JVET-AL0099 AHG9: On design of display rectangles SEI message [J. Nam, H. Tan, J. Lee, C. Kim, J. Lim, S. Kim (LGE)]

This contribution proposes modifications that are asserted to provide improvement to some aspects of display rectangles (DR) SEI message in JVET-AK2032. The proposed modifications can be summarized as follows:

1. Match the number of display rectangles for position updates

Option 1: add a constraint that when dr_pos_updates_flag is equal to 1, the value of dr_num_rect_minus1 shall be equal to the value of the corresponding syntax element in the previous DR SEI message.

Option 2: signal two syntax elements (i.e., dr_assigned_rect_id[ i ] and dr_rect_id[ i ]) to specify rectangle identifier for each display rectangle.

Agreed on option 2 of item 1, with an additional condition for presence of dr_rect_id[ i ] only when dr_pos_updates_flag equal to 1.

2. Simplify loop for signalling syntax elements.

Option 1: Signal one loop over the number of display rectangles.

Option 2: Signal one loop over the number of display rectangles for syntax elements to be signalled when dr_pos_updates_flag is equal to 0.

Agreed to use option 2 of item 2 for the combining the second and third loops, while keeping the first loop containing the aspect ratio separate.

3. Signal a syntax element (i.e., dr_rect_pos_update_flag[ i ]) to specify that the display rectangle position of the i-th rectangle is updated or not when dr_pos_updates_flag is equal to 1.

Agreed on item 3.

4. Signal sign values of top-left position for display rectangle only when the corresponding values are greater than 0.

Agreed on item 4.

5. Derive the variables DrRectTopLeftX[ i ] and DrRectTopLeftX[ i ] by scaling with DrUnitSize.

It was suggested that the unit size of the width and height requires less fine granularity than the position.

There is an issue with the semantics of dr_rect_top_left_x[ ] and dr_rect_top_left_y[ ].

Decision: Remove the reference to DrUnitSize units.

6. Remove the current constraint on DrUnitSize value related to SubWidthC and SubHeightC, while add the constraints on the DrRectWidth[ i ], DrRectHeight[ i ], DrFillWidth[ i ], DrFillHeight[ i ], DrRectTopLeftX[ i ] and DrRectTopLeftY[ i ] values related to SubWidthC and SubHeightC.

Agreed on item 6.

7. Signal the syntax elements (i.e., dr_fill_content_width[ i ], dr_fill_content_region_top_left_x[ i ], dr_fill_content_region_top_left_y[ i ]) for filling parameters when spatial extrapolation filling method is used.

It was suggested that it is not necessary to signal these syntax elements.

Editorial issue: Semantics for the Extrapolate fill method (method 1) should be checked for if there is a bug, because the cropped decoded picture size can be used to determine the regions that need filling.

8. Derive the variables DrFillTopLeftX[ i ] and DrFillTopLeftX[ i ] by scaling with DrUnitSize.

Decision: Adopt as noted above.