Editing Chroma subsampling (section)

==== 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>