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Since it would be too complex to compress video frames by comparing every single pixel in the Frame against every other pixel, modern encoders instead divide frames into Macroblocks that usually consist of a 16x16 pixel grid. Each Macroblock is individually compressed. While this makes encoding much more efficient, it can also lead to encoding artifacts caused by adjacent Macroblocks being compressed very differently. This can lead to Macroblock boundaries becoming visible.
MPEG-1, MPEG-2, MPEG-4, VC-1, and many other video standards and formats use Macroblocks.