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。
- 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 $
- 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:
$\begin{align*} Upper &= (N_{pixel} \times 3) \times 2^{LAMBDA\_ACCURACY\_BITS} + LAMBDA\_DIST \\ Lower &= (N_{pixel} \times \frac{1}{4}) \times 2^{LAMBDA\_ACCURACY\_BITS} + LAMBDA\_DIST \end{align*}$
- 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 $
- 首先定义上界与下界:
- 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 search
EPZS_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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