Image properties-based adaptive filtering method and device in image coding system

1. A restoring picture filtering method performed by a video decoder, comprising:

calculating an activity of an image of a target region of a restoring picture;

determining a directivity of the image of the target region;

selecting a particular filter within a filter set based on the activity and the directivity of the image of the target region; and

generating a filtered restoring picturebased on the selected filter,

wherein a, b, c, d, e, and f represent directivity values of samples of the same row or column in order from a sample on left or top, wherein b, c, d, and e represent directivity values of the samples in the target region, and wherein a and f represent the directivity values of the peripheral samples of the target region, when the target region has a size of 4×4, and

wherein the directivity of the image of the target region that has a size of 4×4 is calculated based on a following equation,

(c+d)*3+(b+e)*2+(a+f).

2. The filtering method of claim 1 , wherein the determining of the directivity includes:

determining the directivity for a sample based on the sample in the target region and four peripheral samples adjacent to the sample; and

determining the directivity for the target region based on the directivity for the sample.

3. The filtering method of claim 2 , wherein the directivity for the sample is determined based on a vertical change degree and a horizontal change degree for the sample, and

the vertical change degree and the horizontal change degree are calculated based on following equations:

Vertical =| E << 1− A−B |, and

Horizontal =| E << 1− C−D |,

wherein Vertical represents the vertical change degree, Horizontal represents the horizontal change degree, E represents a value of the sample, A, B, C, and D represent values of the four peripheral samples, vertical bars || on both sides of the equation represent an absolute value, and << represents an arithmetic left shift.

4. The filtering method of claim 1 , wherein the activity of the image for the target region includes horizontal activity and vertical activity.

5. The filtering method of claim 1 , wherein the determining of the directivity includes:

determining the directivity for the sample based on the sample in the target region and eight peripheral samples adjacent to the sample; and

determining the directivity for the target region based on the directivity for the sample.

6. The filtering method of claim 5 , wherein the directivity of the image for the target region indicates one of a horizontal direction, a vertical direction, an upper right diagonal direction, an upper left diagonal direction, and a non-direction.

7. The filtering method of claim 1 , wherein a size of the target region is variable.

8. The filtering method of claim 1 , further comprising:

receiving a variable region size enabled flag; and

receiving region size information when a value of the variable region size enabled flag indicates 1,

wherein the size of the target region is determined based on the region size information.

9. The filtering method of claim 8 , wherein the region size information is classified into five values of indexes 0 to 4.

10. The filtering method of claim 9 , wherein the indexes 0 to 4 are represented as binarization values “0”, “100”, “101”, “110”, and “111”, respectively.

11. The filtering method of claim 1 , further comprising:

receiving a diagonal activity enabled flag,

wherein when a value of the diagonal activity enabled flag indicates 1, the directivity of the image for the target region is determined by considering a diagonal direction.

12. The filtering method of claim 1 , wherein the filter set is configured based on an image resolution,

wherein, based on the image resolution being SD, the filter set includes a 3×3 filter, a 5×5 filter, and a 7×7 filter,

wherein, based on the image resolution being HD, the filter set includes the 7×7 filter, a 9×7 filter, a 9×9 filter, and a 11×11 filter,

wherein, based on the image resolution being 4K, the filter set includes the 9×9 filter, the 11×11 filter, and a 13×13 filter, and

wherein SD represents standard definition, HD represents high definition, and 4K represents 4K ultra high definition.

13. The filtering method of claim 12 , further comprising:

receiving a variable filter set enabled flag; and

receiving at least one of filter number information and filter tap information when a value of the variable filter set enabled flag indicates 1,

wherein the filter set is determined based on at least one of the filter number information and the filter tap information.

14. The filtering method of claim 13 , further comprising:

receiving a variable filter shape enabled flag,

wherein a case where the value of the variable filter shape enabled flag indicates 0 and a case where the value of the variable filter shape enabled flag indicates 1 are different in shape of the filter indicated by the filter tap information.