Editing Compression artifact (section)

=== Block boundary artifacts ===
[[File:The macroblocking effect (JPEG).png|344px|right|thumb|Block coding artifacts in a JPEG image. Flat blocks are caused by coarse quantization. Discontinuities at transform block boundaries are visible.]]

At low bit rates, any [[lossy]] block-based coding scheme introduces visible artifacts in pixel blocks and at block boundaries. These boundaries can be transform block boundaries, prediction block boundaries, or both, and may coincide with [[macroblock]] boundaries. The term ''macroblocking'' is commonly used regardless of the artifact's cause. Other names include blocking,<ref name="amiriImageCompressionUsing2018">{{Cite journal| doi = 10.1007/s11042-017-4763-1| issn = 1573-7721| volume = 77| issue = 7| pages = 8677–8693| last1 = Amiri| first1 = Sekine Asadi| last2 = Hassanpour| first2 = Hamid| title = Image compression using JPEG with reduced blocking effects via adaptive down-sampling and self-learning image sparse representation| journal = Multimedia Tools and Applications| access-date = 2024-03-08| date = 2018-04-01| url = https://doi.org/10.1007/s11042-017-4763-1| url-access = subscription}}</ref> tiling,<ref>{{cite book|url=https://books.google.com/books?id=STFAx4jqU5IC&q=tiling+mpeg&pg=PA343|title=The MPEG handbook by John Watkinson|isbn=9780240805788|last1=Watkinson|first1=John|year=2004|publisher=Taylor & Francis }}</ref> mosaicing, pixelating, quilting, and checkerboarding.

Block-artifacts are a result of the very principle of [[Transform coding#Digital|block transform]] coding. The transform (for example the discrete cosine transform) is applied to a block of pixels, and to achieve lossy compression, the transform coefficients of each block are [[Quantization (image processing)|quantized]]. The lower the bit rate, the more coarsely the coefficients are represented and the more coefficients are quantized to zero. Statistically, images have more low-[[Spatial frequency|frequency]] than high-frequency content, so it is the low-frequency content that remains after quantization, which results in blurry, low-resolution blocks. In the most extreme case only the DC-coefficient, that is the coefficient which represents the average color of a block, is retained, and the transform block is only a single color after reconstruction.

Because this quantization process is applied individually in each block, neighboring blocks quantize coefficients differently. This leads to discontinuities at the block boundaries. These are most visible in flat areas, where there is little detail to mask the effect.