Techniques for intra prediction for 360 image and video coding

1. A method of decoding a coded image or a coded picture in a coded video sequence, into a picture representing a projection on a planar surface of a non-planar surface, the method being performed by a decoder employing intra prediction, and the method comprising:

obtaining an intra prediction mode indicative of an intra prediction direction, for a block of the picture;

determining at least one non-straight curve representative of a straight line on the non-planar surface, based on the projection and the obtained intra prediction mode;

creating, for the block, prediction samples along the determined at least one non-straight curve, from at least one boundary sample along the determined at least one non-straight curve;

performing a reverse projection on the block of the picture, the block having a first aspect ratio that has a first width and a first height;

identifying a rectangle corresponding to a surface area on the non-planar surface of the block on which the reverse projection is performed, based on at least one property of the projection and a position of the block in the picture;

rounding a decimal number of a second height of a second aspect ratio of the identified rectangle to a nearest power of two that is defined for non-square blocks in the video compression standard; and

based on the decimal number of the second height of the second aspect ratio of the identified rectangle being rounded to the nearest power of two, and on the second aspect ratio of the identified rectangle having the second height shorter than the first height of the first aspect ratio of the block and having a second width wider than the first width of the first aspect ratio, removing, from at least one intra prediction direction available for decoding the block, one or more intra prediction directions toward a bottom left corner of the block.

2. The method of claim 1 , wherein the determining the at least one non-straight curve comprises determining a shape of the at least one non-straight curve to be a shape of the projection.

3. The method of claim 1 , wherein the determining the at least one non-straight curve comprises determining a position of the at least one non-straight curve to correspond to the position of the block in the picture.

4. The method of claim 1 , wherein the determining the at least one non-straight curve comprises determining a direction of the at least one non-straight curve to be the intra prediction direction, from a start point close to the at least one boundary sample to an end point close to one of the prediction samples.

5. A non-transitory computer-readable storage medium storing a program comprising instructions to cause at least one processor to perform the method of claim 1 .

6. A method of decoding a coded image or a coded picture in a coded video sequence, into a picture representing a projection on a planar surface of a non-planar surface, the method being performed by a decoder employing intra prediction, the method comprising:

performing a reverse projection on a block of the picture, the block having a first aspect ratio that has a first width and a first height;

identifying a rectangle corresponding to a surface area on the non-planar surface of the block on which the reverse projection is performed, based on at least one property of the projection and a position of the block in the picture;

rounding a decimal number of a second height of a second aspect ratio of the identified rectangle to a nearest power of two that is defined for non-squared blocks in the video compression standard; and

based on the decimal number of the second height of the second aspect ratio of the identified rectangle being rounded to the nearest power of two, and on the second aspect ratio of the identified rectangle having the second height shorter than the first height of the first aspect ratio of the block and having a second width wider than the first width of the first aspect ratio, removing, from at least one intra prediction direction available for decoding the block, one or more intra prediction directions toward a bottom left corner of the block.

7. The method of claim 6 , further comprising, based on the decimal number of the second height of the second aspect ratio of the identified rectangle being rounded to the nearest power of two, and on the second aspect ratio of the identified rectangle having the second height shorter than the first height of the first aspect ratio of the block and having the second width wider than the first width of the first aspect ratio, adding, to the at least one intra prediction direction, one or more intra prediction directions toward a top right corner of the identified rectangle.

8. A non-transitory computer-readable storage medium storing a program comprising instructions to cause at least one processor to perform the method of claim 6 .

9. An apparatus for decoding a coded image or a coded picture in a coded video sequence, into a picture representing a projection on a planar surface of a non-planar surface, the apparatus being a decoder employing intra prediction, and the apparatus comprising:

at least one memory configured to store computer program code; and

at least one processor configured to access the at least one memory and operate according to the computer program code, the computer program code comprising:

obtaining code configured to cause the at least one processor to obtain an intra prediction mode indicative of an intra prediction direction, for a block of the picture;

determining code configured to cause the at least one processor to determine at least one non-straight curve representative of a straight line on the non-planar surface, based on the projection and the obtained intra prediction mode;

creating code configured to cause the at least one processor to create, for the block, prediction samples along the determined at least one non-straight curve, from at least one boundary sample along the determined at least one non-straight curve;

first performing code configured to cause the at least one processor to perform a reverse projection on the block of the picture, the block having a first aspect ratio that has a first width and a first height;

identifying code configured to cause the at least one processor to identify a rectangle corresponding to a surface area on the non-planar surface of the block on which the reverse projection is performed, based on at least one property of the projection and a position of the block in the picture;

rounding code configured to cause the at least one processor to round a decimal number of a second height of a second aspect ratio of the identified rectangle to a nearest power of two that is defined for non-square blocks in the video compression standard; and

second performing code configured to cause the at least one processor to, based on the decimal number of the second height of the second aspect ratio of the identified rectangle being rounded to the nearest power of two, and on the second aspect ratio of the identified rectangle having the second height shorter than the first height of the first aspect ratio of the block and having a second width wider than the first width of the first aspect ratio, remove, from at least one intra prediction direction available for decoding the block, one or more intra prediction directions toward a bottom left corner of the block.

10. The apparatus of claim 9 , wherein the determining code is further configured to cause the at least one processor to determine a shape of the at least one non-straight curve to be a shape of the projection.

11. The apparatus of claim 9 , wherein the determining code is further configured to cause the at least one processor to determine a position of the at least one non-straight curve to correspond to the position of the block in the picture.

12. The apparatus of claim 9 , wherein the determining code is further configured to cause the at least one processor to determine a direction of the at least one non-straight curve to be the intra prediction direction, from a start point close to the at least one boundary sample to an end point close to one of the prediction samples.

13. The apparatus of claim 9 , wherein the second performing code is further configured to cause the at least one processor to, based on the decimal number of the second height of the second aspect ratio of the identified rectangle being rounded to the nearest power of two, and on the second aspect ratio of the identified rectangle having the second height shorter than the first height of the first aspect ratio of the block and having the second width wider than the first width of the first aspect ratio, add, to the at least one intra prediction direction, one or more intra prediction directions toward a top right corner of the identified rectangle.