[x264-devel] reconfigure bitrate in ABR is by design or not

[Zhenan Lin]

Hi, dear experts,

 

We find that bitrate can be reconfigured in ABR. However, there are some
codes and comments showing that users can ONLY reconfigure bitrate in CBR:

 

x264_encoder_reconfig_apply @ encoder.c:

    /* Supported reconfiguration options (1-pass only):

     * vbv-maxrate

     * vbv-bufsize

     * crf

     * bitrate (CBR only) */

    if( !ret && rc_reconfig )

        x264_ratecontrol_init_reconfigurable( h, 0 );

 

x264_ratecontrol_init_reconfigurable @ ratecontrol.c:

        /* We don't support changing the ABR bitrate right now,

           so if the stream starts as CBR, keep it CBR. */

        if( rc->b_vbv_min_rate )

            h->param.rc.i_vbv_max_bitrate = h->param.rc.i_bitrate;

..

        if( rc->b_vbv_min_rate )

            rc->bitrate = (double)h->param.rc.i_bitrate * kilobit_size;

 

That means that we can ONLY reconfigure bitrate only when b_vbv_min_rate=1, 

and it was set when we initialized the encoder (b_init = 1) with CBR
(i_vbv_max_bitrate<=i_bitrate)

 

x264_validate_parameters @ encoder.c:

        else if( h->param.rc.i_vbv_max_bitrate < h->param.rc.i_bitrate &&

                 h->param.rc.i_rc_method == X264_RC_ABR )

        {

            x264_log( h, X264_LOG_WARNING, "max bitrate less than average
bitrate, assuming CBR\n" );

            h->param.rc.i_bitrate = h->param.rc.i_vbv_max_bitrate;

 

x264_ratecontrol_init_reconfigurable @ ratecontrol.c:

        if( b_init )

        {

                     ..

            rc->b_vbv_min_rate = !rc->b_2pass

                          && h->param.rc.i_rc_method == X264_RC_ABR

                          && h->param.rc.i_vbv_max_bitrate <=
h->param.rc.i_bitrate;

        }

 

However, if we initialize the encoder with ABR (i.e. --bitrate 499
--vbv-maxrate 500 --vbv-bufsize 500),

then we call x264_encoder_reconfig to reconfigure bitrate (i.e
param.rc.i_bitrate = 100, param.rc.i_vbv_max_bitrate = 100, param.rc.
i_vbv_buffer_size = 500),

the new values in param.rc will be copy to h->reconfig_h->param.rc DIRECTLY
in x264_encoder_try_reconfig:

 

int x264_encoder_reconfig( x264_t *h, x264_param_t *param )

{

    h = h->thread[h->thread[0]->i_thread_phase];

    x264_param_t param_save = h->reconfig_h->param;

    h->reconfig_h->param = h->param;

 

    int rc_reconfig;

    int ret = x264_encoder_try_reconfig( h->reconfig_h, param, &rc_reconfig
);

    if( !ret )

        h->reconfig = 1;

    else

        h->reconfig_h->param = param_save;

 

    return ret;

}

 

x264_encoder_try_reconfig at encoder.c:

    /* VBV can't be turned on if it wasn't on to begin with */

    if( h->param.rc.i_vbv_max_bitrate > 0 && h->param.rc.i_vbv_buffer_size >
0 &&

          param->rc.i_vbv_max_bitrate > 0 &&   param->rc.i_vbv_buffer_size >
0 )

    {

        *rc_reconfig |= h->param.rc.i_vbv_max_bitrate !=
param->rc.i_vbv_max_bitrate;

       *rc_reconfig |= h->param.rc.i_vbv_buffer_size !=
param->rc.i_vbv_buffer_size;

        *rc_reconfig |= h->param.rc.i_bitrate != param->rc.i_bitrate;

        COPY( rc.i_vbv_max_bitrate );

        COPY( rc.i_vbv_buffer_size );

        COPY( rc.i_bitrate );

}

 

After that, h->reconfig=1, and x264_encoder_reconfig_apply will be called in
x264_encoder_encode,

Then, the new values in h->reconfig_h->param.rc will be copy to h->param.rc
DIRECTLY in x264_encoder_try_reconfig.

As a result, the checking "if the stream starts as CBR, keep it CBR"in
x264_ratecontrol_init_reconfigurable will not take effects because new
values have been set into h->param.

 

X264_encoder_encode at encoder.c

    if( h->reconfig )

    {

        x264_encoder_reconfig_apply( h, &h->reconfig_h->param );

        h->reconfig = 0;

    }

 

int x264_encoder_reconfig_apply( x264_t *h, x264_param_t *param )

{

    int rc_reconfig;

    int ret = x264_encoder_try_reconfig( h, param, &rc_reconfig );

 

    mbcmp_init( h );

    if( !ret )

        x264_sps_init_reconfigurable( h->sps, &h->param );

 

    /* Supported reconfiguration options (1-pass only):

     * vbv-maxrate

     * vbv-bufsize

     * crf

     * bitrate (CBR only) */

    if( !ret && rc_reconfig )

        x264_ratecontrol_init_reconfigurable( h, 0 );

 

    return ret;

}

 

Therefore, we can change the bitrate in ABR indeed.

 

I wonder whether reconfigure bitrate in ABR is by design (the comments and
checking codes are legacy) or not (this reconfiguration behavior is
unexpected) ?

 

Thanks,

Zhenan

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