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Overall architecture

Previous: Introduction
Next: Rate Distortion Optimisation

Overall, the codec is a classic motion-compensated hybrid codec. The coder has the architecture shown below, whilst the decoder performs the inverse operations.

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Figure: Overall hybrid encoder architecture

There are four main elements or modules to the coder:

  • Transform and scaling involves taking frame data and applying a transform (in this case the wavelet transform) and scaling the coefficients to perform quantisation;
  • Entropy coding is applied to quantised transform coefficients and to motion vector (MV) data and performs lossless compression on them;
  • Motion estimation (ME) involves finding matches for frame data from previously coded frames, trading off accuracy with motion vector bit rate;
  • Motion compensation (MC) involves using the motion vectors to predict the current frame, in such a way as to minimise the cost of encoding the residual data.

The following sections describe these modules in more detail, after first describing the rate-distortion framework used throughout the encoder.

Dirac supports interlaced coding by means of coding whole fields rather than frames. Unlike MPEG codecs there is no complex macroblock-by-macroblock switching. Such complexity is not justified as interlace (an analogue compression technique) becomes a legacy system.

The codec can support any frame dimensions and common chroma formats (444, 422, 420 but not 411) by means of picture padding. The padding ensures that the wavelet transform can be applied properly.

Previous: Introduction
Next: Rate Distortion Optimisation