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之前已经说过,在JM8.6中写码流函数为WriteAnnexbNALU,如果编码一帧,在WriteAnnexbNALU前打个断点,跟踪代码可以看到该函数被调用了3次,但是奇怪的是,在工程中搜索后,貌似找不到调用WriteAnnexbNALU函数的地方. 在搜索过程中,可以发现这样一个语句:
WriteNALU = WriteAnnexbNALU;
原来,这里是对函数指针进行了一个赋值操作,而WriteNALU又是一个全局变量,这样就清晰了,查找WriteNALU,很快就知道,WriteAnnexbNALU函数3次被调用的地方分别为:
下面代码在start_sequence函数中
nalu = NULL; nalu = GenerateSeq_parameter_set_NALU (); len += WriteNALU (nalu); // 第一处 FreeNALU (nalu); nalu = NULL; nalu = GeneratePic_parameter_set_NALU (); len += WriteNALU (nalu); // 第二处 FreeNALU (nalu);
下面代码在writeUnit函数中
stat->bit_ctr += WriteNALU (nalu); // 第三处跟踪码流后,发现写码流部分的调用关系是这样的:(千万要注意WriteNALU是函数指针,实际上调用的是WriteAnnexbNALU,鼠标右键点击后,不可能进入WriteNALU“函数”)
main -> start_sequence -> WriteNALU 接着调WriteAnnexbNALU
main -> encode_one_frame -> frame_picture接着调writeout_picture -> writeUnit -> WriteAnnexbNALU
最后来欣赏一下WriteAnnexbNALU函数:
/*! ******************************************************************************************** * \brief * Writes a NALU to the Annex B Byte Stream * * \return * number of bits written * *********************************************************************************************/int WriteAnnexbNALU (NALU_t *n){ int BitsWritten = 0; // 本次写入的比特长度 assert (n != NULL); assert (n->forbidden_bit == 0); assert (f != NULL); assert (n->startcodeprefix_len == 3 || n->startcodeprefix_len == 4);// printf ("WriteAnnexbNALU: writing %d bytes w/ startcode_len %d\n", n->len+1, n->startcodeprefix_len); if (n->startcodeprefix_len > 3) { putc (0, f); BitsWritten += 8; // 官方代码中是BitsWritten =+ 8;虽然结果刚好相同,但我认为官方代码系敲键盘时的手误 } putc (0, f); putc (0, f); putc (1, f); BitsWritten += 24; n->buf[0] = n->forbidden_bit << 7 | n->nal_reference_idc << 5 | n->nal_unit_type;// printf ("First Byte %x, nal_ref_idc %x, nal_unit_type %d\n", n->buf[0], n->nal_reference_idc, n->nal_unit_type); if (n->len != fwrite (n->buf, 1, n->len, f)) { printf ("Fatal: cannot write %d bytes to bitstream file, exit (-1)\n", n->len); exit (-1); } BitsWritten += n->len * 8; fflush (f);#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", n->startcodeprefix_len == 4?"long":"short", n->len, n->forbidden_bit, n->nal_reference_idc, n->nal_unit_type); fflush (p_trace);#endif return BitsWritten;}
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