在前面的过程中，我们得到了编码图像，编码器开始H264视频编码VCL。首先初始化有关参数，包括帧类型的获取、创建多参考帧的列表、初始化码流控制、初始化写码流结构和写SPS、PPS头结构信息。
1.初始化相关参数
若为IDR帧，则意味着一个新图像片的开始。在H264中为了防止编码错误扩散，规定当前片不以本片以外的其他帧为参考，因而编码器遇到IDR帧则需要重置参考帧区域。同时，将SEI、SPS、PPS分别单独写入NAL单元。这三个参数集集合了编解码的核心参数，直接关系解码端能否正常解码。
若不是IDR帧，依据帧类型设定当前NAL单元的类型和图像片的类型。接着进行一系列初始化参数的操作，包括建立关于参考帧的list0和list1，初始化码流控制得到量化步长、参考帧等相关信息，初始化写比特流环境变量等。
在函数x264_lookahead_put_frame（）中，将fenc放入lookanead.next.list[]队列，再调用函数x264_lookahead_get_frame（）判断帧的类型。
x264_lookahead_get_frame（）：
//通过lookahead分析帧类型
void x264_lookahead_get_frames( x264_t *h )
{
if( h->param.i_sync_lookahead )
{ /* 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
{ /* We are not running a lookahead thread, so perform all the slicetype decide on the fly */
//currect[]必须为空，next不能为空？
if( h->frames.current[0] || !h->lookahead->next.i_size )
return;
//分析lookahead->next->list帧的类型
x264_stack_align( x264_slicetype_decide, h );
//更新lookahead->last_nonb
x264_lookahead_update_last_nonb( h, h->lookahead->next.list[0] );
int shift_frames = h->lookahead->next.list[0]->i_bframes + 1;
//lookahead->next.list移动到lookahead->ofbuf.list
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. */
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 ); //lookahead->ofbuf.list帧移动到frames->current
x264_lookahead_encoder_shift( h );
}
}
函数 x264_lookahead_get_frame（）里调用了x264_slicetype_decide（）函数来判断帧的类型：
//确定帧的类型（I、B、P）
void x264_slicetype_decide( x264_t *h )
{
x264_frame_t *frames[X264_BFRAME_MAX+2];
x264_frame_t *frm;
int bframes;
int brefs; if( !h->lookahead->next.i_size )
return; int lookahead_size = h->lookahead->next.i_size; //遍历next队列
for( int i = 0; i < h->lookahead->next.i_size; i++ )
{
if( h->param.b_vfr_input )
{
if( lookahead_size-- > 1 )
h->lookahead->next.list[i]->i_duration = 2 * (h->lookahead->next.list[i+1]->i_pts - h->lookahead->next.list[i]->i_pts);
else
h->lookahead->next.list[i]->i_duration = h->i_prev_duration;
}
else
h->lookahead->next.list[i]->i_duration = delta_tfi_divisor[h->lookahead->next.list[i]->i_pic_struct];
h->i_prev_duration = h->lookahead->next.list[i]->i_duration;
h->lookahead->next.list[i]->f_duration = (double)h->lookahead->next.list[i]->i_duration
* h->sps->vui.i_num_units_in_tick
/ h->sps->vui.i_time_scale; if( h->lookahead->next.list[i]->i_frame > h->i_disp_fields_last_frame && lookahead_size > 0 )
{
h->lookahead->next.list[i]->i_field_cnt = h->i_disp_fields;
h->i_disp_fields += h->lookahead->next.list[i]->i_duration;
h->i_disp_fields_last_frame = h->lookahead->next.list[i]->i_frame;
}
else if( lookahead_size == 0 )
{
h->lookahead->next.list[i]->i_field_cnt = h->i_disp_fields;
h->lookahead->next.list[i]->i_duration = h->i_prev_duration;
}
} if( h->param.rc.b_stat_read )
{
//b_stat_read在2pass模式的第2遍才不为0 /* Use the frame types from the first pass */
for( int i = 0; i < h->lookahead->next.i_size; i++ )
h->lookahead->next.list[i]->i_type =
x264_ratecontrol_slice_type( h, h->lookahead->next.list[i]->i_frame );
}
else if( (h->param.i_bframe && h->param.i_bframe_adaptive)
|| h->param.i_scenecut_threshold
|| h->param.rc.b_mb_tree
|| (h->param.rc.i_vbv_buffer_size && h->param.rc.i_lookahead) )
x264_slicetype_analyse( h, 0 );//分析帧的类型（I、B、P）
//=========================================================================== for( bframes = 0, brefs = 0;; bframes++ )
{
//从next队列取出1个
frm = h->lookahead->next.list[bframes];
//BREF的处理
if( frm->i_type == X264_TYPE_BREF && h->param.i_bframe_pyramid < X264_B_PYRAMID_NORMAL &&
brefs == h->param.i_bframe_pyramid )
{
//BREF改成B
frm->i_type = X264_TYPE_B;
x264_log( h, X264_LOG_WARNING, "B-ref at frame %d incompatible with B-pyramid %s \n",
frm->i_frame, x264_b_pyramid_names[h->param.i_bframe_pyramid] );
}
/* pyramid with multiple B-refs needs a big enough dpb that the preceding P-frame stays available.
smaller dpb could be supported by smart enough use of mmco, but it's easier just to forbid it. */
else if( frm->i_type == X264_TYPE_BREF && h->param.i_bframe_pyramid == X264_B_PYRAMID_NORMAL &&
brefs && h->param.i_frame_reference <= (brefs+3) )
{
frm->i_type = X264_TYPE_B;
x264_log( h, X264_LOG_WARNING, "B-ref at frame %d incompatible with B-pyramid %s and %d reference frames\n",
frm->i_frame, x264_b_pyramid_names[h->param.i_bframe_pyramid], h->param.i_frame_reference );
}
//Keyframe处理
if( frm->i_type == X264_TYPE_KEYFRAME )
frm->i_type = h->param.b_open_gop ? X264_TYPE_I : X264_TYPE_IDR; /* Limit GOP size */
if( (!h->param.b_intra_refresh || frm->i_frame == 0) && frm->i_frame - h->lookahead->i_last_keyframe >= h->param.i_keyint_max )
{
if( frm->i_type == X264_TYPE_AUTO || frm->i_type == X264_TYPE_I )
frm->i_type = h->param.b_open_gop && h->lookahead->i_last_keyframe >= 0 ? X264_TYPE_I : X264_TYPE_IDR;
int warn = frm->i_type != X264_TYPE_IDR;
if( warn && h->param.b_open_gop )
warn &= frm->i_type != X264_TYPE_I;
if( warn )
{
x264_log( h, X264_LOG_WARNING, "specified frame type (%d) at %d is not compatible with keyframe interval\n", frm->i_type, frm->i_frame );
frm->i_type = h->param.b_open_gop && h->lookahead->i_last_keyframe >= 0 ? X264_TYPE_I : X264_TYPE_IDR;
}
}
if( frm->i_type == X264_TYPE_I && frm->i_frame - h->lookahead->i_last_keyframe >= h->param.i_keyint_min )
{
if( h->param.b_open_gop )
{
h->lookahead->i_last_keyframe = frm->i_frame; // Use display order
if( h->param.b_bluray_compat )
h->lookahead->i_last_keyframe -= bframes; // Use bluray order
frm->b_keyframe = 1;
}
else
frm->i_type = X264_TYPE_IDR;
}
if( frm->i_type == X264_TYPE_IDR )
{
/* Close GOP */
//设置当前帧为“上一个关键帧”
h->lookahead->i_last_keyframe = frm->i_frame;
frm->b_keyframe = 1;
if( bframes > 0 )
{
bframes--;
h->lookahead->next.list[bframes]->i_type = X264_TYPE_P;
}
} if( bframes == h->param.i_bframe ||
!h->lookahead->next.list[bframes+1] )
{
if( IS_X264_TYPE_B( frm->i_type ) )
x264_log( h, X264_LOG_WARNING, "specified frame type is not compatible with max B-frames\n" );
if( frm->i_type == X264_TYPE_AUTO
|| IS_X264_TYPE_B( frm->i_type ) )
frm->i_type = X264_TYPE_P;
} if( frm->i_type == X264_TYPE_BREF )
brefs++; if( frm->i_type == X264_TYPE_AUTO )
frm->i_type = X264_TYPE_B; else if( !IS_X264_TYPE_B( frm->i_type ) ) break;
} if( bframes )
h->lookahead->next.list[bframes-1]->b_last_minigop_bframe = 1;
h->lookahead->next.list[bframes]->i_bframes = bframes; /* insert a bref into the sequence */
if( h->param.i_bframe_pyramid && bframes > 1 && !brefs )
{
h->lookahead->next.list[bframes/2]->i_type = X264_TYPE_BREF;
brefs++;
} /* calculate the frame costs ahead of time for x264_rc_analyse_slice while we still have lowres */
if( h->param.rc.i_rc_method != X264_RC_CQP )
{
x264_mb_analysis_t a;
int p0, p1, b;
p1 = b = bframes + 1; x264_lowres_context_init( h, &a ); frames[0] = h->lookahead->last_nonb;
memcpy( &frames[1], h->lookahead->next.list, (bframes+1) * sizeof(x264_frame_t*) );
if( IS_X264_TYPE_I( h->lookahead->next.list[bframes]->i_type ) )
p0 = bframes + 1;
else // P
p0 = 0; x264_slicetype_frame_cost( h, &a, frames, p0, p1, b, 0 ); if( (p0 != p1 || bframes) && h->param.rc.i_vbv_buffer_size )
{
/* We need the intra costs for row SATDs. */
x264_slicetype_frame_cost( h, &a, frames, b, b, b, 0 ); /* We need B-frame costs for row SATDs. */
p0 = 0;
for( b = 1; b <= bframes; b++ )
{
if( frames[b]->i_type == X264_TYPE_B )
for( p1 = b; frames[p1]->i_type == X264_TYPE_B; )
p1++;
else
p1 = bframes + 1;
x264_slicetype_frame_cost( h, &a, frames, p0, p1, b, 0 );
if( frames[b]->i_type == X264_TYPE_BREF )
p0 = b;
}
}
} /* Analyse for weighted P frames */
if( !h->param.rc.b_stat_read && h->lookahead->next.list[bframes]->i_type == X264_TYPE_P
&& h->param.analyse.i_weighted_pred >= X264_WEIGHTP_SIMPLE )
{
x264_emms();
x264_weights_analyse( h, h->lookahead->next.list[bframes], h->lookahead->last_nonb, 0 );
} /* shift sequence to coded order.
use a small temporary list to avoid shifting the entire next buffer around */
int i_coded = h->lookahead->next.list[0]->i_frame;
if( bframes )
{
int idx_list[] = { brefs+1, 1 };
for( int i = 0; i < bframes; i++ )
{
int idx = idx_list[h->lookahead->next.list[i]->i_type == X264_TYPE_BREF]++;
frames[idx] = h->lookahead->next.list[i];
frames[idx]->i_reordered_pts = h->lookahead->next.list[idx]->i_pts;
}
frames[0] = h->lookahead->next.list[bframes];
frames[0]->i_reordered_pts = h->lookahead->next.list[0]->i_pts;
memcpy( h->lookahead->next.list, frames, (bframes+1) * sizeof(x264_frame_t*) );
} for( int i = 0; i <= bframes; i++ )
{
h->lookahead->next.list[i]->i_coded = i_coded++;
if( i )
{
x264_calculate_durations( h, h->lookahead->next.list[i], h->lookahead->next.list[i-1], &h->i_cpb_delay, &h->i_coded_fields );
h->lookahead->next.list[0]->f_planned_cpb_duration[i-1] = (double)h->lookahead->next.list[i]->i_cpb_duration *
h->sps->vui.i_num_units_in_tick / h->sps->vui.i_time_scale;
}
else
x264_calculate_durations( h, h->lookahead->next.list[i], NULL, &h->i_cpb_delay, &h->i_coded_fields );
}
}
在这个函数里又调用了另一个重要函数：x264_slicetype_analyse（），用来分析帧的类型。
//分析帧的类型（I、B、P）
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_mb_count = NUM_MBS;
int cost1p0, cost2p0, cost1b1, cost2p1;
// 确定最大的搜索长度
// 在我的调试当中， h->lookahead->next.i_size = 4
int i_max_search = X264_MIN( h->lookahead->next.i_size, X264_LOOKAHEAD_MAX );
int 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 )
return;
//frames[0]指向上一次的非B帧
frames[0] = h->lookahead->last_nonb;
//frames[] 依次指向 lookahead->next链表中的帧
for( framecnt = 0; framecnt < i_max_search && h->lookahead->next.list[framecnt]->i_type == X264_TYPE_AUTO; framecnt++ )
frames[framecnt+1] = h->lookahead->next.list[framecnt]; x264_lowres_context_init( h, &a ); if( !framecnt )
{
if( h->param.rc.b_mb_tree )
x264_macroblock_tree( h, &a, frames, 0, keyframe );
return;
} keyint_limit = h->param.i_keyint_max - frames[0]->i_frame + h->lookahead->i_last_keyframe - 1;
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. */
if( (h->param.analyse.b_psy && h->param.rc.b_mb_tree) || 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;
} int num_bframes = 0;
int num_analysed_frames = num_frames;
int reset_start;
//通过scenecut()函数判断是否有场景切换，从而确定I帧
if( h->param.i_scenecut_threshold && scenecut( h, &a, frames, 0, 1, 1, orig_num_frames, i_max_search ) )
{
frames[1]->i_type = X264_TYPE_I;
return;
}#if HAVE_OPENCL
x264_opencl_slicetype_prep( h, frames, num_frames, a.i_lambda );
#endif
//允许有B帧的时候
if( h->param.i_bframe )
{
if( h->param.i_bframe_adaptive == X264_B_ADAPT_TRELLIS )
{
if( num_frames > 1 )
{
char best_paths[X264_BFRAME_MAX+1][X264_LOOKAHEAD_MAX+1] = {"","P"};
int best_path_index = num_frames % (X264_BFRAME_MAX+1); /* Perform the frametype analysis. */
for( int j = 2; j <= num_frames; j++ )
x264_slicetype_path( h, &a, frames, j, best_paths ); num_bframes = strspn( best_paths[best_path_index], "B" );
/* Load the results of the analysis into the frame types. */
for( int j = 1; j < num_frames; j++ )
frames[j]->i_type = best_paths[best_path_index][j-1] == 'B' ? X264_TYPE_B : X264_TYPE_P;
}
frames[num_frames]->i_type = X264_TYPE_P;
}
else if( h->param.i_bframe_adaptive == X264_B_ADAPT_FAST )
{
for( int i = 0; i <= num_frames-2; )
{
//i+2作为Ｐ帧编码的代价
//注：i+2始终为P帧
cost2p1 = x264_slicetype_frame_cost( h, &a, frames, i+0, i+2, i+2, 1 );
if( frames[i+2]->i_intra_mbs[2] > i_mb_count / 2 )
{
frames[i+1]->i_type = X264_TYPE_P;
frames[i+2]->i_type = X264_TYPE_P;
i += 2;
continue;
}#if HAVE_OPENCL
if( h->param.b_opencl )
{
int b_work_done = 0;
b_work_done |= x264_opencl_precalculate_frame_cost(h, frames, a.i_lambda, i+0, i+2, i+1 );
b_work_done |= x264_opencl_precalculate_frame_cost(h, frames, a.i_lambda, i+0, i+1, i+1 );
b_work_done |= x264_opencl_precalculate_frame_cost(h, frames, a.i_lambda, i+1, i+2, i+2 );
if( b_work_done )
x264_opencl_flush( h );
}
#endif
//计算代价
//x264_slicetype_frame_cost(,,,p0,p1,b,)
//p0 b p1
//p1!=b为B帧，否则为P帧 // i + 1 作为B帧编码的代价
cost1b1 = x264_slicetype_frame_cost( h, &a, frames, i+0, i+2, i+1, 0 );
// i + 1 作为P帧编码的代价
cost1p0 = x264_slicetype_frame_cost( h, &a, frames, i+0, i+1, i+1, 0 );
// i + 2 作为P帧编码的代价
cost2p0 = x264_slicetype_frame_cost( h, &a, frames, i+1, i+2, i+2, 0 );
//如果i+1作为P帧编码的代价 + i+2作为P帧编码的代价
//小于 i+1作为B帧编码的代价 + i+2作为P帧编码的代价
if( cost1p0 + cost2p0 < cost1b1 + cost2p1 )
{
//那么i+1将作为P帧编码
//然后直接continue
frames[i+1]->i_type = X264_TYPE_P;
i += 1;
continue;
} // arbitrary and untuned
#define INTER_THRESH 300
#define P_SENS_BIAS (50 - h->param.i_bframe_bias) // i+1 将作为B帧编码
frames[i+1]->i_type = X264_TYPE_B; int j;
for( j = i+2; j <= X264_MIN( i+h->param.i_bframe, num_frames-1 ); j++ )
{
int pthresh = X264_MAX(INTER_THRESH - P_SENS_BIAS * (j-i-1), INTER_THRESH/10);
// 预测j+1作为P帧编码代价
int pcost = x264_slicetype_frame_cost( h, &a, frames, i+0, j+1, j+1, 1 );
// 如果pcost 满足下述条件， 则确定了一个P帧，跳出循环
if( pcost > pthresh*i_mb_count || frames[j+1]->i_intra_mbs[j-i+1] > i_mb_count/3 )
break;
// 否则就是B帧
frames[j]->i_type = X264_TYPE_B;
}
// 将j帧确定为P帧
frames[j]->i_type = X264_TYPE_P;
i = j;
}
// 最后一帧确定为P帧
frames[num_frames]->i_type = X264_TYPE_P;
num_bframes = 0;
// 确定有多少个B帧
while( num_bframes < num_frames && frames[num_bframes+1]->i_type == X264_TYPE_B )
num_bframes++;
}
else
{
// 确定多少B帧
num_bframes = X264_MIN(num_frames-1, h->param.i_bframe);
// 每num_bframes + 1一个P帧， 其余皆为B帧
for( int j = 1; j < num_frames; j++ )
frames[j]->i_type = (j%(num_bframes+1)) ? X264_TYPE_B : X264_TYPE_P;
// 最后一帧为P帧
frames[num_frames]->i_type = X264_TYPE_P;
} /* Check scenecut on the first minigop. */
// 如果B帧中， 有帧有场景切换， 则改变其为P帧
for( int j = 1; j < num_bframes+1; j++ )
if( h->param.i_scenecut_threshold && scenecut( h, &a, frames, j, j+1, 0, orig_num_frames, i_max_search ) )
{
frames[j]->i_type = X264_TYPE_P;
num_analysed_frames = j;
break;
} reset_start = keyframe ? 1 : X264_MIN( num_bframes+2, num_analysed_frames+1 );
}
else
{
//h->param.i_bframe为 0
//则所有的帧皆为P帧
for( int j = 1; j <= num_frames; j++ )
frames[j]->i_type = X264_TYPE_P;
reset_start = !keyframe + 1;
num_bframes = 0;
} /* Perform the actual macroblock tree analysis.
* Don't go farther than the maximum keyframe interval; this helps in short GOPs. */
if( h->param.rc.b_mb_tree )
x264_macroblock_tree( h, &a, frames, X264_MIN(num_frames, h->param.i_keyint_max), keyframe ); /* Enforce keyframe limit. */
if( !h->param.b_intra_refresh )
for( int i = keyint_limit+1; i <= num_frames; i += h->param.i_keyint_max )
{
//迫使为I帧
frames[i]->i_type = X264_TYPE_I;
reset_start = X264_MIN( reset_start, i+1 );
if( h->param.b_open_gop && h->param.b_bluray_compat )
while( IS_X264_TYPE_B( frames[i-1]->i_type ) )
i--;
} if( vbv_lookahead )
x264_vbv_lookahead( h, &a, frames, num_frames, keyframe ); /* Restore frametypes for all frames that haven't actually been decided yet. */
for( int j = reset_start; j <= num_frames; j++ )
frames[j]->i_type = X264_TYPE_AUTO;#if HAVE_OPENCL
x264_opencl_slicetype_end( h );
#endif
}
获取了帧的类型后，调用函数x264_slice_init（）创建Slice Header。
//创建Slice Header
static inline void x264_slice_init( x264_t *h, int i_nal_type, int i_global_qp )
{
/* ------------------------ Create slice header ----------------------- */
if( i_nal_type == NAL_SLICE_IDR )
{
//I帧 //对x264_slice_header_t进行赋值
x264_slice_header_init( h, &h->sh, h->sps, h->pps, h->i_idr_pic_id, h->i_frame_num, i_global_qp ); /* alternate id */
if( h->param.i_avcintra_class )
{
switch( h->i_idr_pic_id )
{
case 5:
h->i_idr_pic_id = 3;
break;
case 3:
h->i_idr_pic_id = 4;
break;
case 4:
default:
h->i_idr_pic_id = 5;
break;
}
}
else
h->i_idr_pic_id ^= 1;
}
else
{
//非IDR帧
x264_slice_header_init( h, &h->sh, h->sps, h->pps, -1, h->i_frame_num, i_global_qp );
//参考帧列表
h->sh.i_num_ref_idx_l0_active = h->i_ref[0] <= 0 ? 1 : h->i_ref[0];
h->sh.i_num_ref_idx_l1_active = h->i_ref[1] <= 0 ? 1 : h->i_ref[1];
if( h->sh.i_num_ref_idx_l0_active != h->pps->i_num_ref_idx_l0_default_active ||
(h->sh.i_type == SLICE_TYPE_B && h->sh.i_num_ref_idx_l1_active != h->pps->i_num_ref_idx_l1_default_active) )
{
h->sh.b_num_ref_idx_override = 1;
}
} if( h->fenc->i_type == X264_TYPE_BREF && h->param.b_bluray_compat && h->sh.i_mmco_command_count )
{
h->b_sh_backup = 1;
h->sh_backup = h->sh;
} h->fdec->i_frame_num = h->sh.i_frame_num; if( h->sps->i_poc_type == 0 )
{
h->sh.i_poc = h->fdec->i_poc;
if( PARAM_INTERLACED )
{
h->sh.i_delta_poc_bottom = h->param.b_tff ? 1 : -1;
h->sh.i_poc += h->sh.i_delta_poc_bottom == -1;
}
else
h->sh.i_delta_poc_bottom = 0;
h->fdec->i_delta_poc[0] = h->sh.i_delta_poc_bottom == -1;
h->fdec->i_delta_poc[1] = h->sh.i_delta_poc_bottom == 1;
}
else
{
/* Nothing to do ? */
}
//主要对mb结构体赋初值
x264_macroblock_slice_init( h );
}
做好一切准备后，接下来就是对视频图像的实际编码了。
2.编码
编码的主要函数是x264_slices_write（）：
//真正的编码——编码1个图像帧
//注意“slice”后面有一个“s”
//它其中又调用了一个x264_slice_write()
//这一点要区分开
static void *x264_slices_write( x264_t *h )
{
int i_slice_num = 0;
int last_thread_mb = h->sh.i_last_mb; /* init stats */
memset( &h->stat.frame, 0, sizeof(h->stat.frame) );
h->mb.b_reencode_mb = 0;
//循环每一个slice（一幅图像可以由多个Slice构成）
while( h->sh.i_first_mb + SLICE_MBAFF*h->mb.i_mb_stride <= last_thread_mb )
{
h->sh.i_last_mb = last_thread_mb;
if( !i_slice_num || !x264_frame_new_slice( h, h->fdec ) )
{
if( h->param.i_slice_max_mbs )
{
if( SLICE_MBAFF )
{
// convert first to mbaff form, add slice-max-mbs, then convert back to normal form
int last_mbaff = 2*(h->sh.i_first_mb % h->mb.i_mb_width)
+ h->mb.i_mb_width*(h->sh.i_first_mb / h->mb.i_mb_width)
+ h->param.i_slice_max_mbs - 1;
int last_x = (last_mbaff % (2*h->mb.i_mb_width))/2;
int last_y = (last_mbaff / (2*h->mb.i_mb_width))*2 + 1;
h->sh.i_last_mb = last_x + h->mb.i_mb_stride*last_y;
}
else
{
h->sh.i_last_mb = h->sh.i_first_mb + h->param.i_slice_max_mbs - 1;
if( h->sh.i_last_mb < last_thread_mb && last_thread_mb - h->sh.i_last_mb < h->param.i_slice_min_mbs )
h->sh.i_last_mb = last_thread_mb - h->param.i_slice_min_mbs;
}
i_slice_num++;
}
else if( h->param.i_slice_count && !h->param.b_sliced_threads )
{
int height = h->mb.i_mb_height >> PARAM_INTERLACED;
int width = h->mb.i_mb_width << PARAM_INTERLACED;
i_slice_num++;
h->sh.i_last_mb = (height * i_slice_num + h->param.i_slice_count/2) / h->param.i_slice_count * width - 1;
}
}
h->sh.i_last_mb = X264_MIN( h->sh.i_last_mb, last_thread_mb );
//真正的编码——编码1个Slice
//x264_stack_align()应该是平台优化过程中内存对齐的工作
//实际上就是调用x264_slice_write()
if( x264_stack_align( x264_slice_write, h ) )
goto fail;
//注意这里对i_first_mb进行了赋值
h->sh.i_first_mb = h->sh.i_last_mb + 1;
// if i_first_mb is not the last mb in a row then go to the next mb in MBAFF order
if( SLICE_MBAFF && h->sh.i_first_mb % h->mb.i_mb_width )
h->sh.i_first_mb -= h->mb.i_mb_stride;
} return (void *)0;fail:
/* Tell other threads we're done, so they wouldn't wait for it */
if( h->param.b_sliced_threads )
x264_threadslice_cond_broadcast( h, 2 );
return (void *)-1;
}
其内部调用了函数x264_slice_write（），x264_slices_write（）的单位是帧，而x264_slice_write（）的单位是Slice，这两个是不一样的，一帧可以有多个Slice，一帧也可以只有一个Slice。
x264_slice_write（）：
static intptr_t x264_slice_write( x264_t *h )
{
int i_skip;
//宏块的序号，以及序号对应的x，y坐标
int mb_xy, i_mb_x, i_mb_y;
/* NALUs other than the first use a 3-byte startcode.
* Add one extra byte for the rbsp, and one more for the final CABAC putbyte.
* Then add an extra 5 bytes just in case, to account for random NAL escapes and
* other inaccuracies. */
int overhead_guess = (NALU_OVERHEAD - (h->param.b_annexb && h->out.i_nal)) + 1 + h->param.b_cabac + 5;
int slice_max_size = h->param.i_slice_max_size > 0 ? (h->param.i_slice_max_size-overhead_guess)*8 : 0;
int back_up_bitstream_cavlc = !h->param.b_cabac && h->sps->i_profile_idc < PROFILE_HIGH;
int back_up_bitstream = slice_max_size || back_up_bitstream_cavlc;
int starting_bits = bs_pos(&h->out.bs);
int b_deblock = h->sh.i_disable_deblocking_filter_idc != 1;
int b_hpel = h->fdec->b_kept_as_ref;
int orig_last_mb = h->sh.i_last_mb;
int thread_last_mb = h->i_threadslice_end * h->mb.i_mb_width - 1;
uint8_t *last_emu_check;
#define BS_BAK_SLICE_MAX_SIZE 0
#define BS_BAK_CAVLC_OVERFLOW 1
#define BS_BAK_SLICE_MIN_MBS 2
#define BS_BAK_ROW_VBV 3
x264_bs_bak_t bs_bak[4];
b_deblock &= b_hpel || h->param.b_full_recon || h->param.psz_dump_yuv;
bs_realign( &h->out.bs ); /* Slice */
//开始输出一个NAL
//后面对应着x264_nal_end()
x264_nal_start( h, h->i_nal_type, h->i_nal_ref_idc );
h->out.nal[h->out.i_nal].i_first_mb = h->sh.i_first_mb; /* Slice header */ //存储宏块像素的缓存fdec_buf和fenc_buf的初始化
//宏块编码缓存p_fenc[0]，p_fenc[1]，p_fenc[2]
//宏块重建缓存p_fdec[0]，p_fdec[1]，p_fdec[2]
//[0]存Y，[1]存U，[2]存V
x264_macroblock_thread_init( h ); /* Set the QP equal to the first QP in the slice for more accurate CABAC initialization. */
h->mb.i_mb_xy = h->sh.i_first_mb;
h->sh.i_qp = x264_ratecontrol_mb_qp( h );
h->sh.i_qp = SPEC_QP( h->sh.i_qp );
h->sh.i_qp_delta = h->sh.i_qp - h->pps->i_pic_init_qp;
//输出 slice header
x264_slice_header_write( &h->out.bs, &h->sh, h->i_nal_ref_idc );
//如果使用CABAC，需要初始化
if( h->param.b_cabac )
{
/* alignment needed */
bs_align_1( &h->out.bs ); /* init cabac */
x264_cabac_context_init( h, &h->cabac, h->sh.i_type, x264_clip3( h->sh.i_qp-QP_BD_OFFSET, 0, 51 ), h->sh.i_cabac_init_idc );
x264_cabac_encode_init ( &h->cabac, h->out.bs.p, h->out.bs.p_end );
last_emu_check = h->cabac.p;
}
else
last_emu_check = h->out.bs.p;
h->mb.i_last_qp = h->sh.i_qp;
h->mb.i_last_dqp = 0;
h->mb.field_decoding_flag = 0;
//宏块位置-纵坐标（初始值）
i_mb_y = h->sh.i_first_mb / h->mb.i_mb_width;
//宏块位置-横坐标（初始值）
i_mb_x = h->sh.i_first_mb % h->mb.i_mb_width;
i_skip = 0; //一个大循环
//对一个slice中每个宏块进行编码
while( 1 )
{
//宏块序号。由i_mb_x和i_mb_y计算而来。
mb_xy = i_mb_x + i_mb_y * h->mb.i_mb_width;
int mb_spos = bs_pos(&h->out.bs) + x264_cabac_pos(&h->cabac);
//一行的开始
if( i_mb_x == 0 )
{
if( x264_bitstream_check_buffer( h ) )
return -1;
if( !(i_mb_y & SLICE_MBAFF) && h->param.rc.i_vbv_buffer_size )
x264_bitstream_backup( h, &bs_bak[BS_BAK_ROW_VBV], i_skip, 1 );
//去块效应滤波、半像素插值、SSIM/PSNR计算等
//一次处理一行宏块
if( !h->mb.b_reencode_mb )
x264_fdec_filter_row( h, i_mb_y, 0 );
} if( back_up_bitstream )
{
if( back_up_bitstream_cavlc )
x264_bitstream_backup( h, &bs_bak[BS_BAK_CAVLC_OVERFLOW], i_skip, 0 );
if( slice_max_size && !(i_mb_y & SLICE_MBAFF) )
{
x264_bitstream_backup( h, &bs_bak[BS_BAK_SLICE_MAX_SIZE], i_skip, 0 );
if( (thread_last_mb+1-mb_xy) == h->param.i_slice_min_mbs )
x264_bitstream_backup( h, &bs_bak[BS_BAK_SLICE_MIN_MBS], i_skip, 0 );
}
} if( PARAM_INTERLACED )
{
if( h->mb.b_adaptive_mbaff )
{
if( !(i_mb_y&1) )
{
/* FIXME: VSAD is fast but fairly poor at choosing the best interlace type. */
h->mb.b_interlaced = x264_field_vsad( h, i_mb_x, i_mb_y );
memcpy( &h->zigzagf, MB_INTERLACED ? &h->zigzagf_interlaced : &h->zigzagf_progressive, sizeof(h->zigzagf) );
if( !MB_INTERLACED && (i_mb_y+2) == h->mb.i_mb_height )
x264_expand_border_mbpair( h, i_mb_x, i_mb_y );
}
}
h->mb.field[mb_xy] = MB_INTERLACED;
} /* load cache */
//将要编码的宏块的周围的宏块的值读进来
//主要是上面、左边块的值
if( SLICE_MBAFF )
x264_macroblock_cache_load_interlaced( h, i_mb_x, i_mb_y );
else
x264_macroblock_cache_load_progressive( h, i_mb_x, i_mb_y );
//分析-帧内预测模式选择、帧间运动估计等
x264_macroblock_analyse( h ); /* encode this macroblock -> be careful it can change the mb type to P_SKIP if needed */
reencode:
//编码-残差DCT变换、量化
x264_macroblock_encode( h );
//输出CABAC
if( h->param.b_cabac )
{
if( mb_xy > h->sh.i_first_mb && !(SLICE_MBAFF && (i_mb_y&1)) )
x264_cabac_encode_terminal( &h->cabac ); if( IS_SKIP( h->mb.i_type ) )
x264_cabac_mb_skip( h, 1 );
else
{
if( h->sh.i_type != SLICE_TYPE_I )
x264_cabac_mb_skip( h, 0 );
//输出
x264_macroblock_write_cabac( h, &h->cabac );
}
}
else
{
//输出CAVLC
if( IS_SKIP( h->mb.i_type ) )
i_skip++;
else
{
if( h->sh.i_type != SLICE_TYPE_I )
{
bs_write_ue( &h->out.bs, i_skip ); /* skip run */
i_skip = 0;
}
//输出
x264_macroblock_write_cavlc( h );
/* If there was a CAVLC level code overflow, try again at a higher QP. */
if( h->mb.b_overflow )
{
h->mb.i_chroma_qp = h->chroma_qp_table[++h->mb.i_qp];
h->mb.i_skip_intra = 0;
h->mb.b_skip_mc = 0;
h->mb.b_overflow = 0;
x264_bitstream_restore( h, &bs_bak[BS_BAK_CAVLC_OVERFLOW], &i_skip, 0 );
goto reencode;
}
}
} int total_bits = bs_pos(&h->out.bs) + x264_cabac_pos(&h->cabac);
int mb_size = total_bits - mb_spos; if( slice_max_size && (!SLICE_MBAFF || (i_mb_y&1)) )
{
/* Count the skip run, just in case. */
if( !h->param.b_cabac )
total_bits += bs_size_ue_big( i_skip );
/* Check for escape bytes. */
uint8_t *end = h->param.b_cabac ? h->cabac.p : h->out.bs.p;
for( ; last_emu_check < end - 2; last_emu_check++ )
if( last_emu_check[0] == 0 && last_emu_check[1] == 0 && last_emu_check[2] <= 3 )
{
slice_max_size -= 8;
last_emu_check++;
}
/* We'll just re-encode this last macroblock if we go over the max slice size. */
if( total_bits - starting_bits > slice_max_size && !h->mb.b_reencode_mb )
{
if( !x264_frame_new_slice( h, h->fdec ) )
{
/* Handle the most obnoxious slice-min-mbs edge case: we need to end the slice
* because it's gone over the maximum size, but doing so would violate slice-min-mbs.
* If possible, roll back to the last checkpoint and try again.
* We could try raising QP, but that would break in the case where a slice spans multiple
* rows, which the re-encoding infrastructure can't currently handle. */
if( mb_xy <= thread_last_mb && (thread_last_mb+1-mb_xy) < h->param.i_slice_min_mbs )
{
if( thread_last_mb-h->param.i_slice_min_mbs < h->sh.i_first_mb+h->param.i_slice_min_mbs )
{
x264_log( h, X264_LOG_WARNING, "slice-max-size violated (frame %d, cause: slice-min-mbs)\n", h->i_frame );
slice_max_size = 0;
goto cont;
}
x264_bitstream_restore( h, &bs_bak[BS_BAK_SLICE_MIN_MBS], &i_skip, 0 );
h->mb.b_reencode_mb = 1;
h->sh.i_last_mb = thread_last_mb-h->param.i_slice_min_mbs;
break;
}
if( mb_xy-SLICE_MBAFF*h->mb.i_mb_stride != h->sh.i_first_mb )
{
x264_bitstream_restore( h, &bs_bak[BS_BAK_SLICE_MAX_SIZE], &i_skip, 0 );
h->mb.b_reencode_mb = 1;
if( SLICE_MBAFF )
{
// set to bottom of previous mbpair
if( i_mb_x )
h->sh.i_last_mb = mb_xy-1+h->mb.i_mb_stride*(!(i_mb_y&1));
else
h->sh.i_last_mb = (i_mb_y-2+!(i_mb_y&1))*h->mb.i_mb_stride + h->mb.i_mb_width - 1;
}
else
h->sh.i_last_mb = mb_xy-1;
break;
}
else
h->sh.i_last_mb = mb_xy;
}
else
slice_max_size = 0;
}
}
cont:
h->mb.b_reencode_mb = 0; /* save cache */
//保存当前宏块的的值，用于以后的宏块的编码
//包括Intra4x4宏块帧内预测模式，DCT非零系数，运动矢量，参考帧序号等等
x264_macroblock_cache_save( h );
//码率控制
if( x264_ratecontrol_mb( h, mb_size ) < 0 )
{
x264_bitstream_restore( h, &bs_bak[BS_BAK_ROW_VBV], &i_skip, 1 );
h->mb.b_reencode_mb = 1;
i_mb_x = 0;
i_mb_y = i_mb_y - SLICE_MBAFF;
h->mb.i_mb_prev_xy = i_mb_y * h->mb.i_mb_stride - 1;
h->sh.i_last_mb = orig_last_mb;
continue;
} /* accumulate mb stats */
//后面很大一段代码都是对stat结构体中的统计信息进行赋值================================
h->stat.frame.i_mb_count[h->mb.i_type]++; int b_intra = IS_INTRA( h->mb.i_type );
int b_skip = IS_SKIP( h->mb.i_type );
if( h->param.i_log_level >= X264_LOG_INFO || h->param.rc.b_stat_write )
{
if( !b_intra && !b_skip && !IS_DIRECT( h->mb.i_type ) )
{
if( h->mb.i_partition != D_8x8 )
h->stat.frame.i_mb_partition[h->mb.i_partition] += 4;
else
for( int i = 0; i < 4; i++ )
h->stat.frame.i_mb_partition[h->mb.i_sub_partition[i]] ++;
if( h->param.i_frame_reference > 1 )
for( int i_list = 0; i_list <= (h->sh.i_type == SLICE_TYPE_B); i_list++ )
for( int i = 0; i < 4; i++ )
{
int i_ref = h->mb.cache.ref[i_list][ x264_scan8[4*i] ];
if( i_ref >= 0 )
h->stat.frame.i_mb_count_ref[i_list][i_ref] ++;
}
}
} if( h->param.i_log_level >= X264_LOG_INFO )
{
if( h->mb.i_cbp_luma | h->mb.i_cbp_chroma )
{
if( CHROMA444 )
{
for( int i = 0; i < 4; i++ )
if( h->mb.i_cbp_luma & (1 << i) )
for( int p = 0; p < 3; p++ )
{
int s8 = i*4+p*16;
int nnz8x8 = M16( &h->mb.cache.non_zero_count[x264_scan8[s8]+0] )
| M16( &h->mb.cache.non_zero_count[x264_scan8[s8]+8] );
h->stat.frame.i_mb_cbp[!b_intra + p*2] += !!nnz8x8;
}
}
else
{
int cbpsum = (h->mb.i_cbp_luma&1) + ((h->mb.i_cbp_luma>>1)&1)
+ ((h->mb.i_cbp_luma>>2)&1) + (h->mb.i_cbp_luma>>3);
h->stat.frame.i_mb_cbp[!b_intra + 0] += cbpsum;
h->stat.frame.i_mb_cbp[!b_intra + 2] += !!h->mb.i_cbp_chroma;
h->stat.frame.i_mb_cbp[!b_intra + 4] += h->mb.i_cbp_chroma >> 1;
}
}
if( h->mb.i_cbp_luma && !b_intra )
{
h->stat.frame.i_mb_count_8x8dct[0] ++;
h->stat.frame.i_mb_count_8x8dct[1] += h->mb.b_transform_8x8;
}
if( b_intra && h->mb.i_type != I_PCM )
{
if( h->mb.i_type == I_16x16 )
h->stat.frame.i_mb_pred_mode[0][h->mb.i_intra16x16_pred_mode]++;
else if( h->mb.i_type == I_8x8 )
for( int i = 0; i < 16; i += 4 )
h->stat.frame.i_mb_pred_mode[1][h->mb.cache.intra4x4_pred_mode[x264_scan8[i]]]++;
else //if( h->mb.i_type == I_4x4 )
for( int i = 0; i < 16; i++ )
h->stat.frame.i_mb_pred_mode[2][h->mb.cache.intra4x4_pred_mode[x264_scan8[i]]]++;
h->stat.frame.i_mb_pred_mode[3][x264_mb_chroma_pred_mode_fix[h->mb.i_chroma_pred_mode]]++;
}
h->stat.frame.i_mb_field[b_intra?0:b_skip?2:1] += MB_INTERLACED;
}
//=========================================================== /* calculate deblock strength values (actual deblocking is done per-row along with hpel) */
//计算去块效应滤波器强度Bs
//这里没有滤波
if( b_deblock )
x264_macroblock_deblock_strength( h ); //如果处理完最后一个宏块，就跳出大循环
if( mb_xy == h->sh.i_last_mb )
break; if( SLICE_MBAFF )
{
i_mb_x += i_mb_y & 1;
i_mb_y ^= i_mb_x < h->mb.i_mb_width;
}
else
i_mb_x++;//宏块序号x加1
//处理完一行宏块
if( i_mb_x == h->mb.i_mb_width )
{
//该处理下一行了
i_mb_y++;//宏块序号y加1
i_mb_x = 0;//宏块序号x设置为0
}
}
if( h->sh.i_last_mb < h->sh.i_first_mb )
return 0; h->out.nal[h->out.i_nal].i_last_mb = h->sh.i_last_mb; //熵编码的收尾工作
if( h->param.b_cabac )
{
x264_cabac_encode_flush( h, &h->cabac );
h->out.bs.p = h->cabac.p;
}
else
{
if( i_skip > 0 )
bs_write_ue( &h->out.bs, i_skip ); /* last skip run */
/* rbsp_slice_trailing_bits */
bs_rbsp_trailing( &h->out.bs );
bs_flush( &h->out.bs );
}
//结束输出一个NAL
//前面对应着x264_nal_start()
if( x264_nal_end( h ) )
return -1;
//多线程并行处理？
if( h->sh.i_last_mb == (h->i_threadslice_end * h->mb.i_mb_width - 1) )
{
h->stat.frame.i_misc_bits = bs_pos( &h->out.bs )
+ (h->out.i_nal*NALU_OVERHEAD * 8)
- h->stat.frame.i_tex_bits
- h->stat.frame.i_mv_bits;
x264_fdec_filter_row( h, h->i_threadslice_end, 0 ); if( h->param.b_sliced_threads )
{
/* Tell the main thread we're done. */
x264_threadslice_cond_broadcast( h, 1 );
/* Do hpel now */
for( int mb_y = h->i_threadslice_start; mb_y <= h->i_threadslice_end; mb_y++ )
x264_fdec_filter_row( h, mb_y, 1 );
x264_threadslice_cond_broadcast( h, 2 );
/* Do the first row of hpel, now that the previous slice is done */
if( h->i_thread_idx > 0 )
{
x264_threadslice_cond_wait( h->thread[h->i_thread_idx-1], 2 );
x264_fdec_filter_row( h, h->i_threadslice_start + (1 << SLICE_MBAFF), 2 );
}
} /* Free mb info after the last thread's done using it */
if( h->fdec->mb_info_free && (!h->param.b_sliced_threads || h->i_thread_idx == (h->param.i_threads-1)) )
{
h->fdec->mb_info_free( h->fdec->mb_info );
h->fdec->mb_info = NULL;
h->fdec->mb_info_free = NULL;
}
} return 0;
}
x264_slice_write（）函数的具体流程：
首先，调用x264_nal_start（）开始输出一个NALU；
接着，x264_macroblock_thread_init（）函数初始化宏块重建像素缓存fdec_buf[]和编码像素缓存fenc_buf[]。
接着调用x264_slice_header_write（）输出Slice Header。
进入一个循环，该循环每执行一次编码一个宏块。
（a）调用x264_fdec_filter_row（）执行滤波模块；
（b）调用x264_macroblock_cache_load_progressive（）将要编码的宏块的周围宏块的信息读进来。
（c）调用x264_macroblock_analyse（）执行分析模块。
（d）调用x264_macroblock_encode（）执行宏块编码信息。
（e）调用x264_macroblock_write_cabac（）/x264_macroblock_write_cavlc（）执行熵编码模块。
（f）调用x264_macroblock_cache_save（）保存当前宏块的信息。
（g）调用x264_ratecontrol_mb（）执行码率控制。
（h）准备处理下一个宏块。
最后调用x264_nal_end（）结束输出一个NALU。
