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The flowchart on the left-hand side depicts a typical signal flow from the original signal down to the resulting bitstream.
In image compression, sub-sampling is typically used to lower the spatial resolution of the chrominances components of colour images using Yxx-colour spaces (YUV, YCbCr), which separates luminance Y (brightness) and chrominances (colour). In video compression sometimes also temporal reduction is applied to reduce the number of pictures per second.
The decorrelation is performed by prediction, transformation or filterbanks. In lossy compression schemes this is usually followed by quantisation, converting the decorrelated signal values to quantisation symbols. The feedback path on the right side of the block diagram is necessary if prediction is combined with quantisation due to the recursive character of prediction.
The quantisation process produces a lot of new correlations between the quantisation symbols, which have to be dissolved by precoding methods. There is a large pool of different techniques, which can be combined. In most applications the compression is finished with entropy coding.
The bypasses on the left side of the chart indicate that - dependent on the application - each of the blocks can be skipped.