Image decoding method and apparatus using projection-type based quantisation parameters in image coding system for 360-degree video

1. A video decoding method performed by a decoding apparatus, the method comprising:

receiving 360-degree video information;

deriving a projection type of a projected picture based on the 360-degree video information;

deriving a weight map of the projected picture based on the projection type;

deriving quantization processing units of the projected picture;

deriving a Decoder side Adaptive Quantization Parameter (DAQP) for each of the quantization processing units based on the weight map; and

decoding the respective quantization processing units based on the DAQP,

wherein the weight map includes weights of samples of the projection picture based on the projection type,

wherein the deriving of the DAQP for each of the quantization processing units based on the weight map includes:

deriving an initial quantization parameter (QP) for the projected picture;

deriving weights of samples of each of the quantization processing units based on the weight map, and deriving an average value of the weights;

deriving the DAQP of each of the quantization processing units based on the initial QP and the average value;

deriving a specific boundary of a face of the projected picture based on the projection type;

deriving an update region adjacent to the specific boundary based on the 360-degree video information; and

deriving updated DAQP by updating the DAQP of the quantization processing unit included in the update region.

2. The video decoding method of claim 1 ,

wherein the 360-degree video information includes DAQP level information indicating a level of the quantization processing units, and

wherein the quantization processing units are derived as blocks corresponding to the level indicated by the DAQP level information.

3. The video decoding method of claim 2 , wherein the DAQP level information indicates a coding tree unit (CTU), a coding unit (CU), or a slice.

4. The video decoding method of claim 1 ,

wherein the 360-degree video information includes projection type information indicating the projection type of the projected picture,

wherein the projection type of the projected picture is derived based on the projection type information, and

wherein the projection type is one of equirectangular projection (ERP), cube map projection (CMP), icosahedral projection (ISP), octahedron projection (OHP), segmented sphere projection (SSP), and equal area projection (EAP).

5. The video decoding method of claim 1 , wherein the DAQP of each of the quantization processing units is derived based on the following equation:

DAQP i =QP init −3 log 2 ( w avg ),

where DAQP i is the DAQP of each of the quantization processing units, QP init is the initial QP for the projected picture, and w avg is the average value of the weights.

6. The video decoding method of claim 1 ,

wherein the 360-degree information includes DAQP depth information indicating a depth of the update region, and

wherein the update region includes quantization processing units as many as a number indicated by the DAQP depth information in an inner direction from the specific boundary.

7. The video decoding method of claim 1 , wherein the updated DAQP is derived based on the following equation:

DAQP i ′=(DAQP i +QP init )>> 1 ,

where DAQP i ′ is the updated DAQP of the quantization processing unit included in the update region, DAQP i is the DAQP of the quantization processing unit included in the update region, and QP init is an initial QP for the projected picture.

8. The video decoding method of claim 1 ,

wherein the 360-degree video information includes a DAQP enabled flag, and

wherein the DAQP enabled flag indicates whether the DAQP for each of the quantization processing units is derived based on the weight map.

9. A decoding apparatus for performing video decoding, the apparatus comprising:

an entropy decoder configured to receive 360-degree video information; and

a de-quantizer configured to derive a projection type of a projected picture based on the 360-degree video information, to derive a weight map of the projected picture based on the projection type, to derive quantization processing units of the projected picture, to derive a decoder side adaptive quantization parameter (DAQP) for each of the quantization processing units based on the weight map, and to decode the respective quantization processing units based on the DAQP,

wherein the weight map includes weights of samples of the projection picture based on the projection type,

wherein the de-quantizer derives an initial quantization parameter (QP) for the projected picture, derives weights of samples of each of the quantization processing units based on the weight map, derives an average value of the weights, derives the DAQP of each of the quantization processing units based on the initial QP and the average value, derives a specific boundary of a face of the projected picture based on the projection type, derives an update region adjacent to the specific boundary based on the 360-degree video information, and derives updated DAQP by updating the DAQP of the quantization processing unit included in the update region.

10. The decoding apparatus of claim 9 ,

wherein the 360-degree video information includes DAQP level information indicating a level of the quantization processing units, and

wherein the quantization processing units are derived as blocks corresponding to the level indicated by the DAQP level information.

11. An encoding apparatus for performing video encoding, the apparatus comprising:

a projection processer configured to obtain 360-degree video data captured by at least one camera, and to obtain a projected picture by processing the 360-degree video data;

a quantizer configured to derive a weight map of the projected picture based on the projection type, to derive quantization processing units of the projected picture, to derive a decoder side adaptive quantization parameter (DAQP) for each of the quantization processing units based on the weight map, and to perform quantization on each of the quantization processing units based on the DAQP; and

an entropy encoder configured to encode and to output 360-degree video information on the projected picture,

wherein the weight map includes weights of samples of the projection picture based on the projection type,

wherein the quantizer derives an initial quantization parameter (QP) for the projected picture, derives weights of samples of each of the quantization processing units based on the weight map, derives an average value of the weights, derives the DAQP of each of the quantization processing units based on the initial QP and the average value, derives a specific boundary of a face of the projected picture based on the projection type, derives an update region adjacent to the specific boundary based on the 360-degree video information, and derives updated DAQP by updating the DAQP of the quantization processing unit included in the update region.