（二十一）x264_lookahead_get_frames函数

x264_lookahead_get_frames函数作用是先判断lookahead队列中的帧的类型，然后将lookahead队列中已经确定类型的一个非B帧和若干个B帧拷贝到frames.currents队列，以备后面编码使用。源码如下：

void x264_lookahead_get_frames( x264_t *h )
{
    if( h->param.i_sync_lookahead )  //多线程时的处理，先忽略
    {   /* We have a lookahead thread, so get frames from there */
        x264_pthread_mutex_lock( &h->lookahead->ofbuf.mutex );
        while( !h->lookahead->ofbuf.i_size && h->lookahead->b_thread_active )
            x264_pthread_cond_wait( &h->lookahead->ofbuf.cv_fill, &h->lookahead->ofbuf.mutex );
        x264_lookahead_encoder_shift( h );
        x264_pthread_mutex_unlock( &h->lookahead->ofbuf.mutex );
    }
    else  //单线程时的处理
    {   /* We are not running a lookahead thread, so perform all the slicetype decide on the fly */

        if( h->frames.current[0] || !h->lookahead->next.i_size )
            return;

        x264_stack_align( x264_slicetype_decide, h );  //对h->lookahead->next.list队列的帧判断类型
        x264_lookahead_update_last_nonb( h, h->lookahead->next.list[0] ); //next.list[0]是非B帧，将它赋给h->lookahead->last_nonb
        //shift_frames等于一个非B帧+i_bframes个B帧
        int shift_frames = h->lookahead->next.list[0]->i_bframes + 1;
        //将lookahead->next的0~shift_frames个元素搬到lookahead->ofbuf中
        x264_lookahead_shift( &h->lookahead->ofbuf, &h->lookahead->next, shift_frames );

        /* For MB-tree and VBV lookahead, we have to perform propagation analysis on I-frames too. */
        //对I帧进行propagation分析
        if( h->lookahead->b_analyse_keyframe && IS_X264_TYPE_I( h->lookahead->last_nonb->i_type ) )
            x264_stack_align( x264_slicetype_analyse, h, shift_frames );

        x264_lookahead_encoder_shift( h ); //将ofbuf数组中一个非B帧+i_bframes个B帧转移到current队列中
    }
}

画了个图辅助：