[x264-devel] Question for intra prediction mode (0-8)

[나태영]

Dear all..

 

I’m analyzing intra prediction part in analyse.c

 

There, I feel so strange for the use of i_pred_mode variable.

 

For 4x4 intra prediction, in x264_mb_analyse_intra function line, we can
see below

 

if( b_merged_satd && i_max >= 6 )

{

int satd[3];

         h->pixf.intra_satd_x3_4x4( p_src_by, p_dst_by, satd );

         if( i_pred_mode < 3 )

                satd[i_pred_mode] -= 3 * a->i_lambda;

         for( i=2; i>=0; i-- )

         COPY2_IF_LT( i_best, satd[i] + 4 * a->i_lambda, a-
>i_predict4x4[idx], i );

         i = 3;

}

else

         i = 0;

 

i_pred_mode is defined by below line.

 

int i_pred_mode = x264_mb_predict_intra4x4_mode( h, idx );

 

 

Here I have a question, “Why the encoder try to decide a best intra mode
among the mode 0,1,2(Vertical, Horizontal, DC) ?”        ß- See the red
part of the code

 

There is no time saving effect, no PSNR preservation…

 

if the QP is 26, a->i_lambda is 25 so satd[i_pred_mode] = -75 initially.

 

It’s a small value for the mode decision but I just wanna know for this
reason.

 

 

Looking forward to hearing from ones who can show me the reason soon….

 

 

Sincerely.

 

 

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