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Quantization scaling
The single pictures in MPEG are saved using a compression method comparable to
JPEG with bitmaps and associated with quality loss. For this single images are divided
into 8 x 8 blocks (view page 371).
Each one of th
ese blocks is then transformed into an 8 x 8 matrix (a table with rows
and columns) using a DCT (discreet cosinus transformation) mathematical method.
Each of these values is produced using all 64 individual pixels of the block, but the
values in the matrix are ordered in such a way that the image information is ordered
according to its importance.
This matrix will then be multiplied by another matrix, i.e. the quantization matrix.
Exactly how and why this matrix must be created is the biggest secret of encoder
programmers, since this determines the quality of the whole encoding process. What
is known is that the result should contain as many zeros as possible! These zeros
correspond to the "unimportant" image elements mentioned and will not be
transmitted in the data stream.
Depending on the encoder parameters regarding the target bit rate, fewer or more
values of the matrix will be declared unimportant by dividing the quantization matrix
by the quantization scaling factor. Since only whole numbers are used, a division can
produce a zero is the remainder is discarded.
This factor is also a direct measure of the sought image quality of the MPEG data
stream, since the "Q" in "Q" factor stands for quantization and quality.