MPEG layers

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(n.) Moving Picture Expert Group layers; levels in an MPEG file that hold information about moving pictures. The levels are in hierarchical order and deal with a series of frames at the top level down to details about regions of a frame. From bottom to top, the different layers in an MPEG stream are as follows:

• Block: An eight-row-by-eight-column (8 x 8) block of pixels. A block is the unit to which discrete cosine transform (DCT) is applied.

• Macroblock: The four 8 x 8 blocks of luminance data and the corresponding 8 x 8 blocks of chrominance data coming from a 16 x 16 section of the luminance component of the picture. A macroblock is the basic unit for motion-compensation prediction.

• Slice: A series of macroblocks that serves as the synchronizing unit for reconstruction of the image data. Typically, a slice is all the blocks in one horizontal picture interval, as well as 16 lines of the picture.

• Picture: Source image or reconstructed data for a single frame consisting of three rectangular matrices of eight-bit numbers that represent the luminance and chrominance signals.

• Group of pictures (GOP): A self-contained sequence of pictures that starts with an I-frame and contains a variable number of P-frames and B-frames. SMPTE time code may be added to mark the first picture in a group at this layer.

• Sequence: A video sequence beginning with a “sequence header,” which is followed by one or more groups of pictures, and ending with a “sequence end” code.