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