帧类型决策-scenecut()

scenecut_internal

/*
	只判断p1相对于p0是否进是两幅不同的场景
	若是若是不同的场景就返回1，否则0

	分别计算将p1作为I帧的开销intra_cost 和 p1作为P帧以p0为参考帧时的开销inter_cost
	若inter_cost >= (1.0 - f_bias) * intra_cost，则认定p1相对p0是不同的场景
*/
static int scenecut_internal( x264_t *h, x264_mb_analysis_t *a, x264_frame_t **frames, int p0, int p1, int real_scenecut )
{
    x264_frame_t *frame = frames[p1];	//取p1帧

    /* Don't do scenecuts on the right view of a frame-packed video. */
    if( real_scenecut && h->param.i_frame_packing == 5 && (frame->i_frame&1) )
        return 0;

    slicetype_frame_cost( h, a, frames, p0, p1, p1 );	//计算当前帧作为P帧的开销

    int icost = frame->i_cost_est[0][0];	//得到帧内预测的开销
    int pcost = frame->i_cost_est[p1-p0][0];//得到帧间预测的开销
    float f_bias;
    int i_gop_size = frame->i_frame - h->lookahead->i_last_keyframe;//得到GOP的大小，即当前帧-上一个关键帧
    float f_thresh_max = h->param.i_scenecut_threshold / 100.0;	//得到场景切换的阈值，即max
    /* magic numbers pulled out of thin air */
    float f_thresh_min = f_thresh_max * 0.25;	//得到场景切换阈值的最小值，即min=max*0.25
    int res;

    if( h->param.i_keyint_min == h->param.i_keyint_max )	//若关键帧之间的最大最小距离相等，即为一个定值
        f_thresh_min = f_thresh_max;	//则最大最小场景切换阈值为定值=max
	
	//根据当前帧距离上一个关键帧的距离来计算f_bias
    if( i_gop_size <= h->param.i_keyint_min / 4 || h->param.b_intra_refresh )
		//当前帧距离上一个关键帧的距离 <= 关键帧之间最小距离的1/4 ，或允许使用帧内预测进行刷新
        f_bias = f_thresh_min / 4;
    else if( i_gop_size <= h->param.i_keyint_min )
		//关键帧之间最小距离的1/4 < 当前帧距离上一个关键帧的距离 <= 关键帧之间最小距离
        f_bias = f_thresh_min * i_gop_size / h->param.i_keyint_min;
    else
    {
        f_bias = f_thresh_min
                 + ( f_thresh_max - f_thresh_min )
                 * ( i_gop_size - h->param.i_keyint_min )
                 / ( h->param.i_keyint_max - h->param.i_keyint_min );
    }

	//P帧的开销 >= （1.0-f_bias） * I帧的开销？
    res = pcost >= (1.0 - f_bias) * icost;

    if( res && real_scenecut )	//若res且real_scenenut，则判定为不同的场景，输出log
    {
        int imb = frame->i_intra_mbs[p1-p0];
        int pmb = NUM_MBS - imb;
        x264_log( h, X264_LOG_DEBUG, "scene cut at %d Icost:%d Pcost:%d ratio:%.4f bias:%.4f gop:%d (imb:%d pmb:%d)\n",
                  frame->i_frame,
                  icost, pcost, 1. - (double)pcost / icost,
                  f_bias, i_gop_size, imb, pmb );
    }

	/* 若P帧的开销 >= （1.0-f_bias） * I帧的开销，则判定为p1相对p0是不同的场景，返回1
	 * 否则认为是相同的场景，返回0. */
    return res;
}

scenecut

static int scenecut( x264_t *h, x264_mb_analysis_t *a, x264_frame_t **frames, int p0, int p1, int real_scenecut, int num_frames, int i_max_search )
{
    /* Only do analysis during a normal scenecut check. */
    if( real_scenecut && h->param.i_bframe )	//允许B帧且real_scenecut?
    {
        int origmaxp1 = p0 + 1;
        /* Look ahead to avoid coding short flashes as scenecuts. */
        if( h->param.i_bframe_adaptive == X264_B_ADAPT_TRELLIS )	//使用viterbi最优路径进行B帧自适应选择
            /* Don't analyse any more frames than the trellis would have covered. */
            origmaxp1 += h->param.i_bframe;	//
        else
            origmaxp1++;
        int maxp1 = X264_MIN( origmaxp1, num_frames );	//选择min作为maxpl

        /* Where A and B are scenes: AAAAAABBBAAAAAA
         * If BBB is shorter than (maxp1-p0), it is detected as a flash
         * and not considered a scenecut. 
		 *
		 * 当B场景的长度小于maxp1-p0，那么将被视为一个场景闪现，不被考虑成场景切换
		 * 提出问题：若AAAAAAAAAAAABBBAAAAAABBBAAAAAABBBAAAAAAAAA呢？这样我觉得第
		 * 一个B定为场景切换帧应该是一个不错的选择，可以考虑优化. */
        for( int curp1 = p1; curp1 <= maxp1; curp1++ )	//遍历p1到maxp1
            if( !scenecut_internal( h, a, frames, p0, curp1, 0 ) )	//若curp1相对于p0没有场景切换
                /* Any frame in between p0 and cur_p1 cannot be a real scenecut. */
				//从p0到当前curp1内的所有帧都不可作为一个场景切换帧
                for( int i = curp1; i > p0; i-- )
                    frames[i]->b_scenecut = 0;

        /* Where A-F are scenes: AAAAABBCCDDEEFFFFFF
         * If each of BB ... EE are shorter than (maxp1-p0), they are
         * detected as flashes and not considered scenecuts.
         * Instead, the first F frame becomes a scenecut.
         * If the video ends before F, no frame becomes a scenecut. 
		 * 
		 * 若每一个场景B/C/.../E都短于maxp1-p0，那么他们都被视为一个场景闪现
		 * 不被考虑成场景切换，第一帧F被视为场景切换. */
        for( int curp0 = p0; curp0 <= maxp1; curp0++ )	//从p0一直遍历到maxp1 
            if( origmaxp1 > i_max_search || (curp0 < maxp1 && scenecut_internal( h, a, frames, curp0, maxp1, 0 )) )
                /* If cur_p0 is the p0 of a scenecut, it cannot be the p1 of a scenecut. */
                    frames[curp0]->b_scenecut = 0;
    }

    /* Ignore frames that are part of a flash, i.e. cannot be real scenecuts. */
    if( !frames[p1]->b_scenecut )	//若p1不被允许作为场景切换
        return 0;

	//返回p1相对p0是否进行了场景切换
    return scenecut_internal( h, a, frames, p0, p1, real_scenecut );
}