boston, MA: Kluwer, 1992). A table or map is also called a codebook, and the binary codes included in it are codewords, respectively. The highest compression using the VQ algorithm is achieved by reducing the number of common block classes, that is, by assuming the similarity of a relatively large number of image blocks and consequently reducing the codebook. As the size of the codebook decreases, the quality of the played video deteriorates. As a result, an artificial “lock” appears on the image. (Vector quantization and signal compression, A. Gersho and R. Gray, Boston, MA: Kluwer, 1992). that is, an assumption about the similarity of a relatively large number of image blocks and, as a consequence, a decrease in the codebook. As the size of the codebook decreases, the quality of the played video deteriorates. As a result, an artificial “lock” appears on the image. (Vector quantization and signal compression, A. Gersho and R. Gray, Boston, MA: Kluwer, 1992). that is, an assumption about the similarity of a relatively large number of image blocks and, as a consequence, a decrease in the codebook. As the size of the codebook decreases, the quality of the played video deteriorates. As a result, an artificial “block” appears in the image. (Vector quantization and signal compression, A. Gersho and R. Gray, Boston, MA: Kluwer, 1992).
A simple example: compare the following three 4 x 4 blocks.
(Block 1)
128 128 128 128
128 128 128 128
128 128 128 128
128 128 128 128
(Block 2)
128 127 128 128
128 128 128 128
128, 128, 127, 128
128 128 128 128
(Box 3)
128 127 126 128
128 128 128 128
127 128 128 128
128 128 128 128
These three blocks are indistinguishable to the human eye. Therefore, the second and third block can be safely replaced with the first. So the codebook will look like this:
Codebook [1] = 128128128128
128 128 128 128
128 128 128 128
128 128 128 128
In conclusion, we note that an important feature of VQ technology is that when compressing video, the same codebook can be used for multiple picture frames.
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Discrete Cosine Transform (DCT)
Compressors using DCT: Motion JPEG; Editable MPEG; MPEG-1; MPEG-2; MPEG-4.
Advantages and disadvantages
