Rectangular area of picture, usually 8 x 8 pixels in size, which are individually subjected to DCT coding as part of a digital picture compression process. Artifact of compression generally showing momentarily as misplaced rectangular areas of picture with distinct boundaries. This is one of the major defects of digital compression, its visibility generally depending on the amount of compression used, the quality of the original signal, and the quality of the coder. The visible blocks may be 8 x 8 DCT blocks or "misplaced blocks"--16 x 16 pixel macroblocks, due to the failure of motion prediction/estimation in encoder or other motion vector system, such as a standards converter. See also: DCT, JPEG, Macroblock, MPEG-2.
The block is a matrix of 8x8 elements. They can be 8x8 adjacent luminance or chrominance samples, or the corresponding DCT coefficients. At the block layer is performed the Discrete Cosine Transform. Video decoding process at the block layer Variable length decoding. The bitstream codewords of the block are decoded to form a vector of quantised DCT coefficient. Inverse scan. The vector elements are put into a two-dimensional array, which is the block, following one of two possible patterns. The pattern is defined by the flag alternate_scan which is set at the picture layer. The scanning purpose is to optimize the entropy coding. Inverse quantisation. DCT coefficients are converted to their original range of values. See also dct quantisation. Inverse DCT. Eventually the IDCT is performed. Now the block elements represent either the image sample (Intra block) or the prediction-error (Non-Intra block).