JM8.6解码端是如何对H.264码流进行读取的？（GetAnnexbNALU 函数）

最新推荐文章于 2022-05-28 10:41:32 发布

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最新推荐文章于 2022-05-28 10:41:32 发布

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分类专栏： S1: C/C++ s6: 视频研究

本文链接：https://blog.youkuaiyun.com/stpeace/article/details/8226239

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本文详细介绍了使用JM8.6解码器对H.264编码的视频文件进行解码的过程，重点解析了从码流中提取NALU单元的实现方法，并通过调试展示了具体的解码信息。

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原始的foreman_part_qcif.yuv文件进行编码后(本人编码的是一帧)，生成了test.264文件，现在要用JM8.6解码器对其进行解码，显然，首先要读取码流，然后对码流进行解析. 读二进制的数据，无非就是用到fread, fgetc这样的函数，在代码中简单搜索一下，就很容易定位到我们感兴趣的地方，即：

while(!feof(bits) && (Buf[pos++]=fgetc(bits))==0);

而这个语句刚好就在GetAnnexbNALU函数中，经跟踪发现，GetAnnexbNALU函数被read_new_slice函数调用，而read_new_slice函数又被decode_one_frame函数调用，decode_one_frame函数进而被main函数调用. 调用关系在代码中为：

int main(int argc, char **argv)
{
......  
    while (decode_one_frame(img, input, snr) != EOS)
    ;
......
}

int decode_one_frame(struct img_par *img,struct inp_par *inp, struct snr_par *snr)
{
  int current_header;
  Slice *currSlice = img->currentSlice;

  img->current_slice_nr = 0;
  img->current_mb_nr = -4711;     // initialized to an impossible value for debugging -- correct value is taken from slice header
  currSlice->next_header = -8888; // initialized to an impossible value for debugging -- correct value is taken from slice header
  img->num_dec_mb = 0;
  img->newframe = 1;

  while ((currSlice->next_header != EOS && currSlice->next_header != SOP))
  {
    current_header = read_new_slice();

    if (current_header == EOS)
    {
      exit_picture();
      return EOS;
    }

    decode_slice(img, inp, current_header);

    img->newframe = 0;
    img->current_slice_nr++;
  }

  exit_picture();

  return (SOP);
}

int read_new_slice()
{
......     
  while (1)
  {
   ......
    if (input->FileFormat == PAR_OF_ANNEXB)
      ret=GetAnnexbNALU (nalu);
    else
      ret=GetRTPNALU (nalu);
   ......
  }
    ...... 
}

从上述跟踪可知：GetAnnexbNALU实际上就是从码流中提取一个NALU的过程，一个slice一般就对应了一个NALU, 在decode_one_frame中，先调用read_new_slice, 进而decode_slice, 逻辑本就该如此.

现在进入GetAnnexbNALU函数看看:

int GetAnnexbNALU (NALU_t *nalu)
{
  int info2, info3, pos = 0;
  int StartCodeFound, rewind;
  char *Buf;
  int LeadingZero8BitsCount=0, TrailingZero8Bits=0;
    
  if ((Buf = (char*)calloc (nalu->max_size , sizeof(char))) == NULL) no_mem_exit("GetAnnexbNALU: Buf");

  // bits是全局的文件指针，已经指向了打开的test.264
  while(!feof(bits) && (Buf[pos++]=fgetc(bits))==0);
  
  if(feof(bits))
  {
    if(pos==0)
    return 0;
    else
    {
      printf( "GetAnnexbNALU can't read start code\n");
      free(Buf);
      return -1;
    }
  }

  if(Buf[pos-1]!=1)
  {
    printf ("GetAnnexbNALU: no Start Code at the begin of the NALU, return -1\n");
    free(Buf);
    return -1;
  }

  if(pos<3)
  {
    printf ("GetAnnexbNALU: no Start Code at the begin of the NALU, return -1\n");
    free(Buf);
    return -1;
  }
  else if(pos==3)
  {
    nalu->startcodeprefix_len = 3;
    LeadingZero8BitsCount = 0;
  }
  else
  {
    LeadingZero8BitsCount = pos-4;
    nalu->startcodeprefix_len = 4;
  }

  //the 1st byte stream NAL unit can has leading_zero_8bits, but subsequent ones are not
  //allowed to contain it since these zeros(if any) are considered trailing_zero_8bits
  //of the previous byte stream NAL unit.
  if(!IsFirstByteStreamNALU && LeadingZero8BitsCount>0)
  {
    printf ("GetAnnexbNALU: The leading_zero_8bits syntax can only be present in the first byte stream NAL unit, return -1\n");
    free(Buf);
    return -1;
  }
  IsFirstByteStreamNALU=0;

  StartCodeFound = 0;
  info2 = 0;
  info3 = 0;

  while (!StartCodeFound)
  {
    if (feof (bits))
    {
      //Count the trailing_zero_8bits
      while(Buf[pos-2-TrailingZero8Bits]==0)
        TrailingZero8Bits++;
      nalu->len = (pos-1)-nalu->startcodeprefix_len-LeadingZero8BitsCount-TrailingZero8Bits;
      
	  // 关键地方：内存复制
	  memcpy (nalu->buf, &Buf[LeadingZero8BitsCount+nalu->startcodeprefix_len], nalu->len);     
      nalu->forbidden_bit = (nalu->buf[0]>>7) & 1;
      nalu->nal_reference_idc = (nalu->buf[0]>>5) & 3;
      nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;

// printf ("GetAnnexbNALU, eof case: pos %d nalu->len %d, nalu->reference_idc %d, nal_unit_type %d \n", pos, nalu->len, nalu->nal_reference_idc, nalu->nal_unit_type);

#if TRACE
  fprintf (p_trace, "\n\nLast NALU in File\n\n");
  fprintf (p_trace, "Annex B NALU w/ %s startcode, len %d, forbidden_bit %d, nal_reference_idc %d, nal_unit_type %d\n\n",
    nalu->startcodeprefix_len == 4?"long":"short", nalu->len, nalu->forbidden_bit, nalu->nal_reference_idc, nalu->nal_unit_type);
  fflush (p_trace);
#endif

      free(Buf);
      return pos-1;
    }
    Buf[pos++] = fgetc (bits);
    info3 = FindStartCode(&Buf[pos-4], 3);
    if(info3 != 1)
      info2 = FindStartCode(&Buf[pos-3], 2);
    StartCodeFound = (info2 == 1 || info3 == 1);
  }

  //Count the trailing_zero_8bits
  if(info3==1)	//if the detected start code is 00 00 01, trailing_zero_8bits is sure not to be present
  {
    while(Buf[pos-5-TrailingZero8Bits]==0)
      TrailingZero8Bits++;
  }
  // Here, we have found another start code (and read length of startcode bytes more than we should
  // have.  Hence, go back in the file
  rewind = 0;
  if(info3 == 1)
    rewind = -4;
  else if (info2 == 1)
    rewind = -3;
  else
    printf(" Panic: Error in next start code search \n");

  if (0 != fseek (bits, rewind, SEEK_CUR))
  {
    snprintf (errortext, ET_SIZE, "GetAnnexbNALU: Cannot fseek %d in the bit stream file", rewind);
    free(Buf);
    error(errortext, 600);
  }

  // Here the leading zeros(if any), Start code, the complete NALU, trailing zeros(if any)
  // and the next start code is in the Buf.
  // The size of Buf is pos, pos+rewind are the number of bytes excluding the next
  // start code, and (pos+rewind)-startcodeprefix_len-LeadingZero8BitsCount-TrailingZero8Bits
  // is the size of the NALU.

  nalu->len = (pos+rewind)-nalu->startcodeprefix_len-LeadingZero8BitsCount-TrailingZero8Bits;
  
  // 关键地方：内存复制
  memcpy (nalu->buf, &Buf[LeadingZero8BitsCount+nalu->startcodeprefix_len], nalu->len);
  nalu->forbidden_bit = (nalu->buf[0]>>7) & 1;
  nalu->nal_reference_idc = (nalu->buf[0]>>5) & 3;
  nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;


//printf ("GetAnnexbNALU, regular case: pos %d nalu->len %d, nalu->reference_idc %d, nal_unit_type %d \n", pos, nalu->len, nalu->nal_reference_idc, nalu->nal_unit_type);
#if TRACE
  fprintf (p_trace, "\n\nAnnex B NALU w/ %s startcode, len %d, forbidden_bit %d, nal_reference_idc %d, nal_unit_type %d\n\n",
    nalu->startcodeprefix_len == 4?"long":"short", nalu->len, nalu->forbidden_bit, nalu->nal_reference_idc, nalu->nal_unit_type);
  fflush (p_trace);
#endif
  
  free(Buf);
 
  return (pos+rewind);
}

可见，GetAnnexbNALU函数正好实现了从test.264中读取一个NALU，调试代码后发现也正是如此，最后给出对应的trace_dec.txt的部分内容:

Annex B NALU w/ long startcode, len 8, forbidden_bit 0, nal_reference_idc 3, nal_unit_type 7

@0      SPS: profile_idc                                      01000010 ( 66)
@8      SPS: constrained_set0_flag                                   0 ( 0)
@9      SPS: constrained_set1_flag                                   0 ( 0)
@10     SPS: constrained_set2_flag                                   0 ( 0)
@11     SPS: reserved_zero_5bits                                 00000 ( 0)
@16     SPS: level_idc                                        00011110 ( 30)
@24     SPS: seq_parameter_set_id                                    1 ( 0)
@25     SPS: log2_max_frame_num_minus4                               1 ( 0)
@26     SPS: pic_order_cnt_type                                      1 ( 0)
@27     SPS: log2_max_pic_order_cnt_lsb_minus4                       1 ( 0)
@28     SPS: num_ref_frames                                    0001011 ( 10)
@35     SPS: gaps_in_frame_num_value_allowed_flag                    0 ( 0)
@36     SPS: pic_width_in_mbs_minus1                           0001011 ( 10)
@43     SPS: pic_height_in_map_units_minus1                    0001001 ( 8)
@50     SPS: frame_mbs_only_flag                                     1 ( 1)
@51     SPS: direct_8x8_inference_flag                               0 ( 0)
@52     SPS: frame_cropping_flag                                     0 ( 0)
@53     SPS: vui_parameters_present_flag                             0 ( 0)

Annex B NALU w/ long startcode, len 5, forbidden_bit 0, nal_reference_idc 3, nal_unit_type 8

@54     PPS: pic_parameter_set_id                                    1 ( 0)
@55     PPS: seq_parameter_set_id                                    1 ( 0)
@56     PPS: entropy_coding_mode_flag                                0 ( 0)
@57     PPS: pic_order_present_flag                                  0 ( 0)
@58     PPS: num_slice_groups_minus1                                 1 ( 0)
@59     PPS: num_ref_idx_l0_active_minus1                      0001010 ( 9)
@66     PPS: num_ref_idx_l1_active_minus1                      0001010 ( 9)
@73     PPS: weighted prediction flag                                0 ( 0)
@74     PPS: weighted_bipred_idc                                    00 ( 0)
@76     PPS: pic_init_qp_minus26                                     1 ( 0)
@77     PPS: pic_init_qs_minus26                                     1 ( 0)
@78     PPS: chroma_qp_index_offset                                  1 ( 0)
@79     PPS: deblocking_filter_control_present_flag                  0 ( 0)
@80     PPS: constrained_intra_pred_flag                             0 ( 0)
@81     PPS: redundant_pic_cnt_present_flag                          0 ( 0)

Last NALU in File

Annex B NALU w/ long startcode, len 2741, forbidden_bit 0, nal_reference_idc 3, nal_unit_type 5

@82     SH: first_mb_in_slice                                        1 ( 0)
@83     SH: slice_type                                         0001000 ( 7)
@90     SH: pic_parameter_set_id                                     1 ( 0)
@91     SH: frame_num                                             0000 ( 0)
@95     SH: idr_pic_id                                               1 ( 0)
@96     SH: pic_order_cnt_lsb                                     0000 ( 0)
@100    SH: no_output_of_prior_pics_flag                             0 ( 0)
@101    SH: long_term_reference_flag                                 0 ( 0)
@102    SH: slice_qp_delta                                       00100 ( 2)

*********** POC: 0 (I/P) MB: 0 Slice: 0 Type 2 **********
@107    mb_type                                                      1 ( 0)
@108    intra4x4_pred_mode                                           1 ( -1)
@109    intra4x4_pred_mode                                        0001 ( 1)
@113    intra4x4_pred_mode                                           1 ( -1)
@114    intra4x4_pred_mode                                        0110 ( 6)
@118    intra4x4_pred_mode                                        0001 ( 1)
@122    intra4x4_pred_mode                                        0111 ( 7)
@126    intra4x4_pred_mode                                           1 ( -1)
@127    intra4x4_pred_mode                                           1 ( -1)
@128    intra4x4_pred_mode                                        0000 ( 0)
@132    intra4x4_pred_mode                                        0000 ( 0)
@136    intra4x4_pred_mode                                        0000 ( 0)
@140    intra4x4_pred_mode                                        0111 ( 7)
@144    intra4x4_pred_mode                                        0101 ( 5)
@148    intra4x4_pred_mode                                           1 ( -1)
@149    intra4x4_pred_mode                                        0111 ( 7)
@153    intra4x4_pred_mode                                        0111 ( 7)

......