x264代码剖析(十八):核心算法之滤波

x264代码剖析(十八):核心算法之滤波

 

        H.264/MPEG-4 AVC视频编码标准中,在编解码器反变换量化后,图像会出现方块效应,主要原因是:1)基于块的帧内和帧间预测残差的DCT变换,变换系数的量化过程相对粗糙,因而反量化过程恢复的变换系数有误差,会造成在图像块边界上的视觉不连续;2)运动补偿可能是从不是同一帧的不同位置上内插样点数据复制而来,因为运动补偿块的匹配不可能是绝对准确的,所以就会在复制块的边界上产生数据不连续;3)参考帧中的存在的不连续也被复制到需要补偿的图像块内。

 

        尽管H.264采用较小的4×4变换尺寸可以降低这种不连续现象,但仍需要一个去方块滤波器,以最大程度提高编码性能。在x264中,x264_slice_write()函数中调用了x264_fdec_filter_row()的源代码。x264_fdec_filter_row()对应着x264中的滤波模块。滤波模块主要完成了下面3个方面的功能:

1)环路滤波(去块效应滤波);

2)半像素内插;

3)视频质量指标PSNRSSIM的计算。

 

        滤波模块对应的函数关系调用图如下:

 

x264代码剖析(十八):核心算法之滤波_第1张图片

 

        下面对x264中的滤波模块对应的主要函数分别进行分析。

 

1x264_slice_write()函数

 

        x264_slice_write()函数中调用了x264_fdec_filter_row()函数,对应于滤波模块。具体的代码分析见《x264代码剖析(九):x264_encoder_encode()函数之x264_slice's'_write()函数》。

 

2x264_fdec_filter_row()函数

 

        x264_fdec_filter_row()函数用于对一行宏块进行滤波。该函数的定义位于encoder\encoder.cx264_fdec_filter_row()完成了三步工作:

1)环路滤波(去块效应滤波)。通过调用x264_frame_deblock_row()函数实现。

2)半像素内插。通过调用x264_frame_filter()函数实现。

3)视频质量SSIMPSNR的计算。PSNR通过调用x264_pixel_ssd_wxh()函数实现,在这里只计算了SSDSSIM的计算则是通过x264_pixel_ssim_wxh()函数实现。

 

        对应的代码分析如下:

 

/******************************************************************/
/******************************************************************/
/*
======Analysed by RuiDong Fang
======Csdn Blog:http://blog.csdn.net/frd2009041510
======Date:2016.04.06
 */
/******************************************************************/
/******************************************************************/

/************====== x264_fdec_filter_row()函数 ======************/
/*
功能:对一行宏块进行滤波-去块效应滤波、半像素插值、SSIM/PSNR计算等
*/
static void x264_fdec_filter_row( x264_t *h, int mb_y, int pass )
{
    /* mb_y is the mb to be encoded next, not the mb to be filtered here */
    int b_hpel = h->fdec->b_kept_as_ref;
    int b_deblock = h->sh.i_disable_deblocking_filter_idc != 1;
    int b_end = mb_y == h->i_threadslice_end;
    int b_measure_quality = 1;
    int min_y = mb_y - (1 << SLICE_MBAFF);
    int b_start = min_y == h->i_threadslice_start;
    /* Even in interlaced mode, deblocking never modifies more than 4 pixels
     * above each MB, as bS=4 doesn't happen for the top of interlaced mbpairs. */
    int minpix_y = min_y*16 - 4 * !b_start;
    int maxpix_y = mb_y*16 - 4 * !b_end;
    b_deblock &= b_hpel || h->param.b_full_recon || h->param.psz_dump_yuv;
    if( h->param.b_sliced_threads )
    {
        switch( pass )
        {
            /* During encode: only do deblock if asked for */
            default:
            case 0:
                b_deblock &= h->param.b_full_recon;
                b_hpel = 0;
                break;
            /* During post-encode pass: do deblock if not done yet, do hpel for all
             * rows except those between slices. */
            case 1:
                b_deblock &= !h->param.b_full_recon;
                b_hpel &= !(b_start && min_y > 0);
                b_measure_quality = 0;
                break;
            /* Final pass: do the rows between slices in sequence. */
            case 2:
                b_deblock = 0;
                b_measure_quality = 0;
                break;
        }
    }
    if( mb_y & SLICE_MBAFF )
        return;
    if( min_y < h->i_threadslice_start )
        return;

    if( b_deblock )
        for( int y = min_y; y < mb_y; y += (1 << SLICE_MBAFF) )
            x264_frame_deblock_row( h, y );	////////////////////////////去块效应滤波

    /* FIXME: Prediction requires different borders for interlaced/progressive mc,
     * but the actual image data is equivalent. For now, maintain this
     * consistency by copying deblocked pixels between planes. */
    if( PARAM_INTERLACED && (!h->param.b_sliced_threads || pass == 1) )
        for( int p = 0; p < h->fdec->i_plane; p++ )
            for( int i = minpix_y>>(CHROMA_V_SHIFT && p); i < maxpix_y>>(CHROMA_V_SHIFT && p); i++ )
                memcpy( h->fdec->plane_fld[p] + i*h->fdec->i_stride[p],
                        h->fdec->plane[p]     + i*h->fdec->i_stride[p],
                        h->mb.i_mb_width*16*sizeof(pixel) );

    if( h->fdec->b_kept_as_ref && (!h->param.b_sliced_threads || pass == 1) )
        x264_frame_expand_border( h, h->fdec, min_y );
    if( b_hpel )
    {
        int end = mb_y == h->mb.i_mb_height;
        /* Can't do hpel until the previous slice is done encoding. */
        if( h->param.analyse.i_subpel_refine )
        {
            x264_frame_filter( h, h->fdec, min_y, end );	////////////////////////////半像素内插
            x264_frame_expand_border_filtered( h, h->fdec, min_y, end );
        }
    }

    if( SLICE_MBAFF && pass == 0 )
        for( int i = 0; i < 3; i++ )
        {
            XCHG( pixel *, h->intra_border_backup[0][i], h->intra_border_backup[3][i] );
            XCHG( pixel *, h->intra_border_backup[1][i], h->intra_border_backup[4][i] );
        }

    if( h->i_thread_frames > 1 && h->fdec->b_kept_as_ref )
        x264_frame_cond_broadcast( h->fdec, mb_y*16 + (b_end ? 10000 : -(X264_THREAD_HEIGHT << SLICE_MBAFF)) );

	//计算编码的质量
    if( b_measure_quality )
    {
        maxpix_y = X264_MIN( maxpix_y, h->param.i_height );
        
		//如果需要打印输出PSNR
		if( h->param.analyse.b_psnr )
        {
            //实际上是计算SSD  
            //输出的时候调用x264_psnr()换算SSD为PSNR  
            /** 
             * 计算PSNR的过程 
             * 
             * MSE = SSD*1/(w*h) 
             * PSNR= 10*log10(MAX^2/MSE) 
             * 
             * 其中MAX指的是图像的灰度级,对于8bit来说就是2^8-1=255 
             */
			for( int p = 0; p < (CHROMA444 ? 3 : 1); p++ )
                h->stat.frame.i_ssd[p] += x264_pixel_ssd_wxh( &h->pixf,
                    h->fdec->plane[p] + minpix_y * h->fdec->i_stride[p], h->fdec->i_stride[p],	//重建帧
                    h->fenc->plane[p] + minpix_y * h->fenc->i_stride[p], h->fenc->i_stride[p],	//编码帧
                    h->param.i_width, maxpix_y-minpix_y );
            if( !CHROMA444 )
            {
                uint64_t ssd_u, ssd_v;
                int v_shift = CHROMA_V_SHIFT;
                x264_pixel_ssd_nv12( &h->pixf,
                    h->fdec->plane[1] + (minpix_y>>v_shift) * h->fdec->i_stride[1], h->fdec->i_stride[1],
                    h->fenc->plane[1] + (minpix_y>>v_shift) * h->fenc->i_stride[1], h->fenc->i_stride[1],
                    h->param.i_width>>1, (maxpix_y-minpix_y)>>v_shift, &ssd_u, &ssd_v );
                h->stat.frame.i_ssd[1] += ssd_u;
                h->stat.frame.i_ssd[2] += ssd_v;
            }
        }

		//如果需要打印输出SSIM
        if( h->param.analyse.b_ssim )
        {
            int ssim_cnt;
            x264_emms();
            /* offset by 2 pixels to avoid alignment of ssim blocks with dct blocks,
             * and overlap by 4 */
            minpix_y += b_start ? 2 : -6;
            h->stat.frame.f_ssim +=
                x264_pixel_ssim_wxh( &h->pixf,
                    h->fdec->plane[0] + 2+minpix_y*h->fdec->i_stride[0], h->fdec->i_stride[0],	//重建帧
                    h->fenc->plane[0] + 2+minpix_y*h->fenc->i_stride[0], h->fenc->i_stride[0],	//编码帧
                    h->param.i_width-2, maxpix_y-minpix_y, h->scratch_buffer, &ssim_cnt );
            h->stat.frame.i_ssim_cnt += ssim_cnt;
        }
    }
}


3x264_frame_deblock_row()函数

 

        x264_frame_deblock_row()用于进行环路滤波(去块效应滤波)。该函数的定义位于common\deblock.cx264_frame_deblock_row()中有一个很长的宏定义“FILTER()”定义了函数调用的方式。FILTER( intra, dir, edge, qp, chroma_qp )中:

1)“intra”指定了是普通滤波(Bs=123)还是强滤波(Bs=4);

2)“dir”指定了滤波器的方向。0为水平滤波器(垂直边界),1为垂直滤波器(水平边界);

(3)“edge”指定了边界的位置。“0”,“1”,“2”,“3”分别代表了水平(或者垂直)的4条边界

 

        对应的代码分析如下:

 

/************====== x264_frame_deblock_row()函数 ======************/
/*
功能:去块效应滤波
*/
void x264_frame_deblock_row( x264_t *h, int mb_y )
{
    int b_interlaced = SLICE_MBAFF;
    int a = h->sh.i_alpha_c0_offset - QP_BD_OFFSET;
    int b = h->sh.i_beta_offset - QP_BD_OFFSET;
    int qp_thresh = 15 - X264_MIN( a, b ) - X264_MAX( 0, h->pps->i_chroma_qp_index_offset );
    int stridey   = h->fdec->i_stride[0];
    int strideuv  = h->fdec->i_stride[1];
    int chroma444 = CHROMA444;
    int chroma_height = 16 >> CHROMA_V_SHIFT;
    intptr_t uvdiff = chroma444 ? h->fdec->plane[2] - h->fdec->plane[1] : 1;

    for( int mb_x = 0; mb_x < h->mb.i_mb_width; mb_x += (~b_interlaced | mb_y)&1, mb_y ^= b_interlaced )
    {
        x264_prefetch_fenc( h, h->fdec, mb_x, mb_y );
        x264_macroblock_cache_load_neighbours_deblock( h, mb_x, mb_y );

        int mb_xy = h->mb.i_mb_xy;
        int transform_8x8 = h->mb.mb_transform_size[mb_xy];
        int intra_cur = IS_INTRA( h->mb.type[mb_xy] );
        uint8_t (*bs)[8][4] = h->deblock_strength[mb_y&1][h->param.b_sliced_threads?mb_xy:mb_x];

		//找到像素数据(宏块的大小是16x16)
        pixel *pixy = h->fdec->plane[0] + 16*mb_y*stridey  + 16*mb_x;
        pixel *pixuv = h->fdec->plane[1] + chroma_height*mb_y*strideuv + 16*mb_x;

        if( mb_y & MB_INTERLACED )
        {
            pixy -= 15*stridey;
            pixuv -= (chroma_height-1)*strideuv;
        }

        int stride2y  = stridey << MB_INTERLACED;
        int stride2uv = strideuv << MB_INTERLACED;
        
		//QP,用于计算环路滤波的门限值alpha和beta 
		int qp = h->mb.qp[mb_xy];
        int qpc = h->chroma_qp_table[qp];
        int first_edge_only = (h->mb.partition[mb_xy] == D_16x16 && !h->mb.cbp[mb_xy] && !intra_cur) || qp <= qp_thresh;

		/* 
         * 滤波顺序如下所示(大方框代表16x16块) 
         * 
         * +--4-+--4-+--4-+--4-+ 
         * 0    1    2    3    | 
         * +--5-+--5-+--5-+--5-+ 
         * 0    1    2    3    | 
         * +--6-+--6-+--6-+--6-+ 
         * 0    1    2    3    | 
         * +--7-+--7-+--7-+--7-+ 
         * 0    1    2    3    | 
         * +----+----+----+----+ 
         * 
         */  
        //一个比较长的宏,用于进行环路滤波  
        //根据不同的情况传递不同的参数  
        //几个参数的含义:  
        //intra:  
        //为“_intra”的时候:  
        //其中的“deblock_edge##intra()”展开为函数deblock_edge_intra()  
        //其中的“h->loopf.deblock_luma##intra[dir]”展开为强滤波汇编函数h->loopf.deblock_luma_intra[dir]()  
        //为“”(空),其中的“deblock_edge##intra()”展开为函数deblock_edge()  
        //其中的“h->loopf.deblock_luma##intra[dir]”展开为普通滤波汇编函数h->loopf.deblock_luma[dir]()  
        //dir:  
        //决定了滤波的方向:0为水平滤波器(垂直边界),1为垂直滤波器(水平边界)

        #define FILTER( intra, dir, edge, qp, chroma_qp )\
        do\
        {\
            if( !(edge & 1) || !transform_8x8 )\
            {\
                deblock_edge##intra( h, pixy + 4*edge*(dir?stride2y:1),\
                                     stride2y, bs[dir][edge], qp, a, b, 0,\
                                     h->loopf.deblock_luma##intra[dir] );\
                if( CHROMA_FORMAT == CHROMA_444 )\
                {\
                    deblock_edge##intra( h, pixuv          + 4*edge*(dir?stride2uv:1),\
                                         stride2uv, bs[dir][edge], chroma_qp, a, b, 0,\
                                         h->loopf.deblock_luma##intra[dir] );\
                    deblock_edge##intra( h, pixuv + uvdiff + 4*edge*(dir?stride2uv:1),\
                                         stride2uv, bs[dir][edge], chroma_qp, a, b, 0,\
                                         h->loopf.deblock_luma##intra[dir] );\
                }\
                else if( CHROMA_FORMAT == CHROMA_420 && !(edge & 1) )\
                {\
                    deblock_edge##intra( h, pixuv + edge*(dir?2*stride2uv:4),\
                                         stride2uv, bs[dir][edge], chroma_qp, a, b, 1,\
                                         h->loopf.deblock_chroma##intra[dir] );\
                }\
            }\
            if( CHROMA_FORMAT == CHROMA_422 && (dir || !(edge & 1)) )\
            {\
                deblock_edge##intra( h, pixuv + edge*(dir?4*stride2uv:4),\
                                     stride2uv, bs[dir][edge], chroma_qp, a, b, 1,\
                                     h->loopf.deblock_chroma##intra[dir] );\
            }\
        } while(0)

        if( h->mb.i_neighbour & MB_LEFT )
        {
            if( b_interlaced && h->mb.field[h->mb.i_mb_left_xy[0]] != MB_INTERLACED )
            {
                int luma_qp[2];
                int chroma_qp[2];
                int left_qp[2];
                x264_deblock_inter_t luma_deblock = h->loopf.deblock_luma_mbaff;
                x264_deblock_inter_t chroma_deblock = h->loopf.deblock_chroma_mbaff;
                x264_deblock_intra_t luma_intra_deblock = h->loopf.deblock_luma_intra_mbaff;
                x264_deblock_intra_t chroma_intra_deblock = h->loopf.deblock_chroma_intra_mbaff;
                int c = chroma444 ? 0 : 1;

                left_qp[0] = h->mb.qp[h->mb.i_mb_left_xy[0]];
                luma_qp[0] = (qp + left_qp[0] + 1) >> 1;
                chroma_qp[0] = (qpc + h->chroma_qp_table[left_qp[0]] + 1) >> 1;
                if( intra_cur || IS_INTRA( h->mb.type[h->mb.i_mb_left_xy[0]] ) )
                {
                    deblock_edge_intra( h, pixy,           2*stridey,  bs[0][0], luma_qp[0],   a, b, 0, luma_intra_deblock );
                    deblock_edge_intra( h, pixuv,          2*strideuv, bs[0][0], chroma_qp[0], a, b, c, chroma_intra_deblock );
                    if( chroma444 )
                        deblock_edge_intra( h, pixuv + uvdiff, 2*strideuv, bs[0][0], chroma_qp[0], a, b, c, chroma_intra_deblock );
                }
                else
                {
                    deblock_edge( h, pixy,           2*stridey,  bs[0][0], luma_qp[0],   a, b, 0, luma_deblock );
                    deblock_edge( h, pixuv,          2*strideuv, bs[0][0], chroma_qp[0], a, b, c, chroma_deblock );
                    if( chroma444 )
                        deblock_edge( h, pixuv + uvdiff, 2*strideuv, bs[0][0], chroma_qp[0], a, b, c, chroma_deblock );
                }

                int offy = MB_INTERLACED ? 4 : 0;
                int offuv = MB_INTERLACED ? 4-CHROMA_V_SHIFT : 0;
                left_qp[1] = h->mb.qp[h->mb.i_mb_left_xy[1]];
                luma_qp[1] = (qp + left_qp[1] + 1) >> 1;
                chroma_qp[1] = (qpc + h->chroma_qp_table[left_qp[1]] + 1) >> 1;
                if( intra_cur || IS_INTRA( h->mb.type[h->mb.i_mb_left_xy[1]] ) )
                {
                    deblock_edge_intra( h, pixy           + (stridey<<offy),   2*stridey,  bs[0][4], luma_qp[1],   a, b, 0, luma_intra_deblock );
                    deblock_edge_intra( h, pixuv          + (strideuv<<offuv), 2*strideuv, bs[0][4], chroma_qp[1], a, b, c, chroma_intra_deblock );
                    if( chroma444 )
                        deblock_edge_intra( h, pixuv + uvdiff + (strideuv<<offuv), 2*strideuv, bs[0][4], chroma_qp[1], a, b, c, chroma_intra_deblock );
                }
                else
                {
                    deblock_edge( h, pixy           + (stridey<<offy),   2*stridey,  bs[0][4], luma_qp[1],   a, b, 0, luma_deblock );
                    deblock_edge( h, pixuv          + (strideuv<<offuv), 2*strideuv, bs[0][4], chroma_qp[1], a, b, c, chroma_deblock );
                    if( chroma444 )
                        deblock_edge( h, pixuv + uvdiff + (strideuv<<offuv), 2*strideuv, bs[0][4], chroma_qp[1], a, b, c, chroma_deblock );
                }
            }
            else
            {
                //左边宏块的qp
				int qpl = h->mb.qp[h->mb.i_mb_xy-1];
                int qp_left = (qp + qpl + 1) >> 1;
                int qpc_left = (qpc + h->chroma_qp_table[qpl] + 1) >> 1;
                //Intra宏块左边宏块的qp
				int intra_left = IS_INTRA( h->mb.type[h->mb.i_mb_xy-1] );
                int intra_deblock = intra_cur || intra_left;

                /* Any MB that was coded, or that analysis decided to skip, has quality commensurate with its QP.
                 * But if deblocking affects neighboring MBs that were force-skipped, blur might accumulate there.
                 * So reset their effective QP to max, to indicate that lack of guarantee. */
                if( h->fdec->mb_info && M32( bs[0][0] ) )
                {
#define RESET_EFFECTIVE_QP(xy) h->fdec->effective_qp[xy] |= 0xff * !!(h->fdec->mb_info[xy] & X264_MBINFO_CONSTANT);
                    RESET_EFFECTIVE_QP(mb_xy);
                    RESET_EFFECTIVE_QP(h->mb.i_mb_left_xy[0]);
                }

                if( intra_deblock )
					//【0】强滤波,水平滤波器(垂直边界)
					FILTER( _intra, 0, 0, qp_left, qpc_left );
                else
                    //【0】普通滤波,水平滤波器(垂直边界)
					FILTER(       , 0, 0, qp_left, qpc_left );
            }
        }
        if( !first_edge_only )
        {
            //普通滤波,水平滤波器(垂直边界)
			FILTER( , 0, 1, qp, qpc );//【1】
            FILTER( , 0, 2, qp, qpc );//【2】
            FILTER( , 0, 3, qp, qpc );//【3】
        }

        if( h->mb.i_neighbour & MB_TOP )
        {
            if( b_interlaced && !(mb_y&1) && !MB_INTERLACED && h->mb.field[h->mb.i_mb_top_xy] )
            {
                int mbn_xy = mb_xy - 2 * h->mb.i_mb_stride;

                for( int j = 0; j < 2; j++, mbn_xy += h->mb.i_mb_stride )
                {
                    int qpt = h->mb.qp[mbn_xy];
                    int qp_top = (qp + qpt + 1) >> 1;
                    int qpc_top = (qpc + h->chroma_qp_table[qpt] + 1) >> 1;
                    int intra_top = IS_INTRA( h->mb.type[mbn_xy] );
                    if( intra_cur || intra_top )
                        M32( bs[1][4*j] ) = 0x03030303;

                    // deblock the first horizontal edge of the even rows, then the first horizontal edge of the odd rows
                    deblock_edge( h, pixy      + j*stridey,  2* stridey, bs[1][4*j], qp_top, a, b, 0, h->loopf.deblock_luma[1] );
                    if( chroma444 )
                    {
                        deblock_edge( h, pixuv          + j*strideuv, 2*strideuv, bs[1][4*j], qpc_top, a, b, 0, h->loopf.deblock_luma[1] );
                        deblock_edge( h, pixuv + uvdiff + j*strideuv, 2*strideuv, bs[1][4*j], qpc_top, a, b, 0, h->loopf.deblock_luma[1] );
                    }
                    else
                        deblock_edge( h, pixuv          + j*strideuv, 2*strideuv, bs[1][4*j], qpc_top, a, b, 1, h->loopf.deblock_chroma[1] );
                }
            }
            else
            {
                int qpt = h->mb.qp[h->mb.i_mb_top_xy];
                int qp_top = (qp + qpt + 1) >> 1;
                int qpc_top = (qpc + h->chroma_qp_table[qpt] + 1) >> 1;
                int intra_top = IS_INTRA( h->mb.type[h->mb.i_mb_top_xy] );
                int intra_deblock = intra_cur || intra_top;

                /* This edge has been modified, reset effective qp to max. */
                if( h->fdec->mb_info && M32( bs[1][0] ) )
                {
                    RESET_EFFECTIVE_QP(mb_xy);
                    RESET_EFFECTIVE_QP(h->mb.i_mb_top_xy);
                }

                if( (!b_interlaced || (!MB_INTERLACED && !h->mb.field[h->mb.i_mb_top_xy])) && intra_deblock )
                {
                    FILTER( _intra, 1, 0, qp_top, qpc_top );//【4】普通滤波,垂直滤波器(水平边界)
                }
                else
                {
                    if( intra_deblock )
                        M32( bs[1][0] ) = 0x03030303;
                    FILTER(       , 1, 0, qp_top, qpc_top );//【4】普通滤波,垂直滤波器(水平边界)
                }
            }
        }

        if( !first_edge_only )
        {
            //普通滤波,垂直滤波器(水平边界) 
			FILTER( , 1, 1, qp, qpc );//【5】
            FILTER( , 1, 2, qp, qpc );//【6】
            FILTER( , 1, 3, qp, qpc );//【7】
        }

        #undef FILTER
    }
}


4x264_frame_filter()函数

 

        x264_frame_filter()用于完成半像素内插的工作。该函数的定义位于common\mc.cx264_frame_filter()调用了汇编函数h->mc.hpel_filter()完成了半像素内插的工作。经过汇编半像素内插函数处理之后,得到的水平半像素内差点存储在x264_frame_tfiltered[][1]中,垂直半像素内差点存储在x264_frame_tfiltered[][2]中,对角线半像素内差点存储在x264_frame_tfiltered[][3]中(整像素点存储在x264_frame_tfiltered[][0]中)。

 

        对应的代码分析如下:

 

/************====== x264_frame_filter()函数 ======************/
/*
功能:半像素内插
*/
void x264_frame_filter( x264_t *h, x264_frame_t *frame, int mb_y, int b_end )
{
    const int b_interlaced = PARAM_INTERLACED;
    int start = mb_y*16 - 8; // buffer = 4 for deblock + 3 for 6tap, rounded to 8
    int height = (b_end ? frame->i_lines[0] + 16*PARAM_INTERLACED : (mb_y+b_interlaced)*16) + 8;

    if( mb_y & b_interlaced )
        return;

    for( int p = 0; p < (CHROMA444 ? 3 : 1); p++ )
    {
        int stride = frame->i_stride[p];
        const int width = frame->i_width[p];
        int offs = start*stride - 8; // buffer = 3 for 6tap, aligned to 8 for simd

		//半像素内插
        if( !b_interlaced || h->mb.b_adaptive_mbaff )
            h->mc.hpel_filter(
                frame->filtered[p][1] + offs,//水平半像素内插
                frame->filtered[p][2] + offs,//垂直半像素内插
                frame->filtered[p][3] + offs,//中间半像素内插
                frame->plane[p] + offs,
                stride, width + 16, height - start,
                h->scratch_buffer );

        if( b_interlaced )
        {
            /* MC must happen between pixels in the same field. */
            stride = frame->i_stride[p] << 1;
            start = (mb_y*16 >> 1) - 8;
            int height_fld = ((b_end ? frame->i_lines[p] : mb_y*16) >> 1) + 8;
            offs = start*stride - 8;
            for( int i = 0; i < 2; i++, offs += frame->i_stride[p] )
            {
                h->mc.hpel_filter(
                    frame->filtered_fld[p][1] + offs,
                    frame->filtered_fld[p][2] + offs,
                    frame->filtered_fld[p][3] + offs,
                    frame->plane_fld[p] + offs,
                    stride, width + 16, height_fld - start,
                    h->scratch_buffer );
            }
        }
    }

    /* generate integral image:
     * frame->integral contains 2 planes. in the upper plane, each element is
     * the sum of an 8x8 pixel region with top-left corner on that point.
     * in the lower plane, 4x4 sums (needed only with --partitions p4x4). */

    if( frame->integral )
    {
        int stride = frame->i_stride[0];
        if( start < 0 )
        {
            memset( frame->integral - PADV * stride - PADH, 0, stride * sizeof(uint16_t) );
            start = -PADV;
        }
        if( b_end )
            height += PADV-9;
        for( int y = start; y < height; y++ )
        {
            pixel    *pix  = frame->plane[0] + y * stride - PADH;
            uint16_t *sum8 = frame->integral + (y+1) * stride - PADH;
            uint16_t *sum4;
            if( h->frames.b_have_sub8x8_esa )
            {
                h->mc.integral_init4h( sum8, pix, stride );
                sum8 -= 8*stride;
                sum4 = sum8 + stride * (frame->i_lines[0] + PADV*2);
                if( y >= 8-PADV )
                    h->mc.integral_init4v( sum8, sum4, stride );
            }
            else
            {
                h->mc.integral_init8h( sum8, pix, stride );
                if( y >= 8-PADV )
                    h->mc.integral_init8v( sum8-8*stride, stride );
            }
        }
    }
}


5x264_pixel_ssd_wxh()函数

 

        x264_pixel_ssd_wxh()用于计算SSD(用于以后计算PSNR)。该函数的定义位于common\pixel.cx264_pixel_ssd_wxh()在计算大部分块的SSD的时候是以16x16的块为单位;当宽度不是16的整数倍的时候,在左侧边缘处不足16像素的地方使用了8x16的块进行计算;当高度不是16的整数倍的时候,在下方不足16像素的地方使用了8x8的块进行计算;当宽高不是8的整数倍的时候,则再单独计算。

 

        对应的代码分析如下:

 

/************====== x264_pixel_ssd_wxh()函数 ======************/
/*
 * 功能:计算SSD(可用于计算PSNR)
 * pix1: 受损数据 
 * pix2: 原始数据 
 * i_width: 图像宽 
 * i_height: 图像高
*/
uint64_t x264_pixel_ssd_wxh( x264_pixel_function_t *pf, pixel *pix1, intptr_t i_pix1,
                             pixel *pix2, intptr_t i_pix2, int i_width, int i_height )
{
    uint64_t i_ssd = 0;//计算结果都累加到i_ssd变量上
    int y;
    int align = !(((intptr_t)pix1 | (intptr_t)pix2 | i_pix1 | i_pix2) & 15);

#define SSD(size) i_ssd += pf->ssd[size]( pix1 + y*i_pix1 + x, i_pix1, \
                                          pix2 + y*i_pix2 + x, i_pix2 );
    /* 
     * SSD计算过程: 
     * 从左上角开始,绝大部分块使用16x16的SSD计算 
     * 右边边界部分可能用16x8的SSD计算 
     * 下边边界可能用8x8的SSD计算 
     * 注意:这么做主要是出于汇编优化的考虑 
     * 
     * +----+----+----+----+----+----+----+----+----+----+-+ 
     * |                   |                   |         | 
     * +                   +                   +         + 
     * |                   |                   |         | 
     * +      16x16        +       16x16       +  8x16   + 
     * |                   |                   |         | 
     * +                   +                   +         + 
     * |                   |                   |         | 
     * +----+----+----+----+----+----+----+----+----+----+-+ 
     * |         | 
     * +   8x8   + 
     * |         | 
     * +----+----+ 
     * +         + 
     */
	for( y = 0; y < i_height-15; y += 16 )
    {
        int x = 0;
        if( align )//大部分使用16x16的SSD 
            for( ; x < i_width-15; x += 16 )
                SSD(PIXEL_16x16);
        for( ; x < i_width-7; x += 8 )//右边边缘部分可能用8x16的SSD 
            SSD(PIXEL_8x16);
    }
    if( y < i_height-7 )//下边边缘部分可能用到8x8的SSD
        for( int x = 0; x < i_width-7; x += 8 )
            SSD(PIXEL_8x8);
#undef SSD

#define SSD1 { int d = pix1[y*i_pix1+x] - pix2[y*i_pix2+x]; i_ssd += d*d; }
    if( i_width & 7 )//如果像素不是16/8的整数倍,边界上的点需要单独算
    {
        for( y = 0; y < (i_height & ~7); y++ )
            for( int x = i_width & ~7; x < i_width; x++ )
                SSD1;
    }
    if( i_height & 7 )
    {
        for( y = i_height & ~7; y < i_height; y++ )
            for( int x = 0; x < i_width; x++ )
                SSD1;
    }
#undef SSD1

    return i_ssd;
}


6x264_pixel_ssim_wxh()函数

 

        x264_pixel_ssim_wxh()用于计算SSIM。该函数的定义位于common\pixel.cx264_pixel_ssim_wxh()中是按照4x4的块对像素进行处理的。使用sum1[]保存上一行块的信息sum0[]保存当前一行块的信息信息包含4个元素:

s1:原始像素之和;

s2:受损像素之和;

ss:原始像素平方之和+受损像素平方之和;

s12:原始像素*受损像素的值的和。

 

        对应的代码分析如下:

 

/************====== x264_pixel_ssd_wxh()函数 ======************/
/*
 * 功能:计算SSIM
 * pix1: 受损数据 
 * pix2: 原始数据 
 * i_width: 图像宽 
 * i_height: 图像高
*/
float x264_pixel_ssim_wxh( x264_pixel_function_t *pf,
                           pixel *pix1, intptr_t stride1,
                           pixel *pix2, intptr_t stride2,
                           int width, int height, void *buf, int *cnt )
{
    /* 
     * SSIM公式 
     * SSIM = ((2*ux*uy+C1)(2*σxy+C2))/((ux^2+uy^2+C1)(σx^2+σy^2+C2)) 
     * 
     * 其中 
     * ux=E(x) 
     * uy=E(y) 
     * σxy=cov(x,y)=E(XY)-ux*uy 
     * σx^2=E(x^2)-E(x)^2 
     * 
     */
	
	int z = 0;
    float ssim = 0.0;

	//这是数组指针,注意和指针数组的区别  
    //数组指针就是指向数组的指针
    int (*sum0)[4] = buf;
	/* 
     * sum0是一个数组指针,其中存储了一个4元素数组的地址 
     * 换句话说,sum0[]中每一个元素对应一个4x4块的信息(该信息包含4个元素)。 
     * 
     * 4个元素中: 
     * [0]原始像素之和 
     * [1]受损像素之和 
     * [2]原始像素平方之和+受损像素平方之和 
     * [3]原始像素*受损像素的值的和 
     * 
     */
    int (*sum1)[4] = sum0 + (width >> 2) + 3;
    //除以4,编程以“4x4块”为单位
	width >>= 2;
    height >>= 2;
    //以8*8的块为单位计算SSIM值。然后以4个像素为step滑动窗口
	for( int y = 1; y < height; y++ )
    {
        //下面这个循环,只有在第一次执行的时候执行2次,处理第1行和第2行的块  
        //后面的都只会执行一次
		for( ; z <= y; z++ )
        {
            //执行完XCHG()之后,sum1[]存储上1行块的值(在上面),而sum0[]等待ssim_4x4x2_core()计算当前行的值(在下面)
			XCHG( void*, sum0, sum1 );
            //获取4x4块的信息(这里并没有代入公式计算SSIM结果)  
            //结果存储在sum0[]中。从左到右每个4x4的块依次存储在sum0[0],sum0[1],sum0[2]...  
            //每次x前进2个块  
            /* 
             * ssim_4x4x2_core():计算2个4x4块 
             * +----+----+ 
             * |    |    | 
             * +----+----+ 
             */
			for( int x = 0; x < width; x+=2 )
                pf->ssim_4x4x2_core( &pix1[4*(x+z*stride1)], stride1, &pix2[4*(x+z*stride2)], stride2, &sum0[x] );
        }
		//x每次增加4,前进4个块  
        //以8*8的块为单位计算  
        /* 
         * sum1[]为上一行4x4块信息,sum0[]为当前行4x4块信息 
         * 示例(line以4x4块为单位) 
         * 第1次运行 
         *       +----+----+----+----+ 
         * 1line |   sum1[] 
         *       +----+----+----+----+ 
         * 2line |   sum0[] 
         *       +----+----+----+----+ 
         * 
         * 第2次运行 
         *       + 
         * 1line | 
         *       +----+----+----+----+ 
         * 2line |   sum1[] 
         *       +----+----+----+----+ 
         * 3line |   sum0[] 
         *       +----+----+----+----+ 
         */
        for( int x = 0; x < width-1; x += 4 )
            ssim += pf->ssim_end4( sum0+x, sum1+x, X264_MIN(4,width-x-1) );
    }
    *cnt = (height-1) * (width-1);
    return ssim;
}

 大笑滤波模块的主要代码分析就到这儿,其实中间有很多实用且有效的函数块,待后面用到时更新。


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