Method of motion information coding

A method and apparatus for motion information coding not only uses the Merge index to indicate the selected Merge candidate, but also refines partial motion information of the selected Merge candidate. An encoder derives a Merge candidate set based on the motion information associated with neighboring blocks. The encoder selects a target Merge candidate from the Merge candidate set and signal a Merge index corresponding to the target Merge candidate. The encoder then predicts a part of the current motion information based on partial motion information associated with the target Merge candidate to generate coded motion information.

TECHNICAL FIELD

The present invention relates to video coding. In particular, the present invention relates to motion information coding based on motion information associated with neighboring blocks.

BACKGROUND

In video coding systems, spatial and temporal redundancy is exploited using spatial and temporal prediction to reduce the information to be transmitted. The spatial and temporal prediction utilizes decoded pixels from the same picture and reference pictures respectively to form prediction for current pixels to be coded. In a conventional coding system, side information associated with spatial and temporal prediction may have to be transmitted, which will take up some bandwidth of the compressed video data. The transmission of motion vectors for temporal prediction may require a noticeable portion of the compressed video data, particularly in low-bitrate applications. To further reduce the bitrate associated with motion vectors, a technique called Motion Vector Prediction (MVP) has been used in the field of video coding in recent years. The MVP technique exploits the statistic redundancy among neighboring motion vectors spatially and temporally. In the rest of this document, MVP may sometimes denote “motion vector prediction” and sometimes denote “motion vector predictor” according to contexts.

In the High-Efficiency Video Coding (HEVC) standard, there are two methods to encode the motion information associated with each Inter prediction unit (PU). One is Merge mode and the other is Advanced Motion Vector Prediction (AMVP) mode. If the Merge mode is used, a Merge index is coded to indicate the Merge candidate selected for the PU from a Merge candidate set, which consists of motion information associated with neighboring blocks. The motion information of the current PU will be the same as the motion information of selected neighboring block. In other words, the current PU inherits motion information from the selected Merge candidate when the Merge mode is used. If the AMVP mode is used for the current PU, the motion information including Inter prediction direction, reference frame index, motion vector predictor index and motion vector difference are coded directly. Compared to the Merge mode, more syntax elements have to be signaled to represent the motion information in the AMVP mode. In other words, more bits are required to encode the motion information in the AMVP mode than the Merge mode. It is desirable to improve the coding efficiency related to motion information coding.

The candidates for the AMVP mode and Merge mode are derived based on the motion information of neighboring blocks. The neighboring blocks include one or more spatial neighboring blocks and temporal neighboring blocks. An example of neighboring blocks used by the HEVC standard are shown inFIG. 1, where B0, B1, B2, A0, and A1, represent spatial neighboring blocks of the current block (110). BCTRrepresents a center co-located temporal neighboring block. If the motion information associated with BCTRis not available, temporal neighboring block BBRis used.

SUMMARY

A method and apparatus for motion information coding using a refined Merge mode are disclosed. Embodiments of the present invention not only uses the Merge index to indicate the selected Merge candidate, but also refines partial motion information of the selected Merge candidate.

An encoder according to the present invention will determine second motion information associated with one or more neighboring blocks of the current block and derive a Merge candidate set consisting of one or more Merge candidates based on the second motion information associated with said one or more neighboring blocks. The encoder selects a target Merge candidate from the Merge candidate set and signal a Merge index corresponding to the target Merge candidate. The encoder then predicts a first part of the current motion information based on partial second motion information associated with the target Merge candidate to generate coded motion information. The encoder signals the coded motion information for the current motion information without other coded information corresponding to remaining part of the current motion information.

A decoder according to the present invention will derive a Merge candidate set consisting of one or more Merge candidates based on the second motion information associated with said one or more neighboring blocks. The decoder will determine a target Merge candidate from the Merge candidate set based on a Merge index signaled in the bitstream. The decoder determines a first part of current motion information based on coded motion information signaled in the bitstream. The remaining part of the current motion information will be inherited from the second motion information associated with the target Merge candidate.

The coded motion information may correspond to Inter prediction direction associated with the target Merge candidate, reference frame index associated with the target Merge candidate, one or more second motion vectors associated with the target Merge candidate, one or more motion vector differences between one or more current motion vectors of the current block and said one or more second motion vectors, or any combination. In one embodiment, the coded motion information corresponds to one or more motion vector differences between one or more current motion vectors of the current block and said one or more second motion vectors. The remaining part of the current motion information inherited from the second motion information associated with the target Merge candidate may correspond to Inter prediction direction, reference frame index and a second motion vector.

DETAILED DESCRIPTION

As mentioned before, among the two motion information coding modes used in the High Efficiency Video Coding (HEVC) standard, AMVP can provide high quality representation of the motion information at the expense of more bits. On the other hand, the Merge mode uses a compact representation that only needs to signal the candidate index to recover the motion information. However, the motion information including Inter prediction direction, reference picture list and motion vector, is restricted to be the same as that of a selected Merge candidate. The present invention discloses a method that utilizes the compact representation of the conventional Merge mode and also refines the motion information quality of the conventional Merge mode. The refinement is achieved by transmitting coded motion information related to partial motion information of the selected merge candidate, such as the motion vector difference (MVD) between the current motion vector and the motion vector of the selected Merge candidate.

It has been observed that, in Inter prediction, most of prediction units (PUs) are coded in the Merge mode, and only a few prediction units are coded in the AMVP mode. In other words, Merge mode is more preferred than the AMVP mode in Inter prediction due to its high coding efficiency. In order to reduce the bits for coding motion information, the present invention utilizes the Merge candidate in Merge mode as a predictor of motion information. This mode is referred as Merge refinement mode or refined Merge mode in this disclosure. In the Merge refinement mode, one Merge index is coded to indicate which Merge candidate is selected as the predictor of motion information. Coded motion information, such as the motion vector difference between the current motion vector and the motion vector of the selected Merge candidate is transmitted. For the Merge refinement mode, Inter prediction direction and reference frame index of the current PU can be set to the same as those of merged motion information. On the other hand, for the AMVP mode, Inter prediction direction and reference frame index of current PU are also transmitted. The Merge refinement mode requires fewer bits than the AMVP mode.

In one embodiment of the Merge refinement mode, the motion vector of the selected Merge candidate as indicated by the Merge index is used as the motion vector predictor for the current PU. The prediction residue, i.e., the motion vector difference is transmitted. The motion vector for the PU can be reconstructed from the sum of motion vector predictor and motion vector difference. Other than the Merge index, only the motion vector difference is coded to refine the merged motion vector. The reconstructed motion vector can be derived as the sum of the motion vector difference transmitted and the motion vector of the selected merge candidate. Inter prediction direction, reference frame index, and motion vector predictor index are directly inherited from those indicated by the Merge index. Therefore, the Merge refinement mode requires fewer transmitted bits than the AMVP mode. On the other hand, the Merge refinement mode results in more accurate motion information representation than the convention Merge mode.

While the coded motion information corresponds to the motion vector difference between the current motion vector and the motion vector of the selected Merge candidate, other motion information can be coded as well. For example, the coded motion information may correspond to Inter prediction direction associated with the selected Merge candidate, reference frame index associated with the selected Merge candidate, one or more second motion vectors associated with the selected Merge candidate, one or more motion vector differences between ore current motion vector(s) of the current block and s motion vector(s) of the selected Merge candidate, or any combination thereof.

In the parsing stage of the Merge refinement mode, only the Merge index is decoded. There is no information regarding whether the Inter prediction direction of the merged motion vector is bi-prediction or uni-prediction. In order to avoid parsing issue, two embodiments are disclosed. In the first embodiment, only one motion vector difference is coded in the Merge refinement mode even if the merged motion vector is bi-prediction. The motion vector difference is used to refine the existing motion vector in the uni-prediction or to refine the pre-defined motion vector in the bi-prediction. In the Merge refinement mode, the motion vector difference should not be zero. Otherwise, the motion information of the PU would have been coded using the Merge mode. An embodiment utilizes this characteristic to further save the coded bins by excluding the zero-valued motion vector difference in a coding table. For example, if the x-component magnitude, MVDxof the motion vector difference is 0, then the y-component magnitude, MVDyof the motion vector difference cannot be zero. To take advantage of this characteristics, (MVDy−1) can be coded instead of MVDyand the range of the y-component magnitude is reduced by 1.

Table 1 shows an example of the syntax structure according to an embodiment of the present invention. As shown in Table 1, when a Merge mode flag, mergeFlag is not asserted, a Merge refinement mode flag, mergeRefinedFlag is included. If the Merge refinement mode flag is asserted, the Merge index, mergeIdx for the selected Merge candidate and the motion vector difference, Mvd are included

In another embodiment, a second flag is used to indicate whether there is another coded motion vector difference in the bitstream. According to this embodiment, two motion vectors in bi-prediction can be refined simultaneously.

FIG. 2illustrates an exemplary flowchart of an encoder incorporating refined Merge mode for motion information coding according to one embodiment of the present invention. The encoder receives current motion information of a current block as shown in step210. The current motion information may be retrieved from memory (e.g., computer memory, buffer (RAM or DRAM) or other media) or from a processor. The encoder then determines second motion information associated with one or more neighboring blocks of the current block in step220. A Merge candidate set consisting of one or more Merge candidates is derived based on the second motion information associated with said one or more neighboring blocks in step230. The encoder selects a target Merge candidate from the Merge candidate set in step240and signals the Merge index corresponding to the target Merge candidate in step250. A first part of the current motion information is predicted based on partial second motion information to generate coded motion information in step260. The coded motion information for the current motion information is signaled without other coded information corresponding to remaining part of the current motion information in step270.

FIG. 3illustrates an exemplary flowchart of a decoder incorporating refined Merge mode for motion information coding according to one embodiment of the present invention. The decoder receives a bitstream including coded data for a current block and neighboring blocks of the current block in step310. The bitstream may be retrieved from memory (e.g., computer memory, buffer (RAM or DRAM) or other media) or from a processor. The decoder also receives second motion information associated with said one or more neighboring blocks of the current block in step320. A Merge candidate set consisting of one or more Merge candidates is derived based on the second motion information associated with said one or more neighboring blocks in step330. The decoder determines a target Merge candidate from the Merge candidate set based on a Merge index signaled in the bitstream in step340. A first part of current motion information is determined based on coded motion information signaled in the bitstream as shown in step350. The coded motion information is generated by predicting the first part of the current motion information based on partial second motion information associated with the target Merge candidate. The coded motion information for the current motion information is signaled without other coded information corresponding to remaining part of the current motion information in step360.

The flowcharts shown above are intended to illustrate an example of motion information coding according to embodiments of the present invention. A person skilled in the art may modify each step, re-arranges the steps, split a step, or combine steps to practice the present invention without departing from the spirit of the present invention.

As shown above, Merge mode motion refinement is used to illustrate an example of explicitly coding partial motion information from a neighboring block and inheriting the remaining motion information. However, the present invention is not restricted to Merge refinement mode, where the motion vector difference (MVD) is explicitly coded and the remaining motion information is inherited from the selected merge candidate. For example, instead of coding all motion information associated with an AMVP mode (i.e., Inter mode), partial motion information may be inherited from a neighboring block.