Editing Chroma subsampling (section)

=== Gamut clipping ===
Another artifact that can occur with chroma subsampling is that out-of-[[gamut]] colors can occur upon chroma reconstruction. Suppose the image consisted of alternating 1-pixel red and black lines and the subsampling omitted the chroma for the black pixels.  Chroma from the red pixels will be reconstructed onto the black pixels, causing the new pixels to have positive red and ''negative'' green and blue values. As displays cannot output negative light (negative light does not exist), these negative values will effectively be clipped, and the resulting luma value will be too high.  Other sub-sampling filters (especially the averaging "box") have a similar issue that is harder to make a simple example out of. Similar artifacts arise in the less artificial example of gradation near a fairly sharp red/black boundary.<ref name=better/>

It is possible for the decoder to deal with out-of-gamut colors by considering how much chroma a given luma value can hold and distribute it into the 4:4:4 intermediate accordingly, termed "in-range chroma reconstruction" by Glenn Chan. The "proportion" method is in spirit similar to Kornelski's luma-weighted average, while the "spill" method resembles [[error diffusion]].<ref name=better/> Improving chroma reconstruction remains an active field of research.<ref>{{cite journal |last1=Chung |first1=Kuo-Liang |last2=Liang |first2=Yan-Cheng |last3=Wang |first3=Ching-Sheng |title=Effective Content-Aware Chroma Reconstruction Method for Screen Content Images |journal=IEEE Transactions on Image Processing |date=March 2019 |volume=28 |issue=3 |pages=1108–1117 |doi=10.1109/TIP.2018.2875340|pmid=30307864 |bibcode=2019ITIP...28.1108C |s2cid=52964340 }}</ref>