Entropy coding motion vector residuals obtained using reference motion vectors

1. A method for decoding a video bitstream, comprising:

determining, for a current block of the video bitstream, a reference motion vector from a number of candidate reference motion vectors, wherein the reference motion vector is associated with a reference block and includes a predicted portion and a residual portion;

selecting a probability context model for the current block by evaluating the residual portion of the reference motion vector with one or more thresholds, wherein the selecting comprises:

comparing a magnitude of the residual portion of the reference motion vector with the one or more thresholds to determine an indication of likelihood of motion intensity for the current block; and

selecting the probability context model from a plurality of probability context models based on the indication of likelihood of motion intensity for the current block; and

entropy decoding, for the current block using a processor, a motion vector residual associated with the current block using the probability context model, wherein the motion vector residual is determined as a difference between a motion vector determined for the current block and the reference motion vector.

2. The method of claim 1 , wherein the current block is located in a current frame of the video bitstream, and the reference block is a previously decoded block located in the current frame, or a block located in a reference frame of the current frame of the video bitstream.

3. The method of claim 2 , wherein the reference frame is selected from a first number of candidate reference frames for the current frame, and the reference frame includes, for the current block, a second number of candidate reference blocks including a co-located block of the current block and at least one of: a block to the right of the co-located block, a block below the co-located block, a block to the bottom-left of the co-located block, and a block to the bottom-right of the co-located block.

4. The method of claim 2 , wherein the reference block is selected from a dynamic list of a varying number of candidate reference blocks, the candidate reference blocks comprising at least one of: a co-located block in the reference frame, a block to the right of the co-located block in the reference frame, a block below the co-located block in the reference frame, a block to the bottom-left of the co-located block in the reference frame, a block to the bottom-right of the co-located block in the reference frame, a block above the current block in the current frame, a block to the left of the current block in the current frame, a block to the above-left of the current block in the current frame, and a block to the above-right of the current block in the current frame.

5. The method of claim 1 , wherein the plurality of probability context models comprises a first probability context model associated with a first category of blocks indicative of strong motion intensity and a second probability context model associated with a second category of blocks indicative of weak motion intensity.

6. The method of claim 1 , wherein the plurality of probability context models is updated based on a histogram of probability context models determined for some or all blocks in a frame.

7. A method for encoding a video stream, comprising:

selecting, for a current block of the video stream, a reference motion vector from a number of candidate reference motion vectors, wherein the reference motion vector is associated with a reference block and includes a predicted portion and a residual portion;

selecting a probability context model for the current block by evaluating the residual portion of the reference motion vector with one or more thresholds, wherein the selecting comprises:

comparing a magnitude of the residual portion of the reference motion vector with the one or more thresholds to determine an indication of likelihood of motion intensity for the current block; and

selecting the probability context model from a plurality of probability context models based on the indication of likelihood of motion intensity for the current block; and

entropy encoding, for the current block using a processor, a motion vector residual associated with the current block using the probability context model, wherein the motion vector residual associated with the current block is determined as a difference between a motion vector determined for the current block and the reference motion vector.

8. The method of claim 7 , wherein the current block is located in a current frame of the video stream, and the reference block is a previously coded block located in the current frame, or a block located in a reference frame of the current frame in the video stream.

9. The method of claim 8 , wherein the reference frame is selected from a first number of candidate reference frames for the current frame, and the reference frame includes, for the current block, a second number of candidate reference blocks including a co-located block of the current block and at least one of the following blocks in the reference frame: a block to the right of the co-located block, a block below the co-located block, a block to the bottom-left of the co-located block, and a block to the bottom-right of the co-located block.

10. The method of claim 8 , wherein the reference block is selected from a dynamic list of a varying number of candidate reference blocks, the candidate reference blocks comprising at least one of: a co-located block in the reference frame, a block to the right of the co-located block in the reference frame, a block below the co-located block in the reference frame, a block to the bottom-left of the co-located block in the reference frame, a block to the bottom-right of the co-located block in the reference frame, a block above the current block in the current frame, a block to the left of the current block in the current frame, a block to the above-left of the current block in the current frame, and a block to the above-right of the current block in the current frame.

11. The method of claim 7 , wherein the plurality of probability context models comprises a first probability context model associated with a first category of blocks indicative of strong motion intensity and a second probability context model associated with a second category of blocks indicative of weak motion intensity.

12. The method of claim 7 , wherein the plurality of probability context models is updated based on a histogram of probability context models determined for some or all blocks in a frame.

13. An apparatus for decoding a video bitstream, comprising:

a memory; and

a processor configured to execute instructions stored in the memory to:

determine, for a current block of the video bitstream, a reference motion vector from a number of candidate reference motion vectors, wherein the reference motion vector is associated with a reference block and includes a predicted portion and a residual portion;

select a probability context model for the current block by evaluating the residual portion of the reference motion vector with one or more thresholds, comprising:

comparing a magnitude of the residual portion of the reference motion vector with the one or more thresholds to determine an indication of likelihood of motion intensity for the current block; and

selecting the probability context model from a plurality of probability context models based on the indication of likelihood of motion intensity for the current block; and

entropy decode, for the current block, a motion vector residual associated with the current block using the probability context model, wherein the motion vector residual associated with the current block is determined as a difference between a motion vector determined for the current block and the reference motion vector.

14. The apparatus of claim 13 , wherein the current block is located in a current frame of the video bitstream, and the reference block is a previously decoded block located in the current frame, or a block located in a reference frame of the current frame in the video bitstream.

15. The apparatus of claim 14 , wherein the reference frame is selected from a first number of candidate reference frames for the current frame, and the reference frame includes, for the current block, a second number of candidate reference blocks including a co-located block of the current block and at least one of the following blocks in the reference frame:

a block to the right of the co-located block, a block below the co-located block, a block to the bottom-left of the co-located block, and a block to the bottom-right of the co-located block.