x264中的提炼半像素函数——refine_subpel 函数分析

//半像素和1/4像素搜索 ,  hpel_iters 半像素搜索次数 ，qpel_iters 1/4像素搜索次数

static void refine_subpel( x264_t *h, x264_me_t *m, int hpel_iters, int qpel_iters, int *p_halfpel_thresh, int b_refine_qpel )

{

    const int bw = x264_pixel_size[m->i_pixel].w;

    const int bh = x264_pixel_size[m->i_pixel].h;

    const uint16_t *p_cost_mvx = m->p_cost_mv - m->mvp[0];

    const uint16_t *p_cost_mvy = m->p_cost_mv - m->mvp[1];

    const int i_pixel = m->i_pixel;

    const int b_chroma_me = h->mb.b_chroma_me && i_pixel <= PIXEL_8x8;

    const int mvy_offset = h->mb.b_interlaced & m->i_ref ? (h->mb.i_mb_y & 1)*4 - 2 : 0;

    ALIGNED_ARRAY_16( pixel, pix,[64*18] ); // really 17x17x2, but round up for alignment

    int bmx = m->mv[0];//做完整像素运动搜索之后预测的运动矢量

    int bmy = m->mv[1];

    int bcost = m->cost;

    int odir = -1, bdir;

    /* try the subpel component of the predicted mv */

    if( hpel_iters && h->mb.i_subpel_refine < 3 )

    {

        int mx = x264_clip3( m->mvp[0], h->mb.mv_min_spel[0]+2, h->mb.mv_max_spel[0]-2 );

        int my = x264_clip3( m->mvp[1], h->mb.mv_min_spel[1]+2, h->mb.mv_max_spel[1]-2 );

        if( (mx-bmx)|(my-bmy) )

            COST_MV_SAD( mx, my );

    }

    /* halfpel diamond search */

    for( int i = hpel_iters; i > 0; i-- )//从做完整像素运动搜索之后预测的运动矢量开始做半像素的diamond搜索

    {

        int omx = bmx, omy = bmy;

        int costs[4];

        int stride = 64; // candidates are either all hpel or all qpel, so one stride is enough

        pixel *src0, *src1, *src2, *src3;

  //得到 omx,moy周围的半像素4个点的地址

        src0 = h->mc.get_ref( pix,    &stride, m->p_fref, m->i_stride[0], omx, omy-2, bw, bh+1, &m->weight[0] );

        src2 = h->mc.get_ref( pix+32, &stride, m->p_fref, m->i_stride[0], omx-2, omy, bw+4, bh, &m->weight[0] );

        src1 = src0 + stride;//src0为中心点的上方点,scr1为中心点的下方点

        src3 = src2 + 1;//src2为中心点的左侧点,scr3为中心点的右侧点

        h->pixf.fpelcmp_x4[i_pixel]( m->p_fenc[0], src0, src1, src2, src3, stride, costs );//计算cost

        COPY2_IF_LT( bcost, costs[0] + p_cost_mvx[omx  ] + p_cost_mvy[omy-2], bmy, omy-2 );

        COPY2_IF_LT( bcost, costs[1] + p_cost_mvx[omx  ] + p_cost_mvy[omy+2], bmy, omy+2 );

        COPY3_IF_LT( bcost, costs[2] + p_cost_mvx[omx-2] + p_cost_mvy[omy  ], bmx, omx-2, bmy, omy );

        COPY3_IF_LT( bcost, costs[3] + p_cost_mvx[omx+2] + p_cost_mvy[omy  ], bmx, omx+2, bmy, omy );

        if( (bmx == omx) & (bmy == omy) )

            break;

    }

    if( !b_refine_qpel && (h->pixf.mbcmp_unaligned[0] != h->pixf.fpelcmp[0] || b_chroma_me) )

    {

        bcost = COST_MAX;

        COST_MV_SATD( bmx, bmy, -1 );

    }

    /* early termination when examining multiple reference frames */

    if( p_halfpel_thresh )

    {

        if( (bcost*7)>>3 > *p_halfpel_thresh )//如果bcost*7/8>阀值

        {

            m->cost = bcost;

            m->mv[0] = bmx;

            m->mv[1] = bmy;

            // don't need cost_mv

            return;

        }

        else if( bcost < *p_halfpel_thresh )

            *p_halfpel_thresh = bcost;

    }

    /* quarterpel diamond search */

    if( h->mb.i_subpel_refine != 1 )

    {

        bdir = -1;//做1/4像素迭代

        for( int i = qpel_iters; i > 0; i-- )

        {

           //判断边界

            if( bmy <= h->mb.mv_min_spel[1] || bmy >= h->mb.mv_max_spel[1] || bmx <= h->mb.mv_min_spel[0] || bmx >= h->mb.mv_max_spel[0] )

                break;

            odir = bdir;

            int omx = bmx, omy = bmy;

            COST_MV_SATD( omx, omy - 1, 0 );//注:在COST_MV_SATD中(dir^1)!=odir 这个条件，用来去除已经做过cost的一个点

            COST_MV_SATD( omx, omy + 1, 1 );

            COST_MV_SATD( omx - 1, omy, 2 );

            COST_MV_SATD( omx + 1, omy, 3 );

            if( (bmx == omx) & (bmy == omy) )

                break;

        }

    }

    /* Special simplified case for subme=1 */

    else if( bmy > h->mb.mv_min_spel[1] && bmy < h->mb.mv_max_spel[1] && bmx > h->mb.mv_min_spel[0] && bmx < h->mb.mv_max_spel[0] )

    {

        int costs[4];

        int omx = bmx, omy = bmy;//只做一次1/4像素搜索

        /* We have to use mc_luma because all strides must be the same to use fpelcmp_x4 */

        h->mc.mc_luma( pix   , 64, m->p_fref, m->i_stride[0], omx, omy-1, bw, bh, &m->weight[0] );

        h->mc.mc_luma( pix+16, 64, m->p_fref, m->i_stride[0], omx, omy+1, bw, bh, &m->weight[0] );

        h->mc.mc_luma( pix+32, 64, m->p_fref, m->i_stride[0], omx-1, omy, bw, bh, &m->weight[0] );

        h->mc.mc_luma( pix+48, 64, m->p_fref, m->i_stride[0], omx+1, omy, bw, bh, &m->weight[0] );

        h->pixf.fpelcmp_x4[i_pixel]( m->p_fenc[0], pix, pix+16, pix+32, pix+48, 64, costs );

        COPY2_IF_LT( bcost, costs[0] + p_cost_mvx[omx  ] + p_cost_mvy[omy-1], bmy, omy-1 );

        COPY2_IF_LT( bcost, costs[1] + p_cost_mvx[omx  ] + p_cost_mvy[omy+1], bmy, omy+1 );

        COPY3_IF_LT( bcost, costs[2] + p_cost_mvx[omx-1] + p_cost_mvy[omy  ], bmx, omx-1, bmy, omy );

        COPY3_IF_LT( bcost, costs[3] + p_cost_mvx[omx+1] + p_cost_mvy[omy  ], bmx, omx+1, bmy, omy );

    }

    m->cost = bcost;//跟新cost 和mv  cost_mv变量

    m->mv[0] = bmx;

    m->mv[1] = bmy;

    m->cost_mv = p_cost_mvx[bmx] + p_cost_mvy[bmy];