What is the name used for the artifact in digital video in which gray (or sometimes purple) blocky "smears" appear on the screen?
Here are three example images: https://imgur.com/a/oZkQwm4
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I would call the 3 images respectively (i) somewhat pixelated or with a few pixelations, (ii) pixelated, and (iii) heavily pixelated.
Pixelate 1 b. intransitive. Of an image: to break up into a number of discrete pixels.
1994 N.Y. Times 7 Aug. ii. 1/2 So finely rendered is the detailed landscape that the image ‘dithered’ or began to ‘pixelate’ into a gridlike pattern not unlike what one would expect if a snapshot had been taken through a screen door.
2. transitive. To blur (parts of a televisual or photographic image) by converting into a number of fairly large, indistinct, pixel-like patches, for purposes of censorship or to maintain the anonymity of the subject.
2003 Church Times 28 May 12/3 Of all the British papers, only The Independent pixelated the faces of all prisoners of war, whichever side they came from.
The artifact can be caused by many things but often the decision of the video process to only update areas of the screen that have most recently changed recently. If other, static, areas are left alone the delay is not so noticeable. This is seen as moving lips appearing over still images of related scenes. The decision mechanism is [often] an "Exclusive OR" or XOR operation. We say the bad image is being X-ORed to death.