帧类型决策-x264_slicetype_analyse()

/*
	对lookahead队列中的帧分析，确定他们的帧类型

	过程：
		1.	若第一帧是AUTO/I，检查其相对于上一个非B帧是否场景切换，若场景切换则将当前帧设置为I，return
		2.	遍历，将所有的关键帧根据openGOP设置成I/IDR
		3.	将所有IDR帧前的AUTO/B帧改为P帧
		4.	若允许使用B帧
			·	使用X264_B_ADAPT_TRELLIS方法自适应设置B帧
			·	使用X264_B_ADAPT_FAST方法自适应设置B帧
			·	不自适应设置B帧
			若不允许使用B帧，则将全部的AUTO/B设置成P
		5.	将最后一帧AUTO/B帧改为P帧
		6.	若允许宏块树分析则进行宏块树分析
		7.	根据关键帧之间的距离限制检查帧类型
		8.	若允许vbv则进行vbv检查
*/
void x264_slicetype_analyse( x264_t *h, int intra_minigop )
{
   
   
    x264_mb_analysis_t a;
    x264_frame_t *frames[X264_LOOKAHEAD_MAX+3] = {
   
    NULL, };
    int num_frames, orig_num_frames, keyint_limit, framecnt;
    int i_max_search = X264_MIN( h->lookahead->next.i_size, X264_LOOKAHEAD_MAX );
    int b_vbv_lookahead = h->param.rc.i_vbv_buffer_size && h->param.rc.i_lookahead;
    /* For determinism we should limit the search to the number of frames lookahead has for sure
     * in h->lookahead->next.list buffer, except at the end of stream.
     * For normal calls with (intra_minigop == 0) that is h->lookahead->i_slicetype_length + 1 frames.
     * And for I-frame calls (intra_minigop != 0) we already removed intra_minigop frames from there. */
    if( h->param.b_deterministic )
        i_max_search = X264_MIN( i_max_search, h->lookahead->i_slicetype_length + 1 - intra_minigop );
    int keyframe = !!intra_minigop;

    assert( h->frames.b_have_lowres );

    if( !h->lookahead->last_nonb )	//若前面没有非B帧，则return
        return;

	//frame[0]存储前面最近的非B帧，frame[1~i_max_search]存储要分析的帧
    frames[0] = h->lookahead->last_nonb;	//取最近的非B帧为frame0
    for( framecnt = 0; framecnt < i_max_search; framecnt++ )	//从list中拷贝到frames中
        frames[framecnt+1] = h->lookahead->next.list[framecnt];

	//低分辨率上下文初始化
    lowres_context_init( h, &a );

    if( !framecnt )	//若framecnt = 0，即i_max_search = 0，则无帧分析，return
    {
   
   
        if( h->param.rc.b_mb_tree )
            macroblock_tree( h, &a, frames, 0, keyframe );
        return;
    }

	//计算keyint_limit = 最大关键帧距离 - 最近的非B帧 + 最近一个关键帧 - 1
    keyint_limit = h->param.i_keyint_max - frames[0]->i_frame + h->lookahead->i_last_keyframe - 1;

	//计算orig_num_frames和num_frames
    orig_num_frames = num_frames = h->param.b_intra_refresh ? framecnt : X264_MIN( framecnt, keyint_limit );

    /* This is important psy-wise: if we have a non-scenecut keyframe,
     * there will be significant visual artifacts if the frames just before
     * go down in quality due to being referenced less, despite it being
     * more RD-optimal. 
	 * 如果我们有一个非场景切换的关键帧，将会有一个明显的视觉影响
	 */
    if( (h->param.analyse.b_psy && h->param.rc.b_mb_tree) || b_vbv_lookahead )
        num_frames = framecnt;
    else if( h->param.b_open_gop && num_frames < framecnt )
        num_frames++;
    else if( num_frames == 0 )
    {
   
   
        frames[1]->i_type = X264_TYPE_I;
        return;
    }

    if( IS_X264_TYPE_AUTO_OR_I( frames[1]->i_type ) &&
        h->param.i_scenecut_threshold && scenecut( h, &a, frames, 0, 1, 1, orig_num_frames, i_max_search ) )
    {
   
   	//若是第一帧是auto/I 且 相对于上一个非B帧有场景切换，则设置为I帧
        if( frames[1]->i_type == X264_TYPE_AUTO )	
            frames[1]->i_type = X264_TYPE_I;	//当前帧定为I帧
        return;
    }

    /* Replace forced keyframes with I/IDR-frames */
    for( int j = 1; j <= num_frames; j++ )
    {
   
   	////遍历检查是否关键帧，依openGOP设定为I/IDR
        if( frames[j]->i_type == X264_TYPE_KEYFRAME )
            frames[j]->i_type = h->param.b_open_gop ? X264_TYPE_I : X264_TYPE_IDR;
    }

    /* Close GOP at IDR-frames */
    for( int j = 2; j <= num_frames; j++ )
    {
   
   	//遍历检查IDR帧，将IDR帧前的AUTO/B帧设置为P帧
        if( frames[j]->i_type == X264_TYPE_IDR && IS_X264_TYPE_AUTO_OR_B( frames[j-1]->i_type ) )
            frames[j-1]->i_type = X264_TYPE_P;
    }

    int num_analysed_frames = num_frames;
    int reset_start;

    if( h->param.i_bframe )	//若允许B帧
    {
   
   
		/*	根据i_bframe_adaptive来进行自适应B帧设置
			i_bframe_adaptive = X2