Method and apparatus for video coding using secondary most probable mode list based on template matching

1 . A method of decoding a current block, performed by a video decoding device, the method comprising:

setting a template of the current block;

determining a corresponding block in a reconstructed region of a current picture based on a matching cost between a template of the corresponding block and the template of the current block;

calculating gradient magnitudes and gradient directions for pixels of the corresponding block;

generating a HoG (Histogram of Gradients) based on the gradient magnitudes and the gradient directions;

deriving intra-prediction modes of the current block based on the HoG; and

generating a prediction block of the current block based on the intra-prediction modes of the current block.

2 . The method of claim 1 , wherein determining the corresponding block includes:

calculating a gradient magnitude of the template of the current block by applying a differential filter to the template;

calculating a gradient magnitude in K×Q blocks for a predefined search region, K and Q being integers greater than or equal to 1; and

determining the corresponding block by calculating the matching cost based on the gradient magnitude of the template of the current block and the gradient magnitude in K×Q blocks.

3 . The method of claim 2 ,

wherein the matching cost is calculated when the gradient magnitude in K×Q blocks is equal to or greater than a preset threshold.

4 . The method of claim 1 , wherein deriving the intra-prediction modes of the current block includes:

accumulating the gradient magnitudes into intra-prediction mode indices according to the gradient directions; and

ordering the intra-prediction mode indices in descending order according to a cumulative gradient magnitude for each intra-prediction mode index.

5 . The method of claim 4 , wherein accumulating the gradient magnitudes includes:

calculating a gradient magnitude value for each of the pixels of the corresponding block, calculating a directionality by using the gradient magnitude value, and then calculating the intra-prediction mode index by using the directionality and a predefined directionality table.

6 . A method of encoding a current block, performed by a video encoding device, the method comprising:

setting a template of the current block;

determining a corresponding block in a reconstructed region of a current based on a matching cost between a template of the corresponding block and the template of the current block;

calculating gradient magnitudes and gradient directions for pixels of the corresponding block;

generating a HoG (Histogram of Gradients) based on the gradient magnitudes and the gradient directions;

deriving intra-prediction modes of the current block based on the HoG; and

generating a prediction block of the current block based on the intra-prediction modes of the current block.

7 . A non-transitory computer-readable recording medium storing a bitstream generated by a video encoding method, the video encoding method comprising:

setting a template of the current block;

determining a corresponding block in a reconstructed region of a current picture based on a matching cost between a template of the corresponding block and the template of the current block;

calculating gradient magnitudes and gradient directions for pixels of the corresponding block;

generating a HoG (Histogram of Gradients) based on the gradient magnitudes and the gradient directions;

deriving intra-prediction modes of the current block based on the HoG; and

generating a prediction block of the current block based on the intra-prediction modes of the current block.

8 . The method of claim 1 , wherein the matching cost includes at least one of Sum of Absolute Difference (SAD) or Sum of Squared Difference (SSD).

9 . The method of claim 6 , wherein the matching cost includes at least one of Sum of Absolute Difference (SAD) or Sum of Squared Difference (SSD).

10 . The non-transitory computer-readable recording medium of claim 7 , wherein the matching cost includes at least one of Sum of Absolute Difference (SAD) or Sum of Squared Difference (SSD).