H264相关知识

1、基本概念

　　I frame ：帧内编码帧 又称intra picture，I 帧通常是每个 GOP（MPEG 所使用的一种视频压缩技术）的第一个帧，经过适度地压缩，做为随机访问的参考点，可以当成图象。I帧可以看成是一个图像经过压缩后的产物。

　　P frame: 前向预测编码帧又称predictive-frame，通过充分将低于图像序列中前面已编码帧的时间冗余信息来压缩传输数据量的编码图像，也叫预测帧；

　　B frame: 双向预测内插编码帧 又称bi-directional interpolated prediction frame，既考虑与源图像序列前面已编码帧，也顾及源图像序列后面已编码帧之间的时间冗余信息来压缩传输数据量的编码图像，也叫双向预测帧；

　　PTS：Presentation Time Stamp。PTS主要用于度量解码后的视频帧什么时候被显示出来

　　DTS：Decode Time Stamp。DTS主要是标识读入内存中的ｂｉｔ流在什么时候开始送入解码器中进行解码。

　　在没有B帧存在的情况下DTS的顺序和PTS的顺序应该是一样的。

ＩＰＢ帧的不同：

　　I frame:自身可以通过视频解压算法解压成一张单独的完整的图片。

　　P frame：需要参考其前面的一个I frame 或者B frame来生成一张完整的图片。

　　B frame:则要参考其前一个I或者P帧及其后面的一个P帧来生成一张完整的图片。

　　两个I frame之间形成一个GOP，在x264中同时可以通过参数来设定bf的大小，即：I 和p或者两个P之间B的数量。

　　通过上述基本可以说明如果有B frame 存在的情况下一个GOP的最后一个frame一定是P.

DTS和PTS的不同：

　　DTS主要用于视频的解码,在解码阶段使用.PTS主要用于视频的同步和输出.在display的时候使用.在没有B frame的情况下.DTS和PTS的输出顺序是一样的.

例子:

　　下面给出一个GOP为15的例子,其解码的参照frame及其解码的顺序都在里面:

　　如上图：I frame 的解码不依赖于任何的其它的帧.而p frame的解码则依赖于其前面的I frame或者P frame.B frame的解码则依赖于其前的最近的一个I frame或者P frame 及其后的最近的一个P frame.

3、NAL全称Network Abstract Layer, 即网络抽象层。
         在H.264/AVC视频编码标准中，整个系统框架被分为了两个层面：视频编码层面（VCL）和网络抽象层面（NAL）。其中，前者负责有效表示视频数据的内容，而后者则负责格式化数据并提供头信息，以保证数据适合各种信道和存储介质上的传输。因此我们平时的每帧数据就是一个NAL单元（SPS与PPS除外）。在实际的H264数据帧中，往往帧前面带有00 00 00 01 或 00 00 01分隔符，一般来说编码器编出的首帧数据为PPS与SPS，接着为I帧……

如下图：

2、如何判断帧类型（是图像参考帧还是I、P帧等）？

     NALU类型是我们判断帧类型的利器，从官方文档中得出如下图：

我们还是接着看最上面图的码流对应的数据来层层分析，以00 00 00 01分割之后的下一个字节就是NALU类型，将其转为二进制数据后，解读顺序为从左往右算，如下:
（1）第1位禁止位，值为1表示语法出错
（2）第2~3位为参考级别
（3）第4~8为是nal单元类型

例如上面00000001后有67,68以及65

其中0x67的二进制码为：
0110 0111
4-8为00111，转为十进制7，参考第二幅图：7对应序列参数集SPS

其中0x68的二进制码为：
0110 1000
4-8为01000，转为十进制8，参考第二幅图：8对应图像参数集PPS

其中0x65的二进制码为：
0110 0101
4-8为00101，转为十进制5，参考第二幅图：5对应IDR图像中的片(I帧)

所以判断是否为I帧的算法为： （NALU类型  & 0001  1111） = 5   即   NALU类型  & 31 = 5

比如0x65 & 31 = 5

4、H264参数语法文档： SPS、PPS、IDR

H.264码流第一个 NALU 是 SPS（序列参数集Sequence Parameter Set）
对应H264标准文档 7.3.2.1 序列参数集的语法进行解析
SPS参数解析// fill sps with content of p

int InterpretSPS (VideoParameters *p_Vid, DataPartition *p, seq_parameter_set_rbsp_t *sps) 
{ 
  unsigned i; 
  unsigned n_ScalingList; 
  int reserved_zero; 
  Bitstream *s = p->bitstream; 
 
  assert (p != NULL); 
  assert (p->bitstream != NULL); 
  assert (p->bitstream->streamBuffer != 0); 
  assert (sps != NULL); 
 
  p_Dec->UsedBits = 0; 
 
  sps->profile_idc = u_v  (8, "SPS: profile_idc", s); 
 
  if ((sps->profile_idc!=BASELINE       ) && 
      (sps->profile_idc!=MAIN           ) && 
      (sps->profile_idc!=EXTENDED       ) && 
      (sps->profile_idc!=FREXT_HP       ) && 
      (sps->profile_idc!=FREXT_Hi10P    ) && 
      (sps->profile_idc!=FREXT_Hi422    ) && 
      (sps->profile_idc!=FREXT_Hi444    ) && 
      (sps->profile_idc!=FREXT_CAVLC444 ) 
#if (MVC_EXTENSION_ENABLE) 
      && (sps->profile_idc!=MVC_HIGH) 
      && (sps->profile_idc!=STEREO_HIGH) 
#endif 
      ) 
  { 
    printf("Invalid Profile IDC (%d) encountered. /n", sps->profile_idc); 
    return p_Dec->UsedBits; 
  } 
 
  sps->constrained_set0_flag = u_1("SPS: constrained_set0_flag", s); 
  sps->constrained_set1_flag = u_1("SPS: constrained_set1_flag", s); 
  sps->constrained_set2_flag = u_1("SPS: constrained_set2_flag", s); 
  sps->constrained_set3_flag = u_1("SPS: constrained_set3_flag", s); 
#if (MVC_EXTENSION_ENABLE) 
  sps->constrained_set4_flag = u_1("SPS: constrained_set4_flag", s); 
  reserved_zero = u_v(3, "SPS: reserved_zero_3bits", s); 
#else 
  reserved_zero = u_v(4, "SPS: reserved_zero_4bits", s); 
#endif 
  assert (reserved_zero==0); 
 
  sps->level_idc = u_v(8, "SPS: level_idc", s); 
 
  sps->seq_parameter_set_id = ue_v("SPS: seq_parameter_set_id", s); 
 
  // Fidelity Range Extensions stuff 
  sps->chroma_format_idc = 1; 
  sps->bit_depth_luma_minus8   = 0; 
  sps->bit_depth_chroma_minus8 = 0; 
  p_Vid->lossless_qpprime_flag   = 0; 
  sps->separate_colour_plane_flag = 0; 
 
  if((sps->profile_idc==FREXT_HP   ) || 
     (sps->profile_idc==FREXT_Hi10P) || 
     (sps->profile_idc==FREXT_Hi422) || 
     (sps->profile_idc==FREXT_Hi444) || 
     (sps->profile_idc==FREXT_CAVLC444) 
#if (MVC_EXTENSION_ENABLE) 
     || (sps->profile_idc==MVC_HIGH) 
     || (sps->profile_idc==STEREO_HIGH) 
#endif 
     ) 
  { 
    sps->chroma_format_idc = ue_v ("SPS: chroma_format_idc", s); 
 
    if(sps->chroma_format_idc == YUV444) 
    { 
      sps->separate_colour_plane_flag= u_1("SPS: separate_colour_plane_flag", s); 
    } 
 
    sps->bit_depth_luma_minus8= ue_v ("SPS: bit_depth_luma_minus8", s); 
    sps->bit_depth_chroma_minus8= ue_v ("SPS: bit_depth_chroma_minus8", s); 
    //checking; 
    if((sps->bit_depth_luma_minus8+8 > sizeof(imgpel)*8)|| (sps->bit_depth_chroma_minus8+8> sizeof(imgpel)*8)) 
      error ("Source picture has higher bit depth than imgpel data type. /n Please recompile with larger data type for imgpel.", 500); 
 
    p_Vid->lossless_qpprime_flag = u_1("SPS: lossless_qpprime_y_zero_flag", s); 
 
    sps->seq_scaling_matrix_present_flag= u_1("SPS: seq_scaling_matrix_present_flag", s); 
     
    if(sps->seq_scaling_matrix_present_flag) 
    { 
      n_ScalingList = (sps->chroma_format_idc != YUV444) ? 8 : 12; 
      for(i=0; iseq_scaling_list_present_flag[i]= u_1("SPS: seq_scaling_list_present_flag", s); 
        if(sps->seq_scaling_list_present_flag[i]) 
        { 
          if(i<6)
　　　　　　　　scaling_list="">ScalingList4x4[i], 16, &sps->UseDefaultScalingMatrix4x4Flag[i], s); 
          else 
　　　　　　　　Scaling_List(sps->ScalingList8x8[i-6], 64, &sps->UseDefaultScalingMatrix8x8Flag[i-6], s); 
        } 
      } 
    } 
  } 
 
  sps->log2_max_frame_num_minus4 = ue_v ("SPS: log2_max_frame_num_minus4", s); 
  sps->pic_order_cnt_type = ue_v ("SPS: pic_order_cnt_type", s); 
 
  if (sps->pic_order_cnt_type == 0) 
    sps->log2_max_pic_order_cnt_lsb_minus4 = ue_v ("SPS: log2_max_pic_order_cnt_lsb_minus4", s); 
  else if (sps->pic_order_cnt_type == 1) 
  { 
    sps->delta_pic_order_always_zero_flag = u_1  ("SPS: delta_pic_order_always_zero_flag", s); 
    sps->offset_for_non_ref_pic= se_v ("SPS: offset_for_non_ref_pic", s); 
    sps->offset_for_top_to_bottom_field = se_v ("SPS: offset_for_top_to_bottom_field", s); 
    sps->num_ref_frames_in_pic_order_cnt_cycle = ue_v ("SPS: num_ref_frames_in_pic_order_cnt_cycle"  , s); 
    for(i=0; inum_ref_frames_in_pic_order_cnt_cycle; i++) 
      sps->offset_for_ref_frame[i] = se_v ("SPS: offset_for_ref_frame[i]", s); 
  } 
  sps->num_ref_frames = ue_v ("SPS: num_ref_frames"                         , s); 
  sps->gaps_in_frame_num_value_allowed_flag  = u_1  ("SPS: gaps_in_frame_num_value_allowed_flag"   , s); 
  sps->pic_width_in_mbs_minus1 = ue_v ("SPS: pic_width_in_mbs_minus1"                , s); 
  sps->pic_height_in_map_units_minus1 = ue_v ("SPS: pic_height_in_map_units_minus1"         , s); 
  sps->frame_mbs_only_flag = u_1  ("SPS: frame_mbs_only_flag"                    , s); 
  if (!sps->frame_mbs_only_flag) 
  { 
    sps->mb_adaptive_frame_field_flag = u_1  ("SPS: mb_adaptive_frame_field_flag"           , s); 
  } 
  sps->direct_8x8_inference_flag = u_1  ("SPS: direct_8x8_inference_flag"              , s); 
  sps->frame_cropping_flag = u_1  ("SPS: frame_cropping_flag"                    , s); 
 
  if (sps->frame_cropping_flag) 
  { 
    sps->frame_cropping_rect_left_offset = ue_v ("SPS: frame_cropping_rect_left_offset"           , s); 
    sps->frame_cropping_rect_right_offset = ue_v ("SPS: frame_cropping_rect_right_offset"          , s); 
    sps->frame_cropping_rect_top_offset = ue_v ("SPS: frame_cropping_rect_top_offset"            , s); 
    sps->frame_cropping_rect_bottom_offset = ue_v ("SPS: frame_cropping_rect_bottom_offset"         , s); 
  } 
  sps->vui_parameters_present_flag = (Boolean) u_1  ("SPS: vui_parameters_present_flag"      , s); 
 
  InitVUI(sps); 
  ReadVUI(p, sps); 
 
  sps->Valid = TRUE; 
  return p_Dec->UsedBits; 

H.264码流第二个 NALU 是 PPS（图像参数集Picture Parameter Set）
对应H264标准文档 7.3.2.2 序列参数集的语法进行解析
PPS参数解析

int InterpretPPS (VideoParameters *p_Vid, DataPartition *p, pic_parameter_set_rbsp_t *pps) 
{ 
  unsigned i; 
  unsigned n_ScalingList; 
  int chroma_format_idc; 
  int NumberBitsPerSliceGroupId; 
  Bitstream *s = p->bitstream; 
 
  assert (p != NULL); 
  assert (p->bitstream != NULL); 
  assert (p->bitstream->streamBuffer != 0); 
  assert (pps != NULL); 
 
  p_Dec->UsedBits = 0; 
 
  pps->pic_parameter_set_id= ue_v ("PPS: pic_parameter_set_id"                   , s); 
  pps->seq_parameter_set_id= ue_v ("PPS: seq_parameter_set_id"                   , s); 
  pps->entropy_coding_mode_flag= u_1  ("PPS: entropy_coding_mode_flag"               , s); 
 
  //! Note: as per JVT-F078 the following bit is unconditional.  If F078 is not accepted, then 
  //! one has to fetch the correct SPS to check whether the bit is present (hopefully there is 
  //! no consistency problem :-( 
  //! The current encoder code handles this in the same way.  When you change this, don't forget 
  //! the encoder!  StW, 12/8/02 
  pps->bottom_field_pic_order_in_frame_present_flag= u_1  ("PPS: bottom_field_pic_order_in_frame_present_flag"                 , s); 
 
  pps->num_slice_groups_minus1= ue_v ("PPS: num_slice_groups_minus1"                , s); 
 
  // FMO stuff begins here 
  if (pps->num_slice_groups_minus1 > 0) 
  { 
    pps->slice_group_map_type= ue_v ("PPS: slice_group_map_type"                , s); 
    if (pps->slice_group_map_type == 0) 
    { 
      for (i=0; i<=pps->num_slice_groups_minus1; i++) 
        pps->run_length_minus1 [i]= ue_v ("PPS: run_length_minus1 [i]"              , s); 
    } 
    else if (pps->slice_group_map_type == 2) 
    { 
      for (i=0; inum_slice_groups_minus1; i++) 
      { 
        //! JVT-F078: avoid reference of SPS by using ue(v) instead of u(v) 
        pps->top_left [i]= ue_v ("PPS: top_left [i]"                        , s); 
        pps->bottom_right [i]= ue_v ("PPS: bottom_right [i]"                    , s); 
      } 
    } 
    else if (pps->slice_group_map_type == 3 || 
             pps->slice_group_map_type == 4 || 
             pps->slice_group_map_type == 5) 
    { 
      pps->slice_group_change_direction_flag= u_1  ("PPS: slice_group_change_direction_flag"      , s); 
      pps->slice_group_change_rate_minus1= ue_v ("PPS: slice_group_change_rate_minus1"         , s); 
    } 
    else if (pps->slice_group_map_type == 6) 
    { 
      if (pps->num_slice_groups_minus1+1 >4) 
        NumberBitsPerSliceGroupId = 3; 
      else if (pps->num_slice_groups_minus1+1 > 2) 
        NumberBitsPerSliceGroupId = 2; 
      else 
        NumberBitsPerSliceGroupId = 1; 
      pps->pic_size_in_map_units_minus1= ue_v ("PPS: pic_size_in_map_units_minus1"               , s); 
      if ((pps->slice_group_id = calloc (pps->pic_size_in_map_units_minus1+1, 1)) == NULL) 
        no_mem_exit ("InterpretPPS: slice_group_id"); 
      for (i=0; i<=pps->pic_size_in_map_units_minus1; i++) 
        pps->slice_group_id[i] = (byte) u_v (NumberBitsPerSliceGroupId, "slice_group_id[i]", s); 
    } 
  } 
 
  // End of FMO stuff  
  pps->num_ref_idx_l0_active_minus1= ue_v ("PPS: num_ref_idx_l0_active_minus1"           , s); 
  pps->num_ref_idx_l1_active_minus1= ue_v ("PPS: num_ref_idx_l1_active_minus1"           , s); 
  pps->weighted_pred_flag= u_1  ("PPS: weighted_pred_flag"                     , s); 
  pps->weighted_bipred_idc= u_v  ( 2, "PPS: weighted_bipred_idc"                , s); 
  pps->pic_init_qp_minus26 = se_v ("PPS: pic_init_qp_minus26"                    , s); 
  pps->pic_init_qs_minus26= se_v ("PPS: pic_init_qs_minus26"                    , s); 
 
  pps->chroma_qp_index_offset= se_v ("PPS: chroma_qp_index_offset"                 , s); 
 
  pps->deblocking_filter_control_present_flag= u_1 ("PPS: deblocking_filter_control_present_flag" , s); 
  pps->constrained_intra_pred_flag= u_1  ("PPS: constrained_intra_pred_flag"            , s); 
  pps->redundant_pic_cnt_present_flag= u_1  ("PPS: redundant_pic_cnt_present_flag"         , s); 
 
  if(more_rbsp_data(s->streamBuffer, s->frame_bitoffset,s->bitstream_length)) // more_data_in_rbsp() 
  { 
    //Fidelity Range Extensions Stuff 
    pps->transform_8x8_mode_flag= u_1  ("PPS: transform_8x8_mode_flag"                , s); 
    pps->pic_scaling_matrix_present_flag=  u_1  ("PPS: pic_scaling_matrix_present_flag"        , s); 
 
    if(pps->pic_scaling_matrix_present_flag) 
    { 
      chroma_format_idc= p_Vid->SeqParSet[pps->seq_parameter_set_id].chroma_format_idc; 
      n_ScalingList = 6 + ((chroma_format_idc != YUV444) ? 2 : 6) * pps->transform_8x8_mode_flag; 
      for(i=0; ipic_scaling_list_present_flag[i]= u_1  ("PPS: pic_scaling_list_present_flag"          , s); 
 
        if(pps->pic_scaling_list_present_flag[i]) 
        { 
          if(i<6)
              scaling_list="">ScalingList4x4[i], 16, &pps->UseDefaultScalingMatrix4x4Flag[i], s); 
          else 
              Scaling_List(pps->ScalingList8x8[i-6], 64, &pps->UseDefaultScalingMatrix8x8Flag[i-6], s); 
        } 
      } 
    } 
    pps->second_chroma_qp_index_offset= se_v ("PPS: second_chroma_qp_index_offset"          , s); 
  } 
  else 
  { 
    pps->second_chroma_qp_index_offset= pps->chroma_qp_index_offset; 
  } 
 
  pps->Valid = TRUE; 
  return p_Dec->UsedBits; 
} 

H.264码流第三个 NALU 是 IDR（即时解码器刷新）
对应H264标准文档 7.3.3 序列参数集的语法进行解析
IDR参数解析

case NALU_TYPE_IDR: 
        img->idr_flag = (nalu->nal_unit_type == NALU_TYPE_IDR); 
        img->nal_reference_idc = nalu->nal_reference_idc; 
        img->disposable_flag = (nalu->nal_reference_idc
== NALU_PRIORITY_DISPOSABLE); 
        currSlice->dp_mode = PAR_DP_1;
//++ dp_mode：数据分割模式；PAR_DP_1=0：没有数据分割 
        currSlice->max_part_nr = 1; 
        currSlice->ei_flag = 0;
//++ 该处赋值直接影响decode_slice()函数中对decode_one_slice()函数的调用 
//++ 该值不为0，表明当前片出错，解码程序将忽略当前片的解码过程，而使用错误隐藏 
        currStream = currSlice->partArr[0].bitstream; 
        currStream->ei_flag = 0;
//++ 此处的赋值为最终赋值，以后不再改变。该值将对每个宏块的ei_flag产生影响 
//++ 参见macroblock.c文件read_one_macroblock()函数的如下语句： 
//++        :if(!dP->bitstream->ei_flag)        :currMB->ei_flag = 0; 
//++ 该值还在macroblock.c文件if(IS_INTRA (currMB)
// && dP->bitstream->ei_flag && img->number)中用到 
        currStream->frame_bitoffset = currStream->read_len = 0; 
        memcpy (currStream->streamBuffer, &nalu->buf[1], nalu->len-1); 
        currStream->code_len = currStream->bitstream_length
= RBSPtoSODB(currStream->streamBuffer, nalu->len-1); 
 
// Some syntax of the Slice Header depends on the parameter set, which depends on 
// the parameter set ID of the SLice header.  Hence, read the pic_parameter_set_id 
// of the slice header first, then setup the active parameter sets, and then read 
// the rest of the slice header 
        BitsUsedByHeader = FirstPartOfSliceHeader();    //++ 参见标准7.3.3 
        UseParameterSet (currSlice->pic_parameter_set_id); 
        BitsUsedByHeader+= RestOfSliceHeader ();    //++ 参见标准7.3.3 
//++ BitsUsedByHeader在程序中没有实际用处，而且BitsUsedByHeader+= RestOfSliceHeader () 
//++ 重复计算了FirstPartOfSliceHeader()所用到的比特数。因为在FirstPartOfSliceHeader() 
//++ 之后，变量UsedBits值并未被置零就代入RestOfSliceHeader()运算，从而RestOfSliceHeader () 
//++ 在返回时，BitsUsedByHeader+= RestOfSliceHeader()多加了一个BitsUsedByHeader值 
 
        FmoInit (active_pps, active_sps); 
 
        if(is_new_picture()) 
        { 
          init_picture(img, input); 
           
          current_header = SOP; 
          //check zero_byte if it is also the first NAL unit in the access unit 
          CheckZeroByteVCL(nalu, &ret); 
        } 
        else 
          current_header = SOS; 
   
        init_lists(img->type, img->currentSlice->structure); 
        reorder_lists (img->type, img->currentSlice); 
 
        if (img->structure==FRAME) 
        { 
          init_mbaff_lists(); 
        } 
 
/*        if (img->frame_num==1) // write a reference list 
        { 
          count ++; 
          if (count==1) 
            for (i=0; i 
 
// From here on, active_sps, active_pps and the slice header are valid 
        if (img->MbaffFrameFlag) 
img->current_mb_nr = currSlice->start_mb_nr << 1="" style="color: #0000ff">else 
img->current_mb_nr = currSlice->start_mb_nr; 
 
        if (active_pps->entropy_coding_mode_flag) 
        { 
          int ByteStartPosition = currStream->frame_bitoffset/8; 
          if (currStream->frame_bitoffset%8 != 0)  
          { 
            ByteStartPosition++; 
          } 
arideco_start_decoding (&currSlice->partArr[0].de_cabac, currStream->streamBuffer
, ByteStartPosition, &currStream->read_len, img->type); 
        } 
// printf ("read_new_slice: returning %s/n", current_header == SOP?"SOP":"SOS"); 
        FreeNALU(nalu); 
        return current_header; 
        break; 
      case NALU_TYPE_DPA: 
//! The state machine here should follow the same ideas as the old readSliceRTP() 
//! basically: 
//! work on DPA (as above) 
//! read and process all following SEI/SPS/PPS/PD/Filler NALUs 
//! if next video NALU is dpB,  
//!   then read and check whether it belongs to DPA, if yes, use it 
//! else 
//!   ;   // nothing 
//! read and process all following SEI/SPS/PPS/PD/Filler NALUs 
//! if next video NALU is dpC 
//!   then read and check whether it belongs to DPA (and DPB, if present), if yes, use it, done 
//! else 
//!   use the DPA (and the DPB if present) 
 
/*  
LC: inserting the code related to DP processing, mainly copying some of the parts 
related to NALU_TYPE_SLICE, NALU_TYPE_IDR. 
*/ 
 
        if(expected_slice_type != NALU_TYPE_DPA) 
        { 
          /* oops... we found the next slice, go back! */ 
          fseek(bits, ftell_position, SEEK_SET); 
          FreeNALU(nalu); 
          return current_header; 
        } 
 
        img->idr_flag          = (nalu->nal_unit_type == NALU_TYPE_IDR); 
        img->nal_reference_idc = nalu->nal_reference_idc; 
        img->disposable_flag   = (nalu->nal_reference_idc == NALU_PRIORITY_DISPOSABLE); 
        currSlice->dp_mode     = PAR_DP_3; 
        currSlice->max_part_nr = 3; 
        currSlice->ei_flag     = 0; 
        currStream             = currSlice->partArr[0].bitstream; 
        currStream->ei_flag    = 0; 
        currStream->frame_bitoffset = currStream->read_len = 0; 
        memcpy (currStream->streamBuffer, &nalu->buf[1], nalu->len-1); 
        currStream->code_len = currStream->bitstream_length
 = RBSPtoSODB(currStream->streamBuffer, nalu->len-1);    //++ 剔除停止比特和填充比特 
         
        BitsUsedByHeader     = FirstPartOfSliceHeader(); 
        UseParameterSet (currSlice->pic_parameter_set_id); 
        BitsUsedByHeader    += RestOfSliceHeader (); 
         
        FmoInit (active_pps, active_sps); 
 
        if(is_new_picture()) 
        { 
          init_picture(img, input); 
          current_header = SOP; 
          CheckZeroByteVCL(nalu, &ret); 
        } 
        else 
          current_header = SOS; 
 
         
        init_lists(img->type, img->currentSlice->structure); 
        reorder_lists (img->type, img->currentSlice); 
         
        if (img->structure==FRAME) 
        { 
          init_mbaff_lists(); 
        } 
 
        // From here on, active_sps, active_pps and the slice header are valid 
        if (img->MbaffFrameFlag) 
              img->current_mb_nr = currSlice->start_mb_nr << 1="" style="color: #0000ff">
        else 
              img->current_mb_nr = currSlice->start_mb_nr; 
        /*  
           LC: 
              Now I need to read the slice ID, which depends on the value of  
              redundant_pic_cnt_present_flag (pag.49).  
        */ 
 
        slice_id_a  = ue_v("NALU:SLICE_A slice_idr", currStream); 
        if (active_pps->entropy_coding_mode_flag) 
        { 
          int ByteStartPosition = currStream->frame_bitoffset/8; 
          if (currStream->frame_bitoffset%8 != 0)  
          { 
            ByteStartPosition++; 
          } 
          arideco_start_decoding (&currSlice->partArr[0].de_cabac, currStream->streamBuffer, ByteStartPosition, &currStream->read_len, img->type); 
        } 
// printf ("read_new_slice: returning %s/n", current_header == SOP?"SOP":"SOS"); 
        break; 
      case NALU_TYPE_DPB: 
        /* LC: inserting the code related to DP processing */ 
 
        currStream             = currSlice->partArr[1].bitstream; 
        currStream->ei_flag    = 0; 
        currStream->frame_bitoffset = currStream->read_len = 0; 
        memcpy (currStream->streamBuffer, &nalu->buf[1], nalu->len-1); 
        currStream->code_len = currStream->bitstream_length = RBSPtoSODB(currStream->streamBuffer, nalu->len-1); 
 
        slice_id_b  = ue_v("NALU:SLICE_B slice_idr", currStream); 
        if (active_pps->redundant_pic_cnt_present_flag) 
          redundant_pic_cnt_b = ue_v("NALU:SLICE_B redudand_pic_cnt", currStream); 
        else 
          redundant_pic_cnt_b = 0; 
         
        /*  LC: Initializing CABAC for the current data stream. */ 
 
        if (active_pps->entropy_coding_mode_flag) 
        { 
          int ByteStartPosition = currStream->frame_bitoffset/8; 
          if (currStream->frame_bitoffset % 8 != 0)  
            ByteStartPosition++; 
           
          arideco_start_decoding (&currSlice->partArr[1].de_cabac, currStream->streamBuffer,  
            ByteStartPosition, &currStream->read_len, img->type); 
           
        } 
 
        /* LC: resilience code to be inserted */ 
        /*         FreeNALU(nalu); */ 
        /*         return current_header; */ 
 
        break; 
IDR参数解析
/* /brief   
 *    read the first part of the header (only the pic_parameter_set_id)   
 * /return   
 *    Length of the first part of the slice header (in bits)   
 */   
//++ 参见标准7.3.3   
int FirstPartOfSliceHeader()   
{   
  Slice *currSlice = img->currentSlice;   
  int dP_nr = assignSE2partition[currSlice->dp_mode][SE_HEADER];   
  DataPartition *partition = &(currSlice->partArr[dP_nr]);   
  Bitstream *currStream = partition->bitstream;   
  int tmp;   
   
  UsedBits= partition->bitstream->frame_bitoffset;
// was hardcoded to 31 for previous start-code. This is better.   
   
  // Get first_mb_in_slice   
  currSlice->start_mb_nr = ue_v ("SH: first_mb_in_slice", currStream);   
   
  tmp = ue_v ("SH: slice_type", currStream);   
     
  if (tmp>4) tmp -=5;   
   
  img->type = currSlice->picture_type = (SliceType) tmp;   
   
  currSlice->pic_parameter_set_id = ue_v ("SH: pic_parameter_set_id", currStream);   
     
  return UsedBits;   
}