Editing Data compression (section)

==== Hybrid block-based transform formats ====
[[File:Hybrid video encoder processing stages.svg|thumb|upright=2|Processing stages of a typical video encoder]]

Many commonly used video compression methods (e.g., those in standards approved by the [[ITU-T]] or [[ISO]]) share the same basic architecture that dates back to [[H.261]] which was standardized in 1988 by the ITU-T. They mostly rely on the DCT, applied to rectangular blocks of neighboring pixels, and temporal prediction using [[motion vector]]s, as well as nowadays also an in-loop filtering step.

In the prediction stage, various [[data deduplication|deduplication]] and difference-coding techniques are applied that help decorrelate data and describe new data based on already transmitted data.

Then rectangular blocks of remaining [[pixel]] data are transformed to the frequency domain. In the main lossy processing stage, frequency domain data gets quantized in order to reduce information that is irrelevant to human visual perception.

In the last stage statistical redundancy gets largely eliminated by an [[entropy coder]] which often applies some form of arithmetic coding.

In an additional in-loop filtering stage various filters can be applied to the reconstructed image signal. By computing these filters also inside the encoding loop they can help compression because they can be applied to reference material before it gets used in the prediction process and they can be guided using the original signal. The most popular example are [[deblocking filter]]s that blur out blocking artifacts from quantization discontinuities at transform block boundaries.