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.
#What is a video codec decoder and encoder how to
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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