Opened 14 months ago
#1619 new defect
[Multilayer Profiles] Potential Mismatch of VTM22.0 & Specification Related To Derivation Process For Merge Motion Vector Difference
Reported by: | ksthey | Owned by: | |
---|---|---|---|
Priority: | minor | Milestone: | |
Component: | VTM | Version: | |
Keywords: | Multilayer | Cc: | 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)
{
}
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?