Editing Chroma subsampling (section)

=== Gamma luminance error ===
{{further|Gamma correction#Scaling and blending}}
Gamma-corrected signals like Y'CbCr have an issue where chroma errors "bleed" into luma. In those signals, a low chroma actually makes a color appear less bright than one with equivalent luma. As a result, when a saturated color blends with an unsaturated or complementary color, a loss of luminance occurs at the border. This can be seen in the example between magenta and green.<ref name=better>{{cite journal |last1=Chan |first1=Glenn |title=Toward Better Chroma Subsampling: Recipient of the 2007 SMPTE Student Paper Award |journal=SMPTE Motion Imaging Journal |date=May 2008 |volume=117 |issue=4 |pages=39–45 |doi=10.5594/J15100 |url=http://www.glennchan.info/articles/technical/chroma/chroma1.htm|doi-access=free }}</ref> This issue persists in HDR video where gamma is generalized into a transfer function "[[EOTF]]". A steeper EOTF shows a stronger luminance loss.<ref name=Larbier>{{cite journal |last1=Larbier |first1=Pierre |title=High Dynamic Range: Compression Challenges |journal=SMPTE 2015 Annual Technical Conference and Exhibition |date=October 2015 |pages=1–15 |doi=10.5594/M001639|isbn=978-1-61482-956-0 }}</ref>

Some proposed corrections of this issue are:
* Luma-weighted average (Kornelski, experiment for mozjpeg)<ref>{{cite web |title=Gamma-correct chroma subsampling · Issue #193 · mozilla/mozjpeg |url=https://github.com/mozilla/mozjpeg/issues/193 |website=GitHub |language=en}}</ref>
* Iterative ''sharp YUV'' method, used by [[WebP]] and optionally [[AVIF]]. Sharp YUV assumes a [[bilinear interpolation|bilinear]] upscaling for chroma.<ref>{{cite web |title=WebP: sharpyuv/sharpyuv.h {{!}} Fossies |url=https://fossies.org/linux/libwebp/sharpyuv/sharpyuv.h |website=fossies.org |quote=Assumes that the image will be upsampled using a bilinear filter. If nearest neighbor is used instead, the upsampled image might look worse than with standard downsampling.}}</ref>
* RGB subsampling in linear space before chroma subsampling (HDRTools)<ref name=Larbier/>
* Iterative or closed-form luma correction to minimize luminance error (HDRTools)<ref>{{cite conference |url=https://norkin.org/pdf/SPIE_2016_HDR_conversion_metrics.pdf |last1=Norkin |first1=Andrey |title=HDR color conversion with varying distortion metrics |date=27 September 2016 |pages=99710E |conference=SPIE Optical Engineering + Applications, 2016 |doi=10.1117/12.2237040}}</ref>

[[Rec. 2020]] defines a "constant luminance" Yc'CbcCrc, which is calculated from linear RGB components and then gamma-encoded. This version does not suffer from the luminance loss by design.<ref name=Recommendation2020>{{cite news |title=BT.2020: Parameter values for ultra-high definition television systems for production and international programme exchange |publisher=[[International Telecommunication Union]] |url=https://www.itu.int/rec/R-REC-BT.2020/en |date=2014-07-17 |access-date=2014-08-31}}</ref>