![]() These blocks are combined together to form a residual macroblock.įor each macroblock, the decoder forms an identical prediction to the one created by the encoder. An inverse transform combines the standard basis patterns, eighted by the re- scaled coefficients, to re-create each block of residual data. Each coefficient is multiplied b an integer value to restore its original scale2. The quantized transform coefficients are re-scaled. This information is then used to reverse the coding process and recreate a sequence of video images. 2.2 Decoder processesĪ video decoder receives the compressed H.264 bitstream, decodes each of the syntax elements and extracts the information described above (quantized transform coefficients, prediction information, etc). The encoded bitstream can then be stored and/or transmitted. Each of these encoding methods produces an efficient, compact binary representation of the information. These values and parameters ( syntax elements) are converted into binary codes using variable length coding and/or arithmetic coding. information about the complete video sequence.information about the structure of the compressed data and the compression tools used during encoding.information to enable the decoder to re-create the prediction.The video coding process produces a number of values that must be encoded to form the compressed bitstream. Setting QP to a low value means that more non-zero coefficients remain after quantization, resulting in better decoded image quality but lower compression. Setting QP to a high value means that more coefficients are set to zero, resulting in high compression at the expense of poor decoded image quality. Typically, the result is a block in which most or all of the coefficients are zero, with a few non-zero coefficients. Quantization reduces the precision of the transform coefficients according to a quantization parameter (QP). each coefficient is divided by an integer value. The output of the transform, a block of transform coefficients, is quantized, i.e. Figure 4 shows how the inverse DCT creates an image block by weighting each basis pattern according to a coefficient value and ombining the weighted basis patterns. When combined, the weighted basis patterns re-create the block of residual samples. The transform outputs a set of coefficients, each of which is a weighting value for a standard basis pattern. 2.1 Encoder processesĪ block of residual samples is transformed using a 4x4 or 8x8 integer transform, an approximate form of the Discrete Cosine Transform ( DCT). ![]() An H.264 video decoder carries out the complementary processes of decoding, inverse transform and reconstruction to produce a decoded video sequence. 2 How does an H.264 codec work ?Īn H.264 video encoder carries out prediction, transform and encoding processes (see Figure 1) to produce a compressed H.264 bitstream. better-quality compressed video) and greater flexibility in compressing, transmitting and storing video. It builds on the concepts of earlier standards such as MPEG-2 and MPEG-4 Visual and offers the potential for better compression efficiency (i.e. The H.264/AVC standard was first published in 2003. Figure 1 shows the encoding and decoding processes and highlights the parts that are covered by the H.264 standard. The standard document does not actually specify how to encode (compress) digital video – this is left to the manufacturer of a video encoder – but in practice the encoder is likely to mirror the steps of the decoding process. It defines a format (syntax) for compressed video and a method for decoding this syntax to produce a displayable video sequence. Recommendation H.264: Advanced Video Coding is a document published by the international standards bodies ITU-T (International Telecommunication Union) and ISO/IEC (International Organisation for Standardisation / International Electrotechnical Commission). An encoder converts video into a compressed format and a decoder converts compressed video back into an uncompressed format. ![]() encoders, decoders and storage media) to inter-operate. Standardising video compression makes it possible for products from different manufacturers (e.g. Video compression (or video coding) is an essential technology for applications such as digital television, DVD-Video, mobile TV, videoconferencing and internet video streaming. H.264 is an industry standard for video compression, the process of converting digital video into a format that takes up less capacity when it is stored or transmitted. ![]()
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