Editing Compression artifact (section)

== Video ==
[[File:Macroblocking error.png|thumb|350px|Example of image with artifacts due to a transmission error]]

When motion prediction is used, as in [[MPEG-1]], [[MPEG-2]] or [[MPEG-4]], compression artifacts tend to remain on several generations of decompressed frames, and move with the [[optic flow]] of the image, leading to a peculiar effect, part way between a painting effect and "grime" that moves with objects in the scene.

Data errors in the compressed bit-stream, possibly due to transmission errors, can lead to errors similar to large quantization errors, or can disrupt the parsing of the data stream entirely for a short time, leading to "break-up" of the picture. Where gross errors have occurred in the bit-stream, decoders continue to apply updates to the damaged picture for a short interval, creating a "ghost image" effect, until receiving the next independently compressed frame. In MPEG picture coding, these are known as "[[I-frame]]s", with the 'I' standing for "intra". Until the next I-frame arrives, the decoder can perform [[error concealment]].

=== Motion compensation block boundary artifacts ===
Block boundary discontinuities can occur at edges of [[motion compensation]] prediction blocks. In motion compensated video compression, the current picture is predicted by shifting blocks (macroblocks, partitions, or prediction units) of pixels from previously decoded frames. If two neighboring blocks use different motion vectors, there will be a discontinuity at the edge between the blocks.

=== Mosquito noise ===
Video compression artifacts include cumulative results of compression of the comprising still images, for instance [[Ringing artifacts#JPEG|ringing]] or other edge busyness in successive still images appear in sequence as a shimmering blur of dots around edges, called '''mosquito noise''', as they resemble mosquitoes swarming around the object.<ref name="dinhpatry">{{cite web|last1=Le Dinh|first1=Phuc-Tue|last2=Patry|first2=Jacques|title=Video compression artifacts and MPEG noise reduction|url=http://www.embedded.com/print/4013028|website=Embedded|access-date=19 February 2016}}</ref><ref>"'''3.9 mosquito noise:''' Form of edge busyness distortion sometimes associated with movement, characterized by moving artifacts and/or blotchy noise patterns superimposed over the objects (resembling a mosquito flying around a person's head and shoulders)."
[http://www.itu.int/rec/T-REC-P.930-199608-I ITU-T Rec. P.930 (08/96) Principles of a reference impairment system for video]</ref> The so-called "mosquito noise" is caused by the block-based [[discrete cosine transform]] (DCT) compression algorithm used in most [[video coding standards]], such as the [[MPEG]] formats.<ref name="pcmag">{{cite web |title=Mosquito noise |url=https://www.pcmag.com/encyclopedia/term/55914/mosquito-noise |website=[[PC Magazine]] |access-date=19 October 2019}}</ref>

=== Video artifact reduction ===
{{Main article|Deblocking filter}}

The artifacts at block boundaries can be reduced by applying a [[deblocking filter]]. As in still image coding, it is possible to apply a deblocking filter to the decoder output as post-processing.

In motion-predicted video coding with a closed prediction loop, the encoder uses the decoder output as the prediction reference from which future frames are predicted. To that end, the encoder conceptually integrates a decoder. If this "decoder" performs a deblocking, the deblocked picture is then used as a reference picture for motion compensation, which improves coding efficiency by preventing a propagation of block artifacts across frames. This is referred to as an in-loop deblocking filter. Standards which specify an in-loop deblocking filter include [[VC-1]], [[H.263]] Annex J, [[H.264/AVC]], and [[H.265/HEVC]].