H.266/VVC中去方塊濾波改進
VVC中，去方塊濾波和HEVC過程類似，主要有以下幾個方面改進：
-
濾波強度依賴於重建像素的平均亮度級。
-
tc表有了擴展。
-
亮度強濾波有了改進。
-
增加了色度強濾波。
依賴重建像素平均亮度級的濾波強度
在HEVC中濾波強度由beta和tc決定，而beta和tc是由平均量化參數QP_L生成。在VVC中爲QP_L增加一個偏移值，偏移量由重建像素的平均亮度級LL決定。
QP_L計算中比HEVC增加了一個偏移值如下：
qpOffset是偏移值，由LL決定。
下面是通過平均亮度級LL計算偏移值的方法：
void LoopFilter::deriveLADFShift( const Pel* src, const int stride, int& shift, const DeblockEdgeDir edgeDir, const SPS sps )
{
uint32_t lumaLevel = 0;
shift = sps.getLadfQpOffset(0);
//!<計算平均亮度級
if (edgeDir == EDGE_VER)
{
lumaLevel = (src[0] + src[3*stride] + src[-1] + src[3*stride - 1]) >> 2;
}
else // (edgeDir == EDGE_HOR)
{
lumaLevel = (src[0] + src[3] + src[-stride] + src[-stride + 3]) >> 2;
}
for ( int k = 1; k < sps.getLadfNumIntervals(); k++ )
{
const int th = sps.getLadfIntervalLowerBound( k );
if ( lumaLevel > th )
{
shift = sps.getLadfQpOffset( k );
}
else
{
break;
}
}
}
#endif
#if LUMA_ADAPTIVE_DEBLOCKING_FILTER_QP_OFFSET
if ( sps.getLadfEnabled() )
{
int iShift = 0;
deriveLADFShift( piTmpSrc + iSrcStep * (iIdx*pelsInPart), iStride, iShift, edgeDir, sps );
iQP += iShift; //!<根據重建像素的平均亮度級LL增加偏移值
}
#endif
tc表擴展
在VVC中最大QP變爲63，與之對應tc表要進行相應擴展。
tC = [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 5, 5, 6, 6, 7, 8, 9,10,11,13,14,16, 18,20,22,25,28,31,35,39,44,50,56,63,70,79,88,99 ]
const uint8_t LoopFilter::sm_tcTable[MAX_QP + 1 + DEFAULT_INTRA_TC_OFFSET] =
{
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,5,5,6,6,7,8,9,10,11,13,14,16,18,20,22,25, 28, 31, 35, 39, 44, 50, 56, 63, 70,79, 88, 99
};
const uint8_t LoopFilter::sm_betaTable[MAX_QP + 1] =
{
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,6,7,8,9,10,11,12,13,14,15,16,17,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50,52,54,56,58,60,62,64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88
};
亮度強濾波
當邊界任一側像素屬於一個“大塊”時使用雙線性濾波器。像素屬於大塊的定義是邊界長度大於等於32。
雙線性濾波如下所示：
相關參數由下表給出：
當下面3個條件都滿足時可以使用雙線性濾波：
條件1：邊界任一側像素屬於一個“大塊”。
條件2：
條件3：
是否使用雙線性濾波判斷條件實現如下：
inline bool LoopFilter::xUseStrongFiltering( Pel* piSrc, const int iOffset, const int d, const int beta, const int tc, bool sidePisLarge, bool sideQisLarge, int maxFilterLengthP, int maxFilterLengthQ ) const
{
const Pel m4 = piSrc[ 0 ];
const Pel m3 = piSrc[-iOffset ];
const Pel m7 = piSrc[ iOffset * 3];
const Pel m0 = piSrc[-iOffset * 4];
int sp3 = abs(m0 - m3);
int sq3 = abs(m7 - m4);
const int d_strong = sp3 + sq3;
if (sidePisLarge || sideQisLarge)
{
Pel mP4;
Pel m11;
if (maxFilterLengthP == 5)
{
mP4 = piSrc[-iOffset * 6];
}
else
{
mP4 = piSrc[-iOffset * 8];
}
if (maxFilterLengthQ == 5)
{
m11 = piSrc[iOffset * 5];
}
else
{
m11 = piSrc[iOffset * 7];
}
if (sidePisLarge)
{
sp3 = (sp3 + abs(m0 - mP4) + 1) >> 1;
}
if (sideQisLarge)
{
sq3 = (sq3 + abs(m11 - m7) + 1) >> 1;
}
return ((sp3 + sq3) < (beta*3 >> 5)) && (d < (beta >> 2)) && (abs(m3 - m4) < ((tc * 5 + 1) >> 1));
}
else
return ( ( d_strong < ( beta >> 3 ) ) && ( d < ( beta >> 2 ) ) && ( abs( m3 - m4 ) < ( ( tc * 5 + 1 ) >> 1 ) ) );
}
雙線性濾波代碼實現如下：
/**
- Deblocking for the luminance component with strong or weak filter
.
\param piSrc pointer to picture data
\param iOffset offset value for picture data
\param tc tc value
\param sw decision strong/weak filter
\param bPartPNoFilter indicator to disable filtering on partP
\param bPartQNoFilter indicator to disable filtering on partQ
\param iThrCut threshold value for weak filter decision
\param bFilterSecondP decision weak filter/no filter for partP
\param bFilterSecondQ decision weak filter/no filter for partQ
\param bitDepthLuma luma bit depth
*/
inline void LoopFilter::xBilinearFilter(Pel* srcP, Pel* srcQ, int offset, int refMiddle, int refP, int refQ, int numberPSide, int numberQSide, const int* dbCoeffsP, const int* dbCoeffsQ, int tc) const
{
int src;
const char tc7[7] = { 6, 5, 4, 3, 2, 1, 1};
const char tc3[3] = { 6, 4, 2 };
const char *tcP = (numberPSide == 3) ? tc3 : tc7;
const char *tcQ = (numberQSide == 3) ? tc3 : tc7;
for (int pos = 0; pos < numberPSide; pos++)
{
src = srcP[-offset*pos];
int cvalue = (tc * tcP[pos]) >>1;
srcP[-offset * pos] = Clip3(src - cvalue, src + cvalue, ((refMiddle*dbCoeffsP[pos] + refP * (64 - dbCoeffsP[pos]) + 32) >> 6));
}
for (int pos = 0; pos < numberQSide; pos++)
{
src = srcQ[offset*pos];
int cvalue = (tc * tcQ[pos]) >> 1;
srcQ[offset*pos] = Clip3(src - cvalue, src + cvalue, ((refMiddle*dbCoeffsQ[pos] + refQ * (64 - dbCoeffsQ[pos]) + 32) >> 6));
}
}
色度強濾波
當色度邊界長度大於等於8，且滿足下面3個條件時，色度邊界使用強濾波。（1）邊界強度和大塊條件。VVC中色度邊界強度計算如下表，當高優先級條件滿足時不再檢查低優先級條件。當BS=2或BS=1且邊界屬於一個大塊時才進行濾波。條件（2）（3）分別和HEVC中濾波開關決策和強濾波決策一樣。
色度強濾波操作如下：
相關代碼實現如下：
inline void LoopFilter::xPelFilterChroma( Pel* piSrc, const int iOffset, const int tc, const bool sw, const bool bPartPNoFilter, const bool bPartQNoFilter, const ClpRng& clpRng, const bool largeBoundary ) const
{
int delta;
const Pel m0 = piSrc[-iOffset * 4];
const Pel m1 = piSrc[-iOffset * 3];
const Pel m2 = piSrc[-iOffset * 2];
const Pel m3 = piSrc[-iOffset];
const Pel m4 = piSrc[0];
const Pel m5 = piSrc[iOffset];
const Pel m6 = piSrc[iOffset * 2];
const Pel m7 = piSrc[iOffset * 3];
if (sw)
{//!<色度強濾波
piSrc[-iOffset * 3] = Clip3(m1 - tc, m1 + tc, ((3 * m0 + 2 * m1 + m2 + m3 + m4 + 4) >> 3)); // p2
piSrc[-iOffset * 2] = Clip3(m2 - tc, m2 + tc, ((2 * m0 + m1 + 2 * m2 + m3 + m4 + m5 + 4) >> 3)); // p1
piSrc[-iOffset * 1] = Clip3(m3 - tc, m3 + tc, ((m0 + m1 + m2 + 2 * m3 + m4 + m5 + m6 + 4) >> 3)); // p0
piSrc[0] = Clip3(m4 - tc, m4 + tc, ((m1 + m2 + m3 + 2 * m4 + m5 + m6 + m7 + 4) >> 3)); // q0
piSrc[iOffset * 1] = Clip3(m5 - tc, m5 + tc, ((m2 + m3 + m4 + 2 * m5 + m6 + 2 * m7 + 4) >> 3)); // q1
piSrc[iOffset * 2] = Clip3(m6 - tc, m6 + tc, ((m3 + m4 + m5 + 2 * m6 + 3 * m7 + 4) >> 3)); // q2
}
else
{
delta = Clip3(-tc, tc, ((((m4 - m3) << 2) + m2 - m5 + 4) >> 3));
piSrc[-iOffset] = ClipPel(m3 + delta, clpRng);
piSrc[0] = ClipPel(m4 - delta, clpRng);
}
if( bPartPNoFilter )
{
if (largeBoundary)
{
piSrc[-iOffset * 3] = m1; // p2
piSrc[-iOffset * 2] = m2; // p1
}
piSrc[-iOffset] = m3;
}
if( bPartQNoFilter )
{
if (largeBoundary)
{
piSrc[iOffset * 1] = m5; // q1
piSrc[iOffset * 2] = m6; // q2
}
piSrc[ 0 ] = m4;
}
}
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