Image encoding method, image decoding method, image encoding apparatus, image decoding apparatus, and image coding apparatus

1. An image encoding method comprising:

generating a first bitstream by encoding an input image, and generating a first decoded image by encoding and decoding the input image; and

generating a second bitstream by encoding the input image using the first decoded image, and generating a second decoded image by encoding and decoding the input image;

wherein the second decoded image has resolution or image quality higher than resolution or image quality of the first decoded image,

the generating of a second bitstream and a second decoded image includes generating a prediction block for a current block by intra prediction,

in the generating of a prediction block, the prediction block is generated using (i) pixels included in the second decoded image as top and left reference pixels for the current block, and (ii) pixels included in the first decoded image as right and bottom reference pixels for the current block,

the intra prediction includes a first mode in which a prediction pixel for a current pixel is generated using reference pixels in an upper direction, a left direction, a lower direction, and a right direction of the current pixel included in the current block, and

in the generating of a prediction block, in the first mode, the prediction pixel is generated using (i) pixels that are in an upper direction and a left direction of the current block and are included in the second decoded image, as reference pixels in an upper direction and a left direction of the current block, and (ii) pixels that are in a right direction and a lower direction of a co-located block of the current pixel and are included in the first decoded image, as reference pixels in a right direction and a lower direction of the current pixel.

2. The image encoding method according to claim 1 ,

wherein the intra prediction includes a DC mode in which prediction pixels for pixels included in the current block are generated using an average of reference pixels adjacent to the current block, and

in the generating of a prediction block, in the DC mode, the prediction pixel is generated using an average of (i) reference pixels that are adjacent to an upper direction and a left direction of the current block and are included in the second decoded image, and (ii) reference pixels that are adjacent to a right direction and a lower direction of a co-located block of the current block and are included in the first decoded image.

3. The image encoding method according to claim 1 ,

wherein the intra prediction includes an Angular mode in which a prediction pixel for a current pixel is generated, using a reference pixel located in a prediction direction of the current pixel included in the current block, and

in the generating of a prediction block, in the Angular mode, the prediction pixel is generated using (i) reference pixels that are in an upper direction, a left direction, a lower left direction, and an upper right direction of the current block and are included in the second decoded image, and (ii) reference pixels that are in a right direction, a lower direction, an upper right direction, and a lower left direction of a co-located block of the current block and are included in the first decoded image.

4. The image encoding method according to claim 1 ,

wherein in the generating of a prediction block:

a plurality of prediction pixels for a plurality of pixels included in the current block are generated; and

prediction pixels at a right end and a lower end in the current block among the prediction pixels are corrected using a co-located pixel of the prediction pixel or a surrounding pixel of the co-located pixel, the collocated pixel and the surrounding pixel being included in the first decoded image.

5. An image decoding method comprising:

generating a first decoded image by decoding a first bitstream; and

generating a second decoded image by decoding a second bitstream using the first decoded image;

wherein the second decoded image has resolution or image quality higher than resolution or image quality of the first decoded image,

the generating of a second decoded image includes generating a prediction block for a current block by intra prediction,

in the generating of a prediction block, the prediction block is generated using pixels included in the second decoded image as top and left reference pixels for the current block, and using pixels included in the first decoded image as reference pixels on right and bottom of the current block,

the intra prediction includes a first mode in which a prediction pixel for a current pixel is generated using reference pixels in an upper direction, a left direction, a lower direction, and a right direction of the current pixel included in the current block, and

in the generating of a prediction block, in the first mode, the prediction pixel is generated using (i) pixels that are in an upper direction and a left direction of the current block and are included in the second decoded image, as reference pixels in an upper direction and a left direction of the current block, and (ii) pixels that are in a right direction and a lower direction of a co-located block of the current pixel and are included in the first decoded image, as reference pixels in a right direction and a lower direction of the current pixel.

6. The image decoding method according to claim 5 ,

wherein the intra prediction includes a DC mode in which prediction pixels for pixels included in the current block are generated using an average of reference pixels adjacent to the current block, and

in the generating of a prediction block, in the DC mode, the prediction pixel is generated using an average of (i) reference pixels that are adjacent to an upper direction and a left direction of the current block and are included in the second decoded image, and (ii) reference pixels that are adjacent to a right direction and a lower direction of a block located at a co-located block of the current block and are included in the first decoded image.

7. The image decoding method according to claim 5 ,

wherein the intra prediction includes an Angular mode in which a prediction pixel for a current pixel is generated using a reference pixel located in a prediction direction of the current pixel included in the current block, and

in the generating of a prediction block, in the Angular mode, the prediction pixel is generated using (i) reference pixels that are in an upper direction, a left direction, a lower left direction, and an upper right direction of the current block and are included in the second decoded image, and (ii) reference pixels that are in a right direction, a lower direction, an upper right direction, and a lower left direction of a co-located block of the current block and are included in the first decoded image.

8. The image decoding method according to claim 5 ,

wherein in the generating of a prediction block:

a plurality of prediction pixels for a plurality of pixels included in the current block are generated; and

prediction pixels at a right end and a lower end in the current block among the prediction pixels are corrected using a co-located pixel of the prediction pixel or a surrounding pixel of the co-located pixel, the co-located pixel and the surrounding pixel being included in the first decoded image.

9. An image encoding apparatus comprising:

a first encoder which generates a first bitstream by encoding an input image, and generates a first decoded image by encoding and decoding the input image; and

a second encoder which generates a second bitstream by encoding the input image using the first decoded image, and generates a second decoded image by encoding and decoding the input image;

wherein the second decoded image has resolution or image quality higher than resolution or image quality of the first decoded image,

the second encoder includes an intra prediction unit configured to generate a prediction block for a current block by intra prediction,

the intra prediction unit is configured to generate the prediction block using (i) pixels included in the second decoded image as top and left reference pixels for the current block, and (ii) pixels included in the first decoded image as right and bottom reference pixels for the current block,

the intra prediction includes a first mode in which a prediction pixel for a current pixel is generated using reference pixels in an upper direction, a left direction, a lower direction, and a right direction of the current pixel included in the current block, and

the intra prediction unit is configured to, in the first mode, generate the prediction pixel using (i) pixels that are in an upper direction and a left direction of the current block and are included in the second decoded image, as reference pixels in an upper direction and a left direction of the current block, and (ii) pixels that are in a right direction and a lower direction of a co-located block of the current pixel and are included in the first decoded image, as reference pixels in a right direction and a lower direction of the current pixel.

10. An image decoding apparatus comprising:

a first decoder which generates a first decoded image by decoding a first bitstream; and

a second decoder which generates a second decoded image by decoding a second bitstream using the first decoded image;

wherein the second decoded image has resolution or image quality higher than the resolution or the image quality of the first decoded image,

the second decoder includes an intra prediction unit configured to generate a prediction block for a current block by intra prediction,

the intra prediction unit is configured to generate the prediction block using (i) pixels included in the second decoded image as top and left reference pixels for the current block, and (ii) pixels included in the first decoded image as right and bottom reference pixels for the current block,

the intra prediction includes a first mode in which a prediction pixel for a current pixel is generated using reference pixels in an upper direction, a left direction, a lower direction, and a right direction of the current pixel included in the current block, and

the intra prediction unit is configured to, in the first mode, generate the prediction pixel using (i) pixels that are in an upper direction and a left direction of the current block and are included in the second decoded image, as reference pixels in an upper direction and a left direction of the current block, and (ii) pixels that are in a right direction and a lower direction of a co-located block of the current pixel and are included in the first decoded image, as reference pixels in a right direction and a lower direction of the current pixel.

11. An image coding apparatus comprising:

the image encoding apparatus according to claim 9 ; and

an image decoding apparatus,

wherein the image decoding apparatus includes:

a first decoder which generates a first decoded image by decoding a first bitstream; and

a second decoder which generates a second decoded image by decoding a second bitstream using the first decoded image;

wherein the second decoded image has resolution or image quality higher than resolution or image quality of the first decoded image,

the second decoder includes an intra prediction unit configured to generate a prediction block for a current block by intra prediction,

the intra prediction unit is configured to generate the prediction block using (i) pixels included in the second decoded image as top and left reference pixels for current block, and (ii) pixels included in the first decoded image as right and bottom reference pixels for the current block,

the intra prediction includes a first mode in which a prediction pixel for a current pixel is generated using reference pixels in an upper direction, a left direction, a lower direction, and a right direction of the current pixel included in the current block, and

the intra prediction unit is configured to, in the first mode, generate the prediction pixel using (i) pixels that are in an upper direction and a left direction of the current block and are included in the second decoded image, as reference pixels in an upper direction and a left direction of the current block, and (ii) pixels that are in a right direction and a lower direction of a co-located block of the current pixel and are included in the first decoded image, as reference pixels in a right direction and a lower direction of the current pixel.