Bi-directional Optical Flow Motion Refinement in Video Coding
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Solution Overview
Problem
Bi-directional Optical Flow (BIO) processes in video coding face challenges with high computational complexity and bandwidth requirements due to extensive division operations and additional reference data access, especially when combined with Overlapped Block Motion Compensation (OBMC).
Innovation Solution
The proposed solution involves using right-shifting operations to eliminate division operations in BIO, reusing small displacements in OBMC regions, modifying processing orders, and applying a boundary-condition-dependent BIO process to reduce computational complexity and bandwidth. Additionally, block-based derivation and simplified gradient calculations are used to reduce computational complexity and memory bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If BIO process uses traditional division operations to derive motion offsets, then measurement precision of motion refinement is improved, but computational complexity increases significantly
Solution Approach 1:
The patent replaces the mechanical division operation with an equivalent mathematical formulation using only multiplication and addition. The motion offset derivation is transformed from a division-based calculation to a multiplication-based calculation, eliminating the need for complex division operations while maintaining the same precision results.
2Manufacturing precision
If BIO process accesses additional reference data for gradient calculation, then manufacturing precision of motion compensation is improved, but bandwidth requirements increase
Solution Approach 1:
The patent performs gradient calculations and motion offset derivations for OBMC regions in advance, before the actual motion compensation process. By pre-computing these values and storing them, the patent eliminates the need to access additional reference data during the main BIO process, thereby reducing bandwidth requirements while maintaining compensation precision.
3Manufacturing precision
If BIO and OBMC are both applied to the same block, then manufacturing precision of video coding is improved, but processing time increases
Solution Approach 1:
The patent merges the BIO and OBMC processes by integrating the motion offset derivation into a single unified calculation. Instead of separately applying BIO then OBMC, the patent combines both operations into one coordinated process that computes motion offsets once and uses them for both BIO refinement and OBMC compensation, thereby reducing processing time while maintaining the precision benefits of both techniques.
Data Source
AI summary
Method and apparatus of using Bi-directional optical flow (BIO) for a true bi-direction predicted block are disclosed. According to one method of the present invention, a division-free BIO process is disclosed, where the x-motion offset and y-motion offset are derived using operations including right-shifting without any division operation. According to another method, a refined predictor is generated for the current block by applying BIO process to the reference blocks, where said applying the BIO process comprises applying a boundary-condition-dependent BIO process conditionally to boundary pixels associated with the reference blocks.