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| 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");
//如果start_code前还有数据，丢弃start_code前leading_zero_8bits
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.
//字节流数据的第一个 NAL 单元才会有leading_zero_8bits;后面的 NALU 不会包含 leading_zero_8bits,
//因为这些 leading_zero_8bits 会被看做前一个 NAL 单元后面的 trailing_zero_8bits
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;
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;
}
//找 start_code，先找0x00000001,后找0x000001
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
//计算 trailing_zero_8bits,如果start_code为0x000001,trailing_zero_8bits 肯定不存在
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;
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);
}
|