Editing Chroma subsampling (section)

===== 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>