JVET-P0158 CE5-related: Align ALF virtual boundary processes in the bottom CTU rows of one picture and one subpicture [C.-Y. Lai, O. Chubach, L. Chen, C.-Y. Chen, T.-D. Chuang, C.-W. Hsu, Y.-W. Huang, S.-M. Lei (MediaTek)]

In VVC Draft 6, to remove the line buffers required in adaptive loop filter (ALF) hardware decoding, the ALF virtual boundary (VB) process is applied to each coding tree unit (CTU) row in a picture except for the bottom CTU row. When a picture consists of independently decodable subpictures and the subpictures may be extracted as a picture, the mismatch may occur for the bottom CTU row handling if the subpicture bottom boundary is not the picture bottom boundary. Two methods are proposed in the contribution to solve this issue.

1. Applying the ALF VB process to all CTU rows in a picture, including the bottom CTU row

2. Disabling the ALF VB process for subpicture bottom CTU rows if the subpicture can be independently decodable (subpic_treated_as_pic_flag[ i ] = 1 and loop_filter_across_subpic_enabled_flag[i]=0 )

BoG recommendation: Adopt method 1, applying ALF VB to all CTU in a picture, including the bottom CTU row.