在H.264之前的标准中，比如H.263，其比特流中的数据是按照一个宏块接一个宏块的方式排列的，一旦发生丢包，很多相邻宏块信息都会丢失，很难进行错误隐藏处理。在H.264中加入了一项新特性：把宏块在比特流中的数据按照一定的映射规则进行排列，而不一定按照原本的光栅扫描顺序排列，这种方称为灵活的宏块重拍FMO(Flexible Macroblock Ordering)。
FMO是基于组的方式将宏块集合起来，把一帧内的宏块划分到不同的条带组（slice group）。当然在最后也需要对条带组按光栅扫描顺序划分条带（slice）。在h.264中，当slice group被设为大于1时才使用FMO。在代码端看来，FMO时，主要是对映射表（数组）的设置，如下第一个表：
对一帧进行FMO：
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
| 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 |
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
| 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 |
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
| 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 |
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
| 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 |
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
得到slice group有三个：
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 | 0 | 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 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| 2 | 2 | 2 | 2 |
然后再对每个slice group按照光栅扫描顺序进行排序，分割出slice，下面为分割slice group 0（假设slice长度为16）
slice 1 in slice group 0:
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
slice 2 in slice group 0:
| 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
FMO在h.264中有7种（FMO0 ~ FMO6）
FMO0
FMO0，交织映射。交织映射是指不同的slice group交替出现。
如下设置了slice group 0 = 10，slice group 1 = 15，slice group 2 = 18
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 0 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 0 | 0 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 | 1 |
JM code如下：
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/*! ************************************************************************ * rief * Generate interleaved slice group map type MapUnit map (type 0) * * param img * Image Parameter to be used for map generation * param pps * Picture Parameter set to be used for map generation ************************************************************************ */static void FmoGenerateType0MapUnitMap (ImageParameters * img, pic_parameter_set_rbsp_t * pps ){ unsigned iGroup, j; unsigned i = 0; do { for( iGroup = 0; (iGroup <= pps->num_slice_groups_minus1) && (i < PicSizeInMapUnits); i += pps->run_length_minus1[iGroup++] + 1) { for( j = 0; j <= pps->run_length_minus1[ iGroup ] && i + j < PicSizeInMapUnits; j++ ) MapUnitToSliceGroupMap[i+j] = iGroup; } } while( i < PicSizeInMapUnits );} |
FMO1
FMO1，分散映射。每个宏块相邻的宏块都不在同一组。x方向上的宏块按照slice group序号递增分配，y方向第一个宏块按照0，slice group numbers/2交替分配。
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
| 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 |
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
| 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 |
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
| 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 |
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
| 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 |
| 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 | 2 | 0 | 1 |
JM code 如下：
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/*! ************************************************************************ * rief * Generate dispersed slice group map type MapUnit map (type 1) * * param img * Image Parameter to be used for map generation * param pps * Picture Parameter set to be used for map generation ************************************************************************ */static void FmoGenerateType1MapUnitMap (ImageParameters * img, pic_parameter_set_rbsp_t * pps ){ unsigned i; for( i = 0; i < PicSizeInMapUnits; i++ ) { MapUnitToSliceGroupMap[i] = ((i%img->PicWidthInMbs)+(((i/img->PicWidthInMbs)*(pps->num_slice_groups_minus1+1))/2)) %(pps->num_slice_groups_minus1+1); }} |
FMO2
FMO2，前后景映射。以整个帧作为背景，该模式可以通过（左上角坐标，右下角坐标）的方式指定前景，每个前景都为一个slice group，最先指定的前景可以覆盖后来指定的前景。
假设有
slice group 0（3,4）（8,8）
slice group 1（1,2）（6,6）
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 | 2 |
| 2 | 1 | 1 | 1 | 1 | 1 | 1 | 2 | 2 | 2 | 2 |
| 2 | 1 | 1 | 1 | 1 | 1 | 1 | 2 | 2 | 2 | 2 |
| 2 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 2 |
| 2 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 2 |
| 2 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 2 |
| 2 | 2 | 2 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 2 |
| 2 | 2 | 2 | 0 | 0 | 0 | 0 | 0 | 0 | 2 | 2 |
JM code如下：
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/*! ************************************************************************ * rief * Generate foreground with left-over slice group map type MapUnit map (type 2) * * param img * Image Parameter to be used for map generation * param pps * Picture Parameter set to be used for map generation ************************************************************************ */static void FmoGenerateType2MapUnitMap (ImageParameters * img, pic_parameter_set_rbsp_t * pps ){ int iGroup; unsigned i, x, y; unsigned yTopLeft, xTopLeft, yBottomRight, xBottomRight; for( i = 0; i < PicSizeInMapUnits; i++ ) MapUnitToSliceGroupMap[ i ] = pps->num_slice_groups_minus1; for( iGroup = pps->num_slice_groups_minus1 - 1 ; iGroup >= 0; iGroup-- ) { yTopLeft = pps->top_left[ iGroup ] / img->PicWidthInMbs; xTopLeft = pps->top_left[ iGroup ] % img->PicWidthInMbs; yBottomRight = pps->bottom_right[ iGroup ] / img->PicWidthInMbs; xBottomRight = pps->bottom_right[ iGroup ] % img->PicWidthInMbs; for( y = yTopLeft; y <= yBottomRight; y++ ) for( x = xTopLeft; x <= xBottomRight; x++ ) MapUnitToSliceGroupMap[ y * img->PicWidthInMbs + x ] = iGroup; }} |
FMO3
FMO3，环形扫描映射。在JM中，环形扫描映射把一帧分为两个slice group，一个是以一帧为背景，另一个是以环作为的前景（其实环可以扩展到更多的group）。环的起点是帧的中心位置，通过指定环的运动方向（顺时针或逆时针）以及环的长度即可得到前景。
以下为顺时针，环长度为32
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 20 | 21 | 22 | 23 | 24 | 25 | 1 | 1 |
| 1 | 1 | 1 | 19 | 6 | 7 | 8 | 9 | 26 | 1 | 1 |
| 1 | 1 | 1 | 18 | 5 | 0 | 1 | 10 | 27 | 1 | 1 |
| 1 | 1 | 1 | 17 | 4 | 3 | 2 | 11 | 28 | 1 | 1 |
| 1 | 1 | 1 | 16 | 15 | 14 | 13 | 12 | 29 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 31 | 30 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
JM code 如下：
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/*! ************************************************************************ * rief * Generate box-out slice group map type MapUnit map (type 3) * * param img * Image Parameter to be used for map generation * param pps * Picture Parameter set to be used for map generation ************************************************************************ */static void FmoGenerateType3MapUnitMap (ImageParameters * img, pic_parameter_set_rbsp_t * pps ){ unsigned i, k; int leftBound, topBound, rightBound, bottomBound; int x, y, xDir, yDir; int mapUnitVacant; unsigned mapUnitsInSliceGroup0 = min((pps->slice_group_change_rate_minus1 + 1) * img->slice_group_change_cycle, PicSizeInMapUnits); for( i = 0; i < PicSizeInMapUnits; i++ ) MapUnitToSliceGroupMap[ i ] = 2; x = ( img->PicWidthInMbs - pps->slice_group_change_direction_flag ) / 2; y = ( img->PicHeightInMapUnits - pps->slice_group_change_direction_flag ) / 2; leftBound = x; topBound = y; rightBound = x; bottomBound = y; xDir = pps->slice_group_change_direction_flag - 1; yDir = pps->slice_group_change_direction_flag; for( k = 0; k < PicSizeInMapUnits; k += mapUnitVacant ) { mapUnitVacant = ( MapUnitToSliceGroupMap[ y * img->PicWidthInMbs + x ] == 2 ); if( mapUnitVacant ) MapUnitToSliceGroupMap[ y * img->PicWidthInMbs + x ] = ( k >= mapUnitsInSliceGroup0 ); if( xDir == -1 && x == leftBound ) { leftBound = max( leftBound - 1, 0 ); x = leftBound; xDir = 0; yDir = 2 * pps->slice_group_change_direction_flag - 1; } else if( xDir == 1 && x == rightBound ) { rightBound = min( rightBound + 1, (int)img->PicWidthInMbs - 1 ); x = rightBound; xDir = 0; yDir = 1 - 2 * pps->slice_group_change_direction_flag; } else if( yDir == -1 && y == topBound ) { topBound = max( topBound - 1, 0 ); y = topBound; xDir = 1 - 2 * pps->slice_group_change_direction_flag; yDir = 0; } else if( yDir == 1 && y == bottomBound ) { bottomBound = min( bottomBound + 1, (int)img->PicHeightInMapUnits - 1 ); y = bottomBound; xDir = 2 * pps->slice_group_change_direction_flag - 1; yDir = 0; } else { x = x + xDir; y = y + yDir; } } } |
FMO4
FMO4，光栅扫描映射。该模式只支持两个slice group，按照光栅扫描顺序来分组，方向有正向与反向之分。
如下为反向FMO4(始于右下角):
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
JM code 如下：
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/*! ************************************************************************ * rief * Generate raster scan slice group map type MapUnit map (type 4) * * param img * Image Parameter to be used for map generation * param pps * Picture Parameter set to be used for map generation ************************************************************************ */static void FmoGenerateType4MapUnitMap (ImageParameters * img, pic_parameter_set_rbsp_t * pps ){ unsigned mapUnitsInSliceGroup0 = min((pps->slice_group_change_rate_minus1 + 1) * img->slice_group_change_cycle, PicSizeInMapUnits); unsigned sizeOfUpperLeftGroup = pps->slice_group_change_direction_flag ? ( PicSizeInMapUnits - mapUnitsInSliceGroup0 ) : mapUnitsInSliceGroup0; unsigned i; for( i = 0; i < PicSizeInMapUnits; i++ ) if( i < sizeOfUpperLeftGroup ) MapUnitToSliceGroupMap[ i ] = pps->slice_group_change_direction_flag; else MapUnitToSliceGroupMap[ i ] = 1 - pps->slice_group_change_direction_flag; } |
FMO5
FMO5，擦式扫描映射。仅支持两个slice group，扫描方式为纵向，也有正反两个方向
如下为正向FMO5（始于左上角）：
| 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 0 | 0 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
JM code 如下：
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/*! ************************************************************************ * rief * Generate wipe slice group map type MapUnit map (type 5) * * param img * Image Parameter to be used for map generation * param pps * Picture Parameter set to be used for map generation *************************************************************************/static void FmoGenerateType5MapUnitMap (ImageParameters * img, pic_parameter_set_rbsp_t * pps ){ unsigned mapUnitsInSliceGroup0 = min((pps->slice_group_change_rate_minus1 + 1) * img->slice_group_change_cycle, PicSizeInMapUnits); unsigned sizeOfUpperLeftGroup = pps->slice_group_change_direction_flag ? ( PicSizeInMapUnits - mapUnitsInSliceGroup0 ) : mapUnitsInSliceGroup0; unsigned i,j, k = 0; for( j = 0; j < img->PicWidthInMbs; j++ ) for( i = 0; i < img->PicHeightInMapUnits; i++ ) if( k++ < sizeOfUpperLeftGroup ) MapUnitToSliceGroupMap[ i * img->PicWidthInMbs + j ] = 1 - pps->slice_group_change_direction_flag; else MapUnitToSliceGroupMap[ i * img->PicWidthInMbs + j ] = pps->slice_group_change_direction_flag; } |
FMO6
FMO6，显示控制映射。可以在配置文件中自由地指定每个宏块所属的slice group。
| 0 | 1 | 0 | 2 | 1 | 1 | 0 | 2 | 1 | 1 | 1 |
| 1 | 2 | 0 | 0 | 0 | 1 | 0 | 2 | 0 | 1 | 0 |
| 3 | 5 | 4 | 1 | 0 | 0 | 3 | 5 | 4 | 0 | 1 |
| 0 | 0 | 5 | 5 | 3 | 1 | 0 | 2 | 5 | 0 | 1 |
| 4 | 1 | 0 | 0 | 1 | 1 | 1 | 1 | 4 | 4 | 2 |
| 0 | 2 | 2 | 3 | 2 | 3 | 2 | 1 | 5 | 2 | 3 |
| 0 | 1 | 2 | 0 | 1 | 1 | 2 | 0 | 5 | 5 | 5 |
| 3 | 2 | 1 | 4 | 1 | 4 | 4 | 0 | 3 | 3 | 2 |
| 4 | 2 | 3 | 5 | 0 | 0 | 1 | 4 | 1 | 2 | 3 |
顺带一提JM支持一帧中最多为8个slice group
JM code 如下：
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/*! ************************************************************************ * rief * Generate explicit slice group map type MapUnit map (type 6) * * param img * Image Parameter to be used for map generation * param pps * Picture Parameter set to be used for map generation ************************************************************************ */static void FmoGenerateType6MapUnitMap (ImageParameters * img, pic_parameter_set_rbsp_t * pps ){ unsigned i; for (i=0; i<PicSizeInMapUnits; i++) { MapUnitToSliceGroupMap[i] = pps->slice_group_id[i]; }}
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