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

2012-11-26 16:12 741 查看

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)

......

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