JVET-C0039 Decoupled Luma/Chroma Transform Trees for Intra [F. Urban, T. Poirier, F. Le Léannec (Technicolor)]
In this contribution, a chroma transform tree decoupled from the luma transform tree is proposed for intra CUs. A chroma split transform flag is proposed to be added, which indicates whether the current chroma block is further split into 4 transform units. In addition, the chroma transform tree cannot be split if the luma transform tree is not split. It is reported that the proposed method provides 0.14% BD-Rate saving in luma and around 1.5% gain in chroma in AI and RA coding conditions, with encoding and decoding time identical to those of the JEM2.0.
In QTBT, luma and chroma have separate trees for intra anyway, and the bug that is reported for JEM2 is not present anymore.
It was claimed that the dependency of the luma and chroma trees might give some compression benefit; however, the contribution does not give evidence on that, as only results for separate trees with dependent coding are presented, but it could be the case that the same gain is achieved with independent coding (note that QTBT performs independent coding of luma and chroma trees, which also has been reported to provide gain compared to using a common tree).
Further study was encouraged to provide evidence that the dependent coding would provide benefit.