Opened 11 months ago

[Multilayer Profiles] Potential Mismatch of VTM22.0 & Specification Related To Derivation Process For Merge Motion Vector Difference

Reported by: Owned by: ksthey minor VTM Multilayer ksuehring, XiangLi, fbossen, jvet@…

Description

In "8.5.2.7 Derivation process for merge motion vector difference", Equation 563:
mMvdL1[ 0 ] = Sign( currPocDiffL0 ) == Sign( currPocDiffL1 ) ? mMvdL0[ 0 ] : −mMvdL0[ 0 ].

The equivalent code is in VTM22.0, InterPrediction.cpp, MergeCtx::getMmvdDeltaMv(), starting from line 2478:
if ((poc1 - currPoc)*(poc0 - currPoc) > 0)
{

deltaMv[1] = deltaMv[0];

}
else
{

deltaMv[1].set(-1 * deltaMv[0].getHor(), -1 * deltaMv[0].getVer());

}

According to specification, definition of Sign(x):
Sign(x) = 1; x > 0
Sign(x) = 0; x == 0
Sign(x) = -1; x < 0

Let's assume:
Result of condition below in specification as r_spec: r_spec = Sign( currPocDiffL0 ) == Sign( currPocDiffL1 )
Result of condition below in VTM22.0 as r_vtm: r_vtm = ((poc1 - currPoc)*(poc0 - currPoc) > 0)

Use combination of currPoc, poc0 & poc1 below as inputs:
1.) currPoc: 1, poc0: 0, poc1: 0

r_spec = 1; r_vtm = 1;

2.) currPoc: 1, poc0: 0, poc1: 2

r_spec = 0; r_vtm = 0;

3.) currPoc: 1, poc0: 2, poc1: 0

r_spec = 0; r_vtm = 0;

4.) currPoc: 1, poc0: 2, poc1: 2

r_spec = 1; r_vtm = 1;

5.) currPoc: 1, poc0: 1, poc1: 0

r_spec = 1; r_vtm = 1;

6.) currPoc: 1, poc0: 0, poc1: 1

r_spec = 1; r_vtm = 1;

7.) currPoc: 1, poc0: 1, poc1: 1

r_spec = 1; r_vtm = 0;

Case 5, 6 & 7 are only possible in multilayer profiles (current picture and reference picture same POC, but different layers).
As shown above, for case 7, results of specification & VTM are different.
So, which is the correct one? specification or VTM?

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