MPEG format: specifications and capabilities Part 2
HOW COMPRESSION HAPPENS
The basic object of encodig in the MPEG standard is a television frame. Since the background of the image remains fairly stable throughout most of the image, and the action takes place only in the foreground, compression begins with the creation of the original frame. The original (Intra) frames are encoded only by intra-frame compression using algorithms similar to those used in JPEG. The frame is divided into 8 × 8 pixel blocks. A discrete cosine transform (DCT) is performed on each block, followed by the quantization of the coefficients obtained. Due to the high spatial correlation of brightness between adjacent pixels in the image, DCT leads to a concentration of the signal in the low-frequency part of the spectrum, which, after quantization, is effectively compressed using variable-length encoding. Predicted frames are processed using direct prediction of previous original or predictable frames. The frame is divided into 16 × 16 pixel macroblocks, each macroblock is assigned the most similar image area of the reference frame, shifted by the displacement vector. This procedure is called motion analysis and compensation. The allowed compression ratio for predictable frames is 3 times that of the original frames. Depending on the nature of the video image, bidirectional interpolated frames are encoded in one of four ways: direct prediction; inverse prediction with motion compensation: used when new image objects appear in the coded frame; motion compensated bidirectional prediction; Intra-frame prediction: when there is a sudden change of scene or a high speed of movement of the elements of the picture. Bidirectional frames are associated with the deepest compression of video data, but since a high compression ratio reduces the accuracy of restoring the original image, bidirectional frames are not used as reference frames. If the DCT coefficients were accurately transmitted, the reconstructed image would completely match the original. However, errors in the recovery of DCT coefficients associated with quantization lead to image distortions. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions. Since a high compression ratio reduces the accuracy of restoring the original image, bidirectional frames are not used as reference frames. If the DCT coefficients were accurately transmitted, the reconstructed image would completely match the original. However, errors in the recovery of DCT coefficients associated with quantization lead to image distortions. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions. Since a high compression ratio reduces the accuracy of restoring the original image, bidirectional frames are not used as reference frames. If the DCT coefficients were accurately transmitted, the reconstructed image would completely match the original. However, errors in the recovery of DCT coefficients associated with quantization lead to image distortions. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions. cause image distortion. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions. lead to image distortion. The coarser the quantization, the less volume the coefficients occupy and the stronger the signal compression, but also more visual distortions.
