Opened 4 months ago

#1564 new defect

Intra prediction ref pixel array bounds too small for wide angle

Reported by: swarrington Owned by:
Priority: minor Milestone:
Component: spec Version: VVC D10 vH
Keywords: Cc: ksuehring, bbross, XiangLi, fbossen, jvet@…

Description

It appears that the range of the ref[] array specified in eq.332 contains insufficient samples for generating the prediction plane in the case of some rectangular regions. It seems it may be an equation error in the max bounds specified in eq.332.

Consider:
nCbW = 64
nCbH = 4
nTbW = 32
nTbH = 4
cIdx = 0
IntraSubPartitionsSplitType = ISP_NO_SPLIT
refIdx = 0
Unmapped predModeIntra = 13

  • eq.304 refW = nTbW*2 = 64
  • 8.4.5.2.7: predModeIntra modified to 78 (whRatio= 5-2=3, predModeIntra < 8+2*whRatio, so predModeIntra += 65 ==> 78)
  • eq.332: max index for ref[] is refW+refIdx+x
    • max x index is (Max(1,nTbW/nTbH)*refIdx+1)=Max(1,32/4)*0+1=1
    • ref is populated up to refW+refIdx+x = 64+0+1 = 65
  • Table 24: intraPredAngle = 256 for predModeIntra=13
  • eq.333: predSamples[x][y] runs, reaching max y of 3
  • eq.333: iIdx = (((y+1+refIdx)*intraPredAngle)>>5)+refIdx = (((3+1+0)*256)>>5)+0 = 32
  • eq.336: predSamples uses ref[x+iIdx+i]
    • iIdx=32 and maximums of x=31 and i=3
    • max index of ref used is x+iIdx+i=31+32+3=66

However, per eq.332, ref is only populated up to max index of 65. So it seems this process is accessing an unitialized value of ref[66].

Examining the VTM code for equivalent bounds handling to eq.332 in xPredIntraAng():

const int log2Ratio = floorLog2(width) - floorLog2(height);
const int s = std::max<int>(0, bIsModeVer ? log2Ratio : -log2Ratio);
const int maxIndex = (multiRefIdx << s) + 2;
const Pel val = refMain[refLength + multiRefIdx];
for (int z = 1; z <= maxIndex; z++)
{

refMain[refLength + multiRefIdx + z] = val;

}

Per the spec eq.332, max index when refIdx==0 is refW+refIdx+x=64+0+(...*0) + 1 == 65, while for VTM maxIndex it is refLength+multiRefIdx+z = 64+0+(0<<...) + 2 == 66.

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