H.264简介

H.264原始码流由NALU组成。

功能分为两层：VCL（video coding layer）、NAL（network abstraction layer）

其中每个NALU之间通过startcode（起始码）进行分隔，起始码分成两种：0x000001或者0x00000001。

如果NALU对应的Slice为一帧的开始就用0x00000001，否则就用0x000001。

NALU = NALU Header + BRSP（Raw Byte Squence Payload）

1、NALU Header（1Byte）

2、RBSP

SODB与RBSP
SODB 数据比特串 -> 是编码后的原始数据.
RBSP 原始字节序列载荷 -> 在原始编码数据的后面添加了 结尾比特。一个 bit“1”若干比特“0”，以便字节对齐。

H.264全面观

1 frame = n slice；1 slice = n Macroblock；1 Macroblock= 16 * 16yuv。

3、Slice

Slice = Slice Header + Slice Data

片是H.264提出的新概念，通过编码图片后切分通过高效的方式整合出来的概念。一张图片有一个或者多个片，而片由NALU装载并进行网络传输的。

但是NALU不一定是切片，这是充分不必要条件，因为 NALU 还有可能装载着其他用作描述视频的信息.

那么为什么要设置片呢?
设置片的目的是为了限制误码的扩散和传输，应使编码片相互间是独立的。某片的预测不能以其他片中的宏块为参考图像，这样某一片中的预测误差才不会传播到其他片中。

可以看到上图中，每个图像中，若干宏块(Macroblock)被排列成片。一个视频图像可编程一个或更多个片，每片包含整数个宏块 (MB),每片至少包含一个宏块。
片有一下五种类型:

4、Macroblock

 1 macroblock = 16*16Y + 8*8 Cb + 8*8 Cr

链接：https://blog.csdn.net/andywang201001/article/details/80274886

typedef enum {
    NALU_TYPE_SLICE = 1,
    NALU_TYPE_DPA = 2,
    NALU_TYPE_DPB = 3,8
    NALU_TYPE_DPC = 4,
    NALU_TYPE_IDR = 5,
    NALU_TYPE_SEI = 6,
    NALU_TYPE_SPS = 7,
    NALU_TYPE_PPS = 8,
    NALU_TYPE_AUD = 9,
    NALU_TYPE_EOSEQ = 10,
    NALU_TYPE_EOSTREAM = 11,
    NALU_TYPE_FILL = 12,
} NaluType;

typedef enum {
    NALU_PRIORITY_DISPOSABLE = 0,
    NALU_PRIRITY_LOW = 1,
    NALU_PRIORITY_HIGH = 2,
    NALU_PRIORITY_HIGHEST = 3
} NaluPriority;


typedef struct
{
    int startcodeprefix_len;      //! 4 for parameter sets and first slice in picture, 3 for everything else (suggested)
    unsigned len;                 //! Length of the NAL unit (Excluding the start code, which does not belong to the NALU)
    unsigned max_size;            //! Nal Unit Buffer size
    int forbidden_bit;            //! should be always FALSE
    int nal_reference_idc;        //! NALU_PRIORITY_xxxx
    int nal_unit_type;            //! NALU_TYPE_xxxx    
    char *buf;                    //! contains the first byte followed by the RBSP
} NALU_t;

FILE *h264bitstream = NULL;                //!< the bit stream file

int info2 = 0, info3 = 0;

static int FindStartCode2(unsigned char *Buf) {
    if (Buf[0] != 0 || Buf[1] != 0 || Buf[2] != 1) return 0; //0x000001?
    else return 1;
}

static int FindStartCode3(unsigned char *Buf) {
    if (Buf[0] != 0 || Buf[1] != 0 || Buf[2] != 0 || Buf[3] != 1) return 0;//0x00000001?
    else return 1;
}


int GetAnnexbNALU(NALU_t *nalu) {
    int pos = 0;
    int StartCodeFound, rewind;
    unsigned char *Buf;

    if ((Buf = (unsigned char*)calloc(nalu->max_size, sizeof(char))) == NULL)
        printf("GetAnnexbNALU: Could not allocate Buf memory
");

    nalu->startcodeprefix_len = 3;

    if (3 != fread(Buf, 1, 3, h264bitstream)) {
        free(Buf);
        return 0;
    }
    info2 = FindStartCode2(Buf);
    if (info2 != 1) {
        if (1 != fread(Buf + 3, 1, 1, h264bitstream)) {
            free(Buf);
            return 0;
        }
        info3 = FindStartCode3(Buf);
        if (info3 != 1) {
            free(Buf);
            return -1;
        }
        else {
            pos = 4;
            nalu->startcodeprefix_len = 4;
        }
    }
    else {
        nalu->startcodeprefix_len = 3;
        pos = 3;
    }
    StartCodeFound = 0;
    info2 = 0;
    info3 = 0;

    while (!StartCodeFound) {
        if (feof(h264bitstream)) {
            nalu->len = (pos - 1) - nalu->startcodeprefix_len;
            memcpy(nalu->buf, &Buf[nalu->startcodeprefix_len], nalu->len);
            nalu->forbidden_bit = nalu->buf[0] & 0x80; //1 bit
            nalu->nal_reference_idc = nalu->buf[0] & 0x60; // 2 bit
            nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;// 5 bit
            free(Buf);
            return pos - 1;
        }
        Buf[pos++] = fgetc(h264bitstream);
        info3 = FindStartCode3(&Buf[pos - 4]);
        if (info3 != 1)
            info2 = FindStartCode2(&Buf[pos - 3]);
        StartCodeFound = (info2 == 1 || info3 == 1);
    }

    // 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 = (info3 == 1) ? -4 : -3;

    if (0 != fseek(h264bitstream, rewind, SEEK_CUR)) {
        free(Buf);
        printf("GetAnnexbNALU: Cannot fseek in the bit stream file");
    }

    // Here the Start code, the complete NALU, 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 is the size of the NALU excluding the start code

    nalu->len = (pos + rewind) - nalu->startcodeprefix_len;
    memcpy(nalu->buf, &Buf[nalu->startcodeprefix_len], nalu->len);//
    nalu->forbidden_bit = nalu->buf[0] & 0x80; //1 bit
    nalu->nal_reference_idc = nalu->buf[0] & 0x60; // 2 bit
    nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;// 5 bit
    free(Buf);

    return (pos + rewind);
}

/**
 * Analysis H.264 Bitstream
 * @param url    Location of input H.264 bitstream file.
 */
int simplest_h264_parser(char *url) {

    NALU_t *n;
    int buffersize = 100000;
    FILE *myout = stdout;

    h264bitstream = fopen(url, "rb+");
    if (h264bitstream == NULL) {
        printf("Open file error
");
        return 0;
    }

    n = (NALU_t*)calloc(1, sizeof(NALU_t));
    if (n == NULL) {
        printf("Alloc NALU Error
");
        return 0;
    }

    n->max_size = buffersize;
    n->buf = (char*)calloc(buffersize, sizeof(char));
    if (n->buf == NULL) {
        free(n);
        printf("AllocNALU: n->buf");
        return 0;
    }

    int data_offset = 0;
    int nal_num = 0;
    printf("-----+-------- NALU Table ------+---------+
");
    printf(" NUM |    POS  |    IDC |  TYPE |   LEN   |
");
    printf("-----+---------+--------+-------+---------+
");

    while (!feof(h264bitstream))
    {
        int data_lenth;
        data_lenth = GetAnnexbNALU(n);

        char type_str[20] = { 0 };
        switch (n->nal_unit_type) {
        case NALU_TYPE_SLICE:sprintf(type_str, "SLICE"); break;
        case NALU_TYPE_DPA:sprintf(type_str, "DPA"); break;
        case NALU_TYPE_DPB:sprintf(type_str, "DPB"); break;
        case NALU_TYPE_DPC:sprintf(type_str, "DPC"); break;
        case NALU_TYPE_IDR:sprintf(type_str, "IDR"); break;
        case NALU_TYPE_SEI:sprintf(type_str, "SEI"); break;
        case NALU_TYPE_SPS:sprintf(type_str, "SPS"); break;
        case NALU_TYPE_PPS:sprintf(type_str, "PPS"); break;
        case NALU_TYPE_AUD:sprintf(type_str, "AUD"); break;
        case NALU_TYPE_EOSEQ:sprintf(type_str, "EOSEQ"); break;
        case NALU_TYPE_EOSTREAM:sprintf(type_str, "EOSTREAM"); break;
        case NALU_TYPE_FILL:sprintf(type_str, "FILL"); break;
        }
        char idc_str[20] = { 0 };
        switch (n->nal_reference_idc >> 5) {
        case NALU_PRIORITY_DISPOSABLE:sprintf(idc_str, "DISPOS"); break;
        case NALU_PRIRITY_LOW:sprintf(idc_str, "LOW"); break;
        case NALU_PRIORITY_HIGH:sprintf(idc_str, "HIGH"); break;
        case NALU_PRIORITY_HIGHEST:sprintf(idc_str, "HIGHEST"); break;
        }

        fprintf(myout, "%5d| %8d| %7s| %6s| %8d|
", nal_num, data_offset, idc_str, type_str, n->len);

        data_offset = data_offset + data_lenth;

        nal_num++;
    }

    //Free
    if (n) {
        if (n->buf) {
            free(n->buf);
            n->buf = NULL;
        }
        free(n);
    }
    return 0;
}

链接：https://blog.csdn.net/leixiaohua1020/article/details/50534369