Method and apparatus for encoding and decoding an omnidirectional video

1. A method for decoding a video, at least one picture of the video being represented as a 3D surface projected onto at least one 2D picture using a projection function, the method comprising, for at least one current block of the 2D picture:

determining whether an absolute value of at least one component of a motion vector (dV) associated with another block of the 2D picture is lower than a value;

responsive to a determination that the absolute value is lower than the value, transforming the motion vector (dV) into a motion vector predictor (dP) of the current block using the projection function;

responsive to a determination that the absolute value is equal to or larger than the value, skipping the transforming of the motion vector or modifying the motion vector by clipping the at least one component so that the absolute value of the at least one component is equal to the value and transforming the modified motion vector into a motion vector predictor of the current block using the projection function; and

decoding the current block from the motion vector predictor (dP).

2. The method of claim 1 , wherein modifying the motion vector comprises clipping dVy to h−Vy in the case where Vy+dVy is larger than the height h of the picture, and clipping dVy to −Vy in the case where Vy+dVy is lower than 0, where dVy is an ordinate of the motion vector (dV) and Vy is an ordinate of a point V of the another block.

3. The method of claim 1 , wherein the value is equal to w/8 or h/4, where w is the width of the picture and h is the height of the picture.

4. The method of claim 1 , further comprising after the transforming, determining whether an absolute value of at least one component difference between the motion vector predictor (dP) and the motion vector (dV) is larger than another value and, in the case where the absolute value is larger than the another value, clipping the at least one component of the motion vector predictor (dP) so that the absolute value of the at least one component difference is equal to the another value.

5. The method of claim 1 , further comprising after the transforming, clipping dPy to h−Py in the case where Py+dPy is larger than the height of the picture and clipping dPy to −Py in the case where Vy+dVy is lower than 0, where h is the height of the picture, dPy is an ordinate of the motion vector predictor (dP) and Py is an ordinate of a point P of the current block.

6. The method of claim 1 , further comprising after the transforming, clipping dPx to w+C−Px in the case where Px+dPx>w+C and clipping dPx to −C−Px in the case where Px+dPx<−C, where w is a width of the picture, C is a value, dPx is an abscissa of the motion vector predictor (dP) and Px is an abscissa of a point P of the current block.

7. The method of claim 1 , wherein the 3D surface is a sphere.

8. The method of claim 7 , wherein transforming the motion vector (dV) associated with another block of the 2D picture into the motion vector predictor (dP) associated with the current block using the projection function comprises:

projecting on the surface a point P of the current block to a point P″′, a point V of the another block to a point V″′ and the point V of the another block displaced by the motion vector (dV) to a point W″′;

determining an intersection Vp between a vector OV″′ and a plane G and an intersection Wp between a vector OW″′ and the plane G, wherein the plane G is a plane tangent in P″′ to the surface, O is the origin of the sphere;

displacing P″′ by dVp to obtain a point Qp in the plane G, wherein dVp is equal to the vector VpWp;

projecting the point Qp in the 2D picture to a point Q; and

determining the motion vector predictor (dP) of the current block as a vector from P to Q.

9. A non-transitory information storage medium storing program code instructions for implementing the method according to claim 1 , when the program is executed by a processor.

10. A method for encoding a video, at least one picture of the video being represented as a 3D surface projected onto at least one 2D picture using a projection function, the method comprising, for at least one current block of the 2D picture:

determining whether an absolute value of at least one component of a motion vector (dV) associated with another block of the 2D picture is lower than a value;

responsive to a determination that the absolute value is lower than the value, transforming the motion vector (dV) into a motion vector predictor (dP) of the current block using the projection function;

responsive to a determination that the absolute value is equal to or larger than the value, skipping the transforming of the motion vector or modifying the motion vector by clipping the at least one component so that the absolute value of the at least one component is equal to the value and transforming the modified motion vector into a motion vector predictor of the current block using the projection function; and

encoding the current block from the motion vector predictor (dP).

11. A non-transitory information storage medium storing program code instructions for implementing the method according to claim 10 , when the program is executed by a processor.

12. An apparatus for decoding a video, at least one picture of the video being represented as a 3D surface projected onto at least one 2D picture using a projection function, the apparatus comprising electronic circuitry adapted for:

determining whether an absolute value of at least one component of a motion vector (dV) associated with another block of the 2D picture is lower than a value;

responsive to a determination that the absolute value is lower than the value, transforming the motion vector (dV) into a motion vector predictor (dP) of the current block using the projection function;

responsive to a determination that the absolute value is equal to or larger than the value, skipping the transforming of the motion vector or modifying the motion vector by clipping the at least one component so that the absolute value of the at least one component is equal to the value and transforming the modified motion vector into a motion vector predictor of the current block using the projection function; and

decoding the current block from the motion vector predictor (dP).

13. An immersive rendering device comprising an apparatus for decoding a video according to claim 12 .

14. An apparatus for encoding a video, at least one picture of the video being represented as a 3D surface projected onto at least one 2D picture using a projection function, the apparatus comprising electronic circuitry adapted for:

determining whether an absolute value of at least one component of a motion vector (dV) associated with another block of the 2D picture is lower than a value;

responsive to a determination that the absolute value is lower than the value, transforming the motion vector (dV) into a motion vector predictor (dP) of the current block using the projection function;

responsive to a determination that the absolute value is equal to or larger than the value, skipping the transforming of the motion vector or modifying the motion vector by clipping the at least one component so that the absolute value of the at least one component is equal to the value and transforming the modified motion vector into a motion vector predictor of the current block using the projection function; and

encoding the current block from the motion vector predictor (dP).