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EPZS搜索过程

 

  EPZS(Enhance Predictive Zonal Search) 增强预测区域搜索,是一种整像素运动估计的搜索算法。

  EPZS采用的是相关性较高的预测方法。这里的相关性较高是指,更多地根据已有的条件,来进行运动向量的预测(如采用相邻块的mv作为当前搜索块的mv进行预测)。

 

Search Set

  在搜索范围内的所有mv,可以被归结为集合$S$,EPZS会从$S$中按照其算法的规律来选择特定的子集$S’$。$S’$可以包含以下几种搜索方式。

 

  1.$S_1$,mvp,(0, 0)

$mv_{prev} ,  mv(0, 0)$

 

 

 

  2.$S_2$,当前块的相邻块mv,包括ABCD四个mv,co-located块mv,做mv_scale,即对于POC的距离进行比例增减。

$mv_a,mv_b,mv_c,mv_d,mv_{co}$

 

 

  3.$S_3$,这种方式基于第0个参考图像的mv,当前参考图像的mv需要由ref0的mv以POC的距离做mv_scale后得到

$mv_{refn} = {mv_{ref0}} \cdot {\frac{POC_{cur}-POC_{n}}{POC_{cur}-POC_{0}}}$

 


 

  4.$S_4$,以mvp或者(0, 0)为中心,呈正方形窗口的mv集合

$mv_{window}$

 

 

  5.$S_5$,由于运动有可能不是匀速地运动,而是以相同加速度来进行,因此可以用前两张编码图像的mv计算出当前mv

$mv_{cur} = mv_{t-1} + (mv_{t-1} - mv_{t-2})$

 

 

 

  6.$S_6$,co-located块的相邻mv,需做mv_scale

$mv_{cola},mv_{colb},mv_{colc},mv_{cold}$

 

 

  7.$S_7$,Memory Prediction即采用同一块位置中,上一个块类型(Block type - 1)做预测得到的mv,采用其相邻mv,做mv_scale

$mv_{preva},mv_{prevb},mv_{prevc},mv_{prevd}$

 

 

  8.$S_8$,如果采用的是HME(Hierarchical Motion Estimation)分层运动估计的编码方式,那么应该选择该次HME运动估计的参考图像,从参考图像中选择co-located块,以及co-located块周边的块(带col块共9个块)的mv作为候选mv

$mv_{hme\_co},mv_{hme\_surround\_of\_co}$

 

 

 

        9.$S_9$,上层块mv,做mv_scale

$mv_{4 \times 4} = mv_{8 \times 4},mv_{8 \times 4} = mv{8 \times 8},…$

 

 

  从如上的mv集合明显看出,基本上,如果当前mv是由参考图像已有mv预测出来的情况,都需要进行mv_scale,以此提高mv预测的准确性。

 

  在预测完所有mv的可能位置后,得到mv集合$S$,由于此处进行的是整像素预测,所以需要对mv求整。接下来需要在$S$中求出mv_cost最小的mv,当然,mv需要在search range内。

 

Termination

  Termination用于判断当前mv预测是否满足停止条件(mv_cost < stopCriterion),满足则代表当前mv预测已经足够准确,可以退出。

  1.在求出第一个子集$S_1$的mv_cost后,进行第一次Termination,记为$T_1$。$T_1$是由当前块类型的像素点个数(如16x16像素点个数为256)与lambda联合组成。JM18.6中$T_1$,即stopCriterion的值为如下形式:

$stopCriterion = (N_{pixel} \times \frac{3}{4}) \times 2^{LAMBDA\_ACCURACY\_BITS} + LAMBDA\_DIST $

 

  2.如果第一个停止条件不能满足,则接下来需要对所有的mv集合进行预测,从中选出mv_cost最小的mv。在得到最小mv后,进行第二次Termination,记为$T_2$。$T_2$由3块类型的相邻块中最小SAD,用$stopCriterion_{T_1}$修正计算得来。

首先定义上界与下界:

$Upper = (N_{pixel} \times 3) \times 2^{LAMBDA\_ACCURACY\_BITS} + LAMBDA\_DIST $

$Lower = (N_{pixel} \times \frac{1}{4}) \times 2^{LAMBDA\_ACCURACY\_BITS} + LAMBDA\_DIST $

上下界修正:

$tmpStopCriterion = Mid( Min(SAD_A,SAD_B,SAD_C), Upper, Lower)$

StopCriterion修正:

  $stopCriterion = \frac{(Max(tmpStopCriterion, stopCriterion) \times 8 + stopCriterion)}{8} + LAMBDA\_DIST $

 

        3.如果第二个停止条件仍然不满足,接下来进行最后的小区域搜索,搜索方式有菱形,正方形,大菱形等。如果搜索得到最佳mv为菱形(或正方形)的中心点,则停止搜索,得到最终的最佳整数mv。可以参考http://www.cnblogs.com/TaigaCon/p/3788984.html

 

        相对于UMHS,EPZS更多地考虑了运动的相关性,基本上采用的都是用已得到的mv预测当前mv,这种方式在规律运动图像的预测中会比UMHS更有效率。但是UMHS的预测更加考虑周全,对搜索范围进行了更细密的搜索,一旦碰上无规律运动图像,UMHS预测可以得到比EPZS更准确的预测效果。因此,根据图像性质来选择不同的搜索算法将能更好地提升编码质量。

 

jm18.6:

/*!************************************************************************ \brief*    FAST Motion Estimation using EPZS*    AMT/HYC************************************************************************/distblk                                            //  ==> minimum motion cost after searchEPZS_motion_estimation (Macroblock * currMB,     // <--  current Macroblock                          MotionVector * pred_mv,  // <--  motion vector predictor in sub-pel units                          MEBlock * mv_block,      // <--  motion vector information                          distblk min_mcost,       // <--  minimum motion cost (cost for center or huge value)                          int lambda_factor        // <--  lagrangian parameter for determining motion cost                          ){  Slice *currSlice = currMB->p_Slice;  VideoParameters *p_Vid = currMB->p_Vid;  InputParameters *p_Inp = currMB->p_Inp;  EPZSParameters *p_EPZS = currSlice->p_EPZS;  PicMotionParams **motion = p_Vid->enc_picture->mv_info;  int blocktype = mv_block->blocktype;  int list = mv_block->list;  int cur_list = list + currMB->list_offset;  short ref = mv_block->ref_idx;  MotionVector *mv = &mv_block->mv[list];  SearchWindow *searchRange = &mv_block->searchRange;  int mapCenter_x = searchRange->max_x - mv->mv_x;  int mapCenter_y = searchRange->max_y - mv->mv_y;  StorablePicture *ref_picture = currSlice->listX[cur_list][ref];  distblk lambda_dist = weighted_cost(lambda_factor, 2);  distblk stopCriterion = p_EPZS->medthres[blocktype] + lambda_dist;  distblk *prevSad = &p_EPZS->distortion[cur_list][blocktype - 1][mv_block->pos_x2];  MotionVector *p_motion = NULL; //only work for EPZSSpatialMem  EPZSStructure *searchPatternF = p_EPZS->searchPattern;  uint16 **EPZSMap = &p_EPZS->EPZSMap[mapCenter_y];  uint16 *EPZSPoint = &p_EPZS->EPZSMap[searchRange->max_y][searchRange->max_x];  MotionVector center = pad_MVs (*mv, mv_block);  MotionVector pred = pad_MVs (*pred_mv, mv_block);  MotionVector tmp = *mv, cand = center;  ++p_EPZS->BlkCount;  if (p_EPZS->BlkCount == 0)    ++p_EPZS->BlkCount;  if (p_Inp->EPZSSpatialMem)  {#if EPZSREF    p_motion = &p_EPZS->p_motion[cur_list][ref][blocktype - 1][mv_block->block_y][mv_block->pos_x2];#else    p_motion = &p_EPZS->p_motion[cur_list][blocktype - 1][mv_block->block_y][mv_block->pos_x2];#endif  }  // Clear EPZSMap  // memset(EPZSMap[0],FALSE,searcharray*searcharray);  // Check median candidate;  //p_EPZS->EPZSMap[0][0] = p_EPZS->BlkCount;  *EPZSPoint = p_EPZS->BlkCount;  //--- initialize motion cost (cost for motion vector) and check ---  //(0,0)作为mv  min_mcost = mv_cost (p_Vid, lambda_factor, &cand, &pred);  //--- add residual cost to motion cost ---  min_mcost += mv_block->computePredFPel (ref_picture, mv_block, DISTBLK_MAX - min_mcost, &cand);  // Additional threshold for ref>0  if ((ref > 0 && currSlice->structure == FRAME) && (*prevSad < distblkmin (p_EPZS->medthres[blocktype] + lambda_dist, min_mcost)))   {//满足返回条件#if EPZSREF    if (p_Inp->EPZSSpatialMem)#else     if (p_Inp->EPZSSpatialMem && ref == 0)#endif     {      *p_motion = tmp;    }    return min_mcost;  }  //! If p_EPZS->medthres satisfied, then terminate, otherwise generate Predictors  //! Condition could be strengthened by consideration distortion of adjacent partitions.  if (min_mcost > stopCriterion)  {    SPoint *p_EPZS_point = p_EPZS->predictor->point;    Boolean checkMedian = FALSE;    distblk second_mcost = DISTBLK_MAX;    distblk mcost;    int prednum = 5;    int conditionEPZS;    MotionVector tmp2 = {0, 0}, tmv;    int pos;    short invalid_refs = 0;    stopCriterion = EPZSDetermineStopCriterion (p_EPZS, prevSad, mv_block, lambda_dist);    if (min_mcost < (stopCriterion >> 1))    {#if EPZSREF      if (p_Inp->EPZSSpatialMem)#else       if (p_Inp->EPZSSpatialMem && ref == 0)#endif       {        *p_motion = tmp;      }      return min_mcost;    }    //! Add Spatial Predictors in predictor list.    //! Scheme adds zero, left, top-left, top, top-right. Note that top-left adds very little    //! in terms of performance and could be removed with little penalty if any.    //当前块相邻mv,做mv_scale    invalid_refs = EPZS_spatial_predictors (p_EPZS, mv_block,       list, currMB->list_offset, ref, motion);    //当前类型的block_type(8x4)的上一个block_type(8x8),相同位置留下来的mv,采用其相邻mv    /*     *           *   +--------+----+----+                      *   |        |    B    |                      *   |        |____A____|                      *   |        |         |                      *   |        |         |                      *   |        +----+----+                      *   |                  |                      *   |                  |                      *   |                  |                      *   |                  |                      *   +------------------+                      *         *   B : 8x4    A 8x8     *     macroblock                           *                                          *memory_mv_b = surrounding_of_mv_a     *采用mv_a的相邻mv     *     */    if (p_Inp->EPZSSpatialMem)      EPZS_spatial_memory_predictors (p_EPZS, mv_block, cur_list, &prednum, ref_picture->size_x >> 2);       //if (p_Inp->HMEEnable == 1 && p_Inp->EPZSUseHMEPredictors == 1 && blocktype == 4)    //if (p_Inp->HMEEnable == 1 && p_Inp->EPZSUseHMEPredictors == 1 && (currSlice->slice_type == P_SLICE || currSlice->slice_type == SP_SLICE || p_Inp->EnableReorderBslice) )    //如果采用HME(Hierarchical Motion Estimate),即B帧的参考图像选择方式有所不同    //选择此时的分层参考图像(Hierarchical ref pic)对应块的相邻mv    if (p_Inp->HMEEnable == 1 && p_Inp->EPZSUseHMEPredictors == 1)      EPZS_hierarchical_predictors (p_EPZS, mv_block, &prednum, ref_picture, currSlice);#if (MVC_EXTENSION_ENABLE)    if ( p_Inp->EPZSTemporal[currSlice->view_id] && blocktype < 5 ) #else    // Temporal predictors    //colocate块的相邻块mv    if (p_Inp->EPZSTemporal && blocktype < 5)#endif    {      EPZS_temporal_predictors (currMB, ref_picture, p_EPZS, mv_block, &prednum, stopCriterion, min_mcost);    }    //! Window Size Based Predictors    //! Basically replaces a Hierarchical ME concept and helps escaping local minima, or    //! determining large motion variations.    //! Following predictors can be adjusted further (i.e. removed, conditioned etc)    //! based on distortion, correlation of adjacent MVs, complexity etc. These predictors    //! and their conditioning could also be moved after all other predictors have been    //! tested. Adaptation could also be based on type of material and coding mode (i.e.    //! field/frame coding,MBAFF etc considering the higher dependency with opposite parity field    //conditionEPZS = ((min_mcost > stopCriterion)    // && (p_Inp->EPZSFixed > 1 || (p_Inp->EPZSFixed && currSlice->slice_type == P_SLICE)));    //conditionEPZS = ((ref == 0) && (blocktype < 5) && (min_mcost > stopCriterion)    //&& (p_Inp->EPZSFixed > 1 || (p_Inp->EPZSFixed && currSlice->slice_type == P_SLICE)));    //conditionEPZS = ((min_mcost > stopCriterion) && ((ref < 2 && blocktype < 4)    conditionEPZS = (p_Inp->EPZSFixed == 3 && (currMB->mb_x == 0 || currMB->mb_y == 0))      || ((min_mcost > 3 * stopCriterion) && ((ref < 2 && blocktype < 4) || (ref < 1 && blocktype == 4)      || ((currSlice->structure != FRAME || currMB->list_offset)      && ref < 3))      && (p_Inp->EPZSFixed > 1 || (p_Inp->EPZSFixed && currSlice->slice_type == P_SLICE)));    //正方形窗口mv    if (conditionEPZS)      EPZSWindowPredictors (mv, p_EPZS->predictor, &prednum,       (p_Inp->EPZSAggressiveWindow != 0) || ((blocktype < 5) && (invalid_refs > 2) && (ref < 1 + (currSlice->structure != FRAME || currMB->list_offset)))      ? p_EPZS->window_predictor_ext : p_EPZS->window_predictor);    //! Blocktype/Reference dependent predictors.    //! Since already mvs for other blocktypes/references have been computed, we can reuse    //! them in order to easier determine the optimal point. Use of predictors could depend    //! on cost,    //conditionEPZS = (ref == 0 || (ref > 0 && min_mcost > stopCriterion) || currSlice->structure != FRAME || currMB->list_offset);    conditionEPZS = (ref == 0 || (ref > 0 && min_mcost > 2 * stopCriterion));    //上层宏块mv,做mv_scale    if (conditionEPZS && currMB->mbAddrX != 0 && p_Inp->EPZSBlockType)      EPZSBlockTypePredictorsMB (currSlice, mv_block, p_EPZS_point, &prednum);    //! Check all predictors        //循环内,对所有的cand mv(S1,S2,S3,S4)做cost.取出最佳mv    for (pos = 0; pos < prednum; ++pos)    {      tmv = p_EPZS_point[pos].motion;      set_integer_mv(&tmv);      //if (((iabs (tmv.mv_x - mv->mv_x) > searchRange->max_x || iabs (tmv.mv_y - mv->mv_y) > searchRange->max_y)) && (tmv.mv_x || tmv.mv_y))      if ((iabs (tmv.mv_x - mv->mv_x) - searchRange->max_x <= 0) && (iabs (tmv.mv_y - mv->mv_y) - searchRange->max_y <= 0))      {        EPZSPoint = &EPZSMap[tmv.mv_y][mapCenter_x + tmv.mv_x];        if (*EPZSPoint != p_EPZS->BlkCount)        {          *EPZSPoint = p_EPZS->BlkCount;          cand = pad_MVs (tmv, mv_block);          //--- set motion cost (cost for motion vector) and check ---          mcost = mv_cost (p_Vid, lambda_factor, &cand, &pred);          if (mcost < second_mcost)          {            mcost += mv_block->computePredFPel (ref_picture, mv_block, second_mcost - mcost, &cand);            //--- check if motion cost is less than minimum cost ---            if (mcost < min_mcost)            {              tmp2 = tmp;              tmp = tmv;              second_mcost = min_mcost;              min_mcost = mcost;              checkMedian = TRUE;            }            //else if (mcost < second_mcost && (tmp.mv_x != tmv.mv_x || tmp.mv_y != tmv.mv_y))            else if (mcost < second_mcost)            {              tmp2 = tmv;              second_mcost = mcost;              checkMedian = TRUE;            }          }        }      }    }    //! Refine using EPZS pattern if needed    //! Note that we are using a conservative threshold method. Threshold    //! could be tested after checking only a certain number of predictors    //! instead of the full set. Code could be easily modified for this task.    if (min_mcost > stopCriterion)    {      const int mv_range = 10;      int patternStop = 0, pointNumber = 0, checkPts, nextLast = 0;      int totalCheckPts = 0, motionDirection = 0;      //! Adapt pattern based on different conditions.      if (p_Inp->EPZSPattern != 0)      {        if ((min_mcost < stopCriterion + ((3 * p_EPZS->medthres[blocktype]) >> 1)))        {          if ((tmp.mv_x == 0 && tmp.mv_y == 0)             || (iabs (tmp.mv_x - mv->mv_x) < (mv_range) && iabs (tmp.mv_y - mv->mv_y) < (mv_range)))            searchPatternF = p_Vid->sdiamond;          else            searchPatternF = p_Vid->square;        }        else if (blocktype > 4 || (ref > 0 && blocktype != 1))          searchPatternF = p_Vid->square;        else          searchPatternF = p_EPZS->searchPattern;      }      //! center on best predictor      center = tmp;      for (;;)      {        totalCheckPts = searchPatternF->searchPoints;        do        {//循环对得到的最佳mv是做小范围搜索:square,dimond,etc.          checkPts = totalCheckPts;          do          {//循环对小范围进行各个像素点的搜索            tmv = add_MVs (center, &(searchPatternF->point[pointNumber].motion));            if (((iabs (tmv.mv_x - mv->mv_x) - searchRange->max_x) <= 0) && ((iabs (tmv.mv_y - mv->mv_y) - searchRange->max_y) <= 0))            {              //EPZSMap用于记录当前点是否被搜索过,一旦搜索过,则不再搜索,这样的话就不会跟//! Check all predictors重复了              EPZSPoint = &EPZSMap[tmv.mv_y][mapCenter_x + tmv.mv_x];              if (*EPZSPoint != p_EPZS->BlkCount)              {                *EPZSPoint = p_EPZS->BlkCount;                cand = pad_MVs (tmv, mv_block);                mcost = mv_cost (p_Vid, lambda_factor, &cand, &pred);                if (mcost < min_mcost)                {                  mcost += mv_block->computePredFPel (ref_picture, mv_block, min_mcost - mcost, &cand);                  if (mcost < min_mcost)                  {                    tmp = tmv;                    min_mcost = mcost;                    motionDirection = pointNumber;                  }                }              }            }            ++pointNumber;            if (pointNumber >= searchPatternF->searchPoints)              pointNumber -= searchPatternF->searchPoints;            checkPts--;          }          while (checkPts > 0);                    //对得到的最佳mv,如果是center(第一次的center是mvp),则停止当前循环          if (nextLast || ((tmp.mv_x == center.mv_x) && (tmp.mv_y == center.mv_y)))          {            patternStop = searchPatternF->stopSearch;            searchPatternF = searchPatternF->nextpattern;            totalCheckPts = searchPatternF->searchPoints;            nextLast = searchPatternF->nextLast;            motionDirection = 0;            pointNumber = 0;          }          else          {            totalCheckPts = searchPatternF->point[motionDirection].next_points;            pointNumber = searchPatternF->point[motionDirection].start_nmbr;            center = tmp;          }        }        while (patternStop != 1);        if ((ref > 0) && (currSlice->structure == FRAME)           && ((4 * *prevSad < min_mcost) || ((3 * *prevSad < min_mcost) && (*prevSad <= stopCriterion))))        {          *mv = tmp;#if EPZSREF          if (p_Inp->EPZSSpatialMem)#else            if (p_Inp->EPZSSpatialMem && ref == 0)#endif            {            *p_motion = tmp;          }          return min_mcost;        }        //! Check Second best predictor with EPZS pattern        conditionEPZS = (checkMedian == TRUE)          && ((currSlice->slice_type == P_SLICE) || (blocktype < 5))          && (min_mcost > stopCriterion) && (p_Inp->EPZSDual > 0);        if (!conditionEPZS)          break;         //如果上方条件有多于0个不满足的,则退出循环        //退出条件1:  EPZSDual = 0 ,即循环只需要执行一次        //        2:  cost足够小了        //        3:  blocktypc = 5,6,7        //        4:  不为P_SLICE        //        5:  checkMedian = false,即由于EPZSDual,而执行完了第二次        pointNumber = 0;        patternStop = 0;        motionDirection = 0;        nextLast = 0;        if ((tmp.mv_x == 0 && tmp.mv_y == 0) || (tmp.mv_x == mv->mv_x && tmp.mv_y == mv->mv_y))        {          if (iabs (tmp.mv_x - mv->mv_x) < (mv_range) && iabs (tmp.mv_y - mv->mv_y) < (mv_range))            searchPatternF = p_Vid->sdiamond;          else            searchPatternF = p_Vid->square;        }        else          searchPatternF = p_EPZS->searchPatternD;        //! Second best. Note that following code is identical as for best predictor.        //由于EPZSDual > 0,开启第二次循环,以原点(当前mv_block所在坐标)为中心,开始搜索        center = tmp2;        checkMedian = FALSE;      }    }  }  if ((ref == 0) || (*prevSad > min_mcost))    *prevSad = min_mcost;#if EPZSREF  if (p_Inp->EPZSSpatialMem)#else    if (p_Inp->EPZSSpatialMem && ref == 0)#endif    {    *p_motion = tmp;    //printf("value %d %d %d %d\n", p_motion->mv_x, p_motion->mv_y, p_motion[cur_list][ref][0][0][0].mv_x, p_motion[list + list_offset][ref][0][0][0].mv_y);    //printf("xxxxx %d %d %d %d\n", p_motion->mv_x, p_motion->mv_y, p_motion[cur_list][ref][blocktype - 1][mv_block->block_y][pic_pix_x2].mv_x, p_motion[cur_list][ref][blocktype - 1][mv_block->block_y][pic_pix_x2].mv_y);  }  *mv = tmp;  return min_mcost;}
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EPZS搜索过程