JVET-L0591 CE9-related: A simplified design of bi-directional optical flow (BIO) [X. Xiu, Y. He, Y. Ye (InterDigital), C.-Y. Lai, Y.-C. Su, T.-D. Chuang, C.-Y. Chen, Y.-W. Huang, S.-M. Lei (MediaTek)] [late]
This was discussed Saturday 2020 (GJS).
In this contribution, methods are proposed to make the existing design of bi-directional optical flow (BIO) tool in BMS-2.1 more implementation friendly. To control the size of the multipliers in BIO, one bit-width control method is applied to ensure BIO can be implemented with at most 15-bit multiplier and the intermediate values are within the 32-bit range. Further, three solutions are provided to reduce the BIO worst-case computational complexity by reducing or avoiding the interpolation of the prediction samples in the extended area of one BIO CU. Additionally, in all three solutions, the BIO is disabled if either of the following is true: 1) the CU has height equal to 4 or the CUs is in size of 4×8, or 2) the CU is coded with sub-block modes. Compared to VTM-2.1 anchor, the performances of the proposed BIO simplifications are summarized as follows:
The existing BIO in the BMS-2.1:
- {Y, U, V} BD-rate saving {1.41%, 0.58%, 0.40%}, EncT=103%, DecT=119%
The bit-width control method + complexity reduction solution one:
- {Y, U, V} BD-rate saving {1.29%, 0.53%, 0.35%}, EncT=100%, DecT=105%
The bit-width control method + complexity reduction solution two:
- {Y, U, V} BD-rate saving {1.24%, 0.48%, 0.37%}, EncT=100%, DecT=102%
The bit-width control method + complexity reduction solution three:
- {Y, U, V} BD-rate saving {1.17%, 0.47%, 0.32%}, EncT=101%, DecT=103%
Based on the simulation results, it is asserted that with the proposed simplification methods, BIO becomes an implementable tool with desirable performance vs. complexity trade-off.
This was described as being mostly similar to JVET-L0256. The contributor said this could be studied later in a CE while adopting JVET-L0256.