JVET-Q0194 Cleanup for checking CTU row boundary location in CCLM [C.-M. Tsai, C.-W. Hsu, Y.-W. Huang, S.-M. Lei (MediaTek)]
In this contribution, the condition of checking the coding tree unit (CTU) row boundary location in luma downsampling process of cross-component linear model (CCLM) modes is corrected. In VVC Draft 7, the CTU row boundary location is checked by evaluating if the top position of the current chroma transform block (TB) is a multiple of luma coding tree block (CTB) size divided by 2, which is not valid for colour formats 4:2:2 and 4:4:4. In the proposed method, the condition is modified to check if the corresponding luma top position of the current chroma TB is a multiple of luma CTB size.
Apparently, there is no problem in the current software, as the results with and without that change are identical. Obviously, the software has some mechanism to identify where the CTU boundary is in the $$$ and 4:2:2 chroma cases.
This was again reviewed Thursday 16 Jan. It was confirmed by other experts that the suggested text change is needed, and also the software needs to be fixed.
Decision (BF/text&SW): Adopt JVET_Q0194