[x264-devel] A question about "x264_frame_init_lowres"

[王小静]

I'm a newbie of x264,and I  have confused by the x264_frame_init_lowres
routine in the file  mc.c.
The routine  is called by x264_encoder_encode when
h->frames.b_have_lowresis true.
I wonder what  does the h->frames.b_have_lowres stand by and what's the
function of x264_frame_init_lowres?
I will be grateful with any reply.Thanks.
the source of this routine is following:
void x264_frame_init_lowres( int cpu, x264_frame_t *frame )
{    // FIXME: tapfilter?
    const int i_stride = frame->i_stride[0];
    const int i_stride2 = frame->i_stride_lowres;
    const int i_width2 = i_stride2 - 64;
    int x, y, i;
    for( y = 0; y < frame->i_lines_lowres - 1; y++ )
    {
        uint8_t *src0 = &frame->plane[0][2*y*i_stride];
        uint8_t *src1 = src0+i_stride;
        uint8_t *src2 = src1+i_stride;
        uint8_t *dst0 = &frame->lowres[0][y*i_stride2];
        uint8_t *dsth = &frame->lowres[1][y*i_stride2];
        uint8_t *dstv = &frame->lowres[2][y*i_stride2];
        uint8_t *dstc = &frame->lowres[3][y*i_stride2];
        for( x = 0; x < i_width2 - 1; x++ )
        {
            dst0[x] = (src0[2*x  ] + src0[2*x+1] + src1[2*x  ] + src1[2*x+1]
+ 2) >> 2;
            dsth[x] = (src0[2*x+1] + src0[2*x+2] + src1[2*x+1] + src1[2*x+2]
+ 2) >> 2;
            dstv[x] = (src1[2*x  ] + src1[2*x+1] + src2[2*x  ] + src2[2*x+1]
+ 2) >> 2;
            dstc[x] = (src1[2*x+1] + src1[2*x+2] + src2[2*x+1] + src2[2*x+2]
+ 2) >> 2;
        }
        dst0[x] = (src0[2*x  ] + src0[2*x+1] + src1[2*x  ] + src1[2*x+1] +
2) >> 2;
        dstv[x] = (src1[2*x  ] + src1[2*x+1] + src2[2*x  ] + src2[2*x+1] +
2) >> 2;
        dsth[x] = (src0[2*x+1] + src1[2*x+1] + 1) >> 1;
        dstc[x] = (src1[2*x+1] + src2[2*x+1] + 1) >> 1;
    }
    for( i = 0; i < 4; i++ )
        memcpy( &frame->lowres[i][y*i_stride2],
&frame->lowres[i][(y-1)*i_stride2], i_width2 );

    for( y = 0; y < 16; y++ )
        for( x = 0; x < 16; x++ )
            frame->i_cost_est[x][y] = -1;

    x264_frame_expand_border_lowres( frame );
}
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