Editing Chroma subsampling (section)

==Types of sampling and subsampling==

===4:4:4===
[[File:Yuvformats444sampling.svg|thumb|4:4:4 sampling]]
Each of the three [[Y'CbCr]] components has the same sample rate, thus there is no chroma subsampling. This scheme is sometimes used in high-end film scanners and cinematic post-production.

"4:4:4" may instead be wrongly referring to [[R'G'B']] color space, which implicitly also does not have any chroma subsampling (except in JPEG R'G'B' can be subsampled). Formats such as [[HDCAM SR]] can record 4:4:4 R'G'B' over dual-link [[HD-SDI]].

===4:2:2===
[[File:Yuvformats422sampling.svg|thumb|4:2:2 sampling]]
The two chroma components are sampled at half the horizontal sample rate of luma: the horizontal chroma resolution is halved. This reduces the bandwidth of an uncompressed video signal by one-third, which means for 8 bit per component without [[Alpha compositing|alpha]] (24 bit per pixel) only 16 bits are enough, as in NV16.

Many high-end digital video formats and interfaces use this scheme:
* [[AVC-Intra|AVC-Intra 100]]
* [[Digital Betacam]]
* [[Betacam SX]]
* [[DV (video format)#DVCPRO50|DVCPRO50]] and [[DVCPRO HD]]
* [[Digital-S]]
* [[CCIR 601]] / [[Serial digital interface]] / [[D-1 (Sony)|D-1]]
* [[ProRes|ProRes (HQ, 422, LT, and Proxy)]]
* [[XDCAM|XDCAM HD422]]
* [[Canon XF-300|Canon MXF HD422]]

===4:1:1===
In 4:1:1 chroma subsampling, the horizontal color resolution is quartered, and the bandwidth is halved compared to no chroma subsampling. Initially, 4:1:1 chroma subsampling of the [[DV (video format)|DV]] format was not considered to be broadcast quality and was only acceptable for low-end and consumer applications.<ref name="dv-betacam">{{cite web |url=http://www.dvcentral.org/DV-Beta.html |title=DV vs. Betacam SP |last=Jennings |first=Roger |author2=Bertel Schmitt |year=1997 |work=DV Central |access-date=2008-08-29 |archive-url=https://web.archive.org/web/20080702013821/http://www.dvcentral.org/DV-Beta.html |archive-date=2008-07-02 |url-status=dead}}</ref><ref name="dv-formats">{{cite web |url=http://www.adamwilt.com/DV-FAQ-tech.html |title=DV, DVCAM & DVCPRO Formats |last=Wilt |first=Adam J. |year=2006 |work=adamwilt.com |access-date=2008-08-29}}</ref> However, [[DV (video format)|DV]]-based formats (some of which use 4:1:1 chroma subsampling) have been used professionally in electronic news gathering and in playout servers. DV has also been sporadically used in feature films and in [[digital cinematography]].

In the [[480i]] "NTSC" system, if the luma is sampled at 13.5&nbsp;MHz, then this means that the '''Cr''' and '''Cb''' signals will each be sampled at 3.375&nbsp;MHz, which corresponds to a maximum [[Nyquist–Shannon sampling theorem|Nyquist]] bandwidth of 1.6875&nbsp;MHz, whereas traditional "high-end broadcast [[NTSC|analog NTSC]] encoder" would have a Nyquist bandwidth of 1.5&nbsp;MHz and 0.5&nbsp;MHz for the [[YIQ|I/Q]] channels. However, in most equipment, especially cheap TV sets and [[VHS]]/[[Betamax]] [[VCR]]s, the chroma channels have only the 0.5&nbsp;MHz bandwidth for both '''Cr''' and '''Cb''' (or equivalently for I/Q). Thus the DV system actually provides a superior color bandwidth compared to the best [[Composite video|composite analog]] specifications for NTSC, despite having only 1/4 of the chroma bandwidth of a "full" digital signal.

Formats that use 4:1:1 chroma subsampling include:
* [[DVCPRO]] / D-7 ([[NTSC]] and [[PAL]])
* [[480i]] "NTSC" [[DV (video format)|DV]] and [[DVCAM]]

===4:2:0===
[[File:Yuvformats420sampling.svg|thumb|4:2:0 with sampling taken at the center of the 2&times;2 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>

===4:1:0===
This ratio is possible, and some [[codec]]s support it, but it is not widely used. This ratio uses half of the vertical and one-fourth the horizontal color resolutions, with only one-eighth of the bandwidth of the maximum color resolutions used. Uncompressed video in this format with 8-bit quantization uses 10 bytes for every macropixel (which is 4×2 pixels) or 10 bit for every pixel. It has the equivalent chrominance bandwidth of a PAL-I or PAL-M signal decoded with a delay line decoder, and still very much superior to NTSC.

===3:1:1===
Used by Sony in their HDCAM High Definition recorders (not HDCAM SR). In the horizontal dimension, luma is sampled horizontally at three quarters of the full HD sampling rate{{snd}} 1440 samples per row instead of 1920. Chroma is sampled at 480 samples per row, a third of the luma sampling rate. In the vertical dimension, both luma and chroma are sampled at the full HD sampling rate (1080 samples vertically).

=== Different Cb and Cr rates ===
A number of legacy schemes allow different subsampling factors in Cb and Cr, similar to how a different amount of bandwidth is allocated to the two chroma values in broadcast systems such as [[CCIR System M]]. These schemes are not expressible in ''J:a:b'' notation. Instead, they adopt a ''Y:Cb:Cr'' notation, with each part describing the amount of resolution for the corresponding component. It is unspecified whether the resolution reduction happens in the horizontal or vertical direction.

* In JPEG, 4:4:2 and 4:2:1 half the vertical resolution of ''Cb'' compared to 4:4:4 and 4:4:0.<ref>{{cite web |title=Support decoding yuv442 and yuv421 jpeg images. · FFmpeg/FFmpeg@387d860 |url=https://github.com/FFmpeg/FFmpeg/commit/387d86077f5237e53cec7a4ed31f5531224feebf |website=GitHub |language=en}}</ref> 
* In another version of {{vanchor|4:2:1}}, '''Cb''' horizontal resolution is half that of '''Cr''' (and a quarter of the horizontal resolution of '''Y''').
* 4:1:0.5 or 4:1:0.25 are variants of 4:1:0 with reduced horizontal resolution on Cb, similar to VHS quality.