Editing Chroma subsampling (section)

===4:2:0===
[[File:Yuvformats420sampling.svg|thumb|4:2:0 with sampling taken at the center of 2&times;2 the square]]
[[File:Yuvformats420samplingMPEG-2.svg|thumb|4:2:0 with sampling taken at the midpoint of the left edge of the 2&times;2 square]]
In 4:2:0, the horizontal sampling is doubled compared to 4:1:1, but as the '''Cb''' and '''Cr''' channels are only sampled on each alternate line in this scheme, the vertical resolution is halved. The data rate is thus the same. This fits reasonably well with the [[PAL]] color encoding system, since this has only half the vertical chrominance resolution of [[NTSC]]. It would also fit extremely well with the [[SECAM]] color encoding system, since like that format, 4:2:0 only stores and transmits one color channel per line (the other channel being recovered from the previous line). However, little equipment has actually been produced that outputs a SECAM analogue video signal. In general, SECAM territories either have to use a PAL-capable display or a [[transcoder]] to convert the PAL signal to SECAM for display.

Different variants of 4:2:0 chroma configurations are found in:
* All [[ISO/IEC]] [[MPEG]] and [[ITU-T]] [[VCEG]] H.26x video coding standards including [[H.262/MPEG-2 Part 2]] implementations (although some profiles of [[MPEG-4 Part 2]] and [[H.264/MPEG-4 AVC]] allow higher-quality sampling schemes such as 4:4:4)
* [[DVD-Video]] and [[Blu-ray Disc]].<ref name=AudioholicsHDMIApril2008>{{cite news |title=HDMI Enhanced Black Levels, xvYCC and RGB |author=Clint DeBoer |publisher=[[Audioholics]] |url=http://www.audioholics.com/tweaks/calibrate-your-system/hdmi-black-levels-xvycc-rgb |date=2008-04-16 |access-date=2013-06-02}}</ref><ref name=TelairityDigitalColorCodingPDF>{{cite news |title=Digital Color Coding |publisher=Telairity |url=http://www.telairity.com/assets/downloads/Digital%20Color%20Coding.pdf |access-date=2013-06-02 |archive-url=https://web.archive.org/web/20140107171831/http://www.telairity.com/assets/downloads/Digital%20Color%20Coding.pdf |archive-date=2014-01-07 |url-status=dead }}</ref>
* [[576i]] "PAL" [[DV (video format)|DV]] and [[DVCAM]]
* [[HDV]]
* [[AVCHD]] and [[AVC-Intra|AVC-Intra 50]]
* [[Apple Intermediate Codec]]
* Most common [[JPEG|JPEG/JFIF]] and [[MJPEG]] implementations
* [[VC-1]]
* [[WebP]]
* [[YJK]],<ref>{{Cite web |last=MSX Licensing Corporation |date=2022 |title=The YJK screen modes |url=http://map.grauw.nl/articles/yjk/ |website=MSX Assembly Page}}</ref><ref>{{Cite book |last=Niemietz |first=Ricardo Cancho |url=http://rs.gr8bit.ru/Documentation/Issues-on-YJK-colour-model-implemented-in-Yamaha-V9958-VDP-chip.pdf |title=Issues on YJK colour model implemented in Yamaha V9958 VDP chip |year=2014}}</ref><ref>{{Cite web |title=VCFe Vortrag vom 2016.04.30 – Homecomputer und Spielkonsolen – Videoarchitekturen als visuelles Medium |url=http://neil.franklin.ch/Articles/20160430_VCFe_Video_als_Medium.html |access-date=2022-11-13 |website=neil.franklin.ch}}</ref> a proprietary [[color space]] implemented by the [[Yamaha V9958]]<ref>{{Cite book |url=https://books.google.com/books?id=8RwSAQAAMAAJ&dq=%22Yamaha+V9958%22+-wikipedia&pg=PA3984 |title=IC Master |date=2001 |publisher=United Technical Publications |language=en}}</ref><ref>{{Cite thesis |title=Arqueología informática: los ordenadores MSX en los inicios de la microinformática doméstica |url=https://riunet.upv.es/handle/10251/70909 |publisher=Universitat Politècnica de València |date=2016-10-03 |degree=Proyecto/Trabajo fin de carrera/grado |first=Sergio |last=Martín Sesma}}</ref><ref>{{Cite web |last=Redazione |date=2008-10-20 |title=MSX – Vari Costruttori- 1983 |url=https://www.cyberludus.com/2008/10/msx-vari-costruttori-1983/ |access-date=2022-11-13 |website=CyberLudus.com |language=it-IT}}</ref> graphic chip on [[MSX2+]] computers.<ref name="auto">{{Cite web |date=1988 |title=V9958 MSX-VIDEO TECHNICAL DATA BOOK |url=http://map.grauw.nl/resources/video/yamaha_v9958_ocr.pdf}}</ref><ref>{{Cite journal |last=Alex |first=Wulms |date=1995 |title=Schermen op MSX – De 2+ schermen |url=http://www.msxarchive.nl/pub/msx/mirrors/hanso/hwdoityourself/msxplus.pdf |journal=MSX Computer & Club Magazine |issue=72}}</ref>

'''Cb''' and '''Cr''' are each subsampled at a factor of 2 both horizontally and vertically. Most digital video formats corresponding to 576i "PAL" use 4:2:0 chroma subsampling.

==== Sampling positions ====
There are four main variants of 4:2:0 schemes, having different horizontal and vertical sampling siting relative to the 2&times;2 "square" of the original input size.<ref name="chroma-subsampling-notation">{{cite web |url=http://www.poynton.com/PDFs/Chroma_subsampling_notation.pdf |title=Chroma Subsampling Notation |last=Poynton |first=Charles |year=2008 |work=Poynton.com |access-date=2008-10-01}}</ref>

* In MPEG-2, MPEG-4, and AVC, ''Cb'' and ''Cr'' are taken on midpoint of the left-edge of the 2&times;2 square. In other words, they have the same horizontal location as the top-left pixel, but is shifted one-half pixel down vertically. Also called "left".<ref name=AvChromaLocation>[https://ffmpeg.org/doxygen/3.1/pixfmt_8h.html#a1f86ed1b6a420faccacf77c98db6c1ff enum AvChromaLocation], ffmpeg 3.1.</ref>
* In JPEG/JFIF, H.261, and MPEG-1, ''Cb'' and ''Cr'' are taken at the center of 2&times;2 the square. In other words, they are offset one-half pixel to the right and one-half pixel down compared to the top-left pixel. Also called "center".<ref name=AvChromaLocation/>
* In HEVC for BT.2020 and [[Rec. 2100#Chroma sample location|BT.2100]] content (in particular on Blu-rays), ''Cb'' and ''Cr'' are sampled at the same location as the group's top-left Y pixel ("co-sited", "co-located"). Also called "top-left". An analogous co-sited sampling is used in MPEG-2 4:2:2.<ref name=AvChromaLocation/>
* In 4:2:0 PAL-DV (IEC 61834-2), ''Cr'' is sampled at the same location as the group's top-left Y pixel, but ''Cb'' is sampled one pixel down.<ref>{{cite web |title=y4minput.c - webm/libvpx - Git at Google |url=https://chromium.googlesource.com/webm/libvpx/+/refs/heads/main/y4minput.c |website=chromium.googlesource.com |quote=420paldv chroma samples are sited like:}}</ref> It is ''also'' called "top-left" in ffmpeg.<ref name=AvChromaLocation/>

===== Interlaced and progressive =====
With [[interlaced]] material, 4:2:0 chroma subsampling can result in motion artifacts if it is implemented the same way as for progressive material. The luma samples are derived from separate time intervals, while the chroma samples would be derived from both time intervals. It is this difference that can result in motion artifacts. The MPEG-2 standard allows for an alternate interlaced sampling scheme, where 4:2:0 is applied to each field (not both fields at once). This solves the problem of motion artifacts, reduces the vertical chroma resolution by half, and can introduce comb-like artifacts in the image.

[[File:444-original-single-field.png]]
<br />Original. This image shows a single field. The moving text has some motion blur applied to it.

[[Image:420-progressive-single-field.png]]
<br />4:2:0 '''progressive''' sampling applied to moving ''interlaced'' material. The chroma leads and trails the moving text. This image shows a single field.

[[File:420-interlaced-single-field.png]]
<br />4:2:0 '''interlaced''' sampling applied to moving ''interlaced'' material. This image shows a single field.

In the 4:2:0 interlaced scheme, however, vertical resolution of the chroma is roughly halved, since the chroma samples effectively describe an area 2 samples wide by 4 samples tall instead of 2×2. As well, the spatial displacement between both fields can result in the appearance of comb-like chroma artifacts.

[[File:420-original444.png]]
<br />Original still image.

[[File:420-progressive-still.png]]
<br />4:2:0 '''progressive''' sampling applied to a still image. Both fields are shown.

[[File:420-interlaced-still.png]]
<br />4:2:0 '''interlaced''' sampling applied to a still image. Both fields are shown.

If the interlaced material is to be de-interlaced, the comb-like chroma artifacts (from 4:2:0 interlaced sampling) can be removed by blurring the chroma vertically.<ref name="chroma-upsampling-error">{{cite web |url=http://www.hometheaterhifi.com/volume_8_2/dvd-benchmark-special-report-chroma-bug-4-2001.html |title=DVD Player Benchmark – Chroma Upsampling Error |last=Munsil |first=Don |author2=Stacey Spears |year=2003 |work=Secrets of Home Theater and High Fidelity |access-date=2008-08-29 |archive-url=https://web.archive.org/web/20080606004931/http://www.hometheaterhifi.com/volume_8_2/dvd-benchmark-special-report-chroma-bug-4-2001.html |archive-date=2008-06-06 |url-status=dead}}</ref>