具体 Box 分析

Avcc Box

介绍

该 Box 则包含了真实 H264 码流的SPS PPS等信息。

工具分析

这里就直接分析字段值

代码定义

因为上面有些字段出现了 bit 部分，所以我们得弄个专门对bit操作的工具。

// TimeBufferBitStream.h

class TimeBufferBitStream {
private:
    void *Buffer;
    int len; // Byte
    int index=0;
    unsigned char BitIndex;
public:
    TimeBufferBitStream(void *b, int len);
    unsigned char GetBit();
    bool IsEnd();
    unsigned char GetUChar();
};

// TimeBufferBitStream.cpp

TimeBufferBitStream::TimeBufferBitStream(void *b, int len)
    :Buffer(b), len(len), index(0), BitIndex(0)
{

}

unsigned char TimeBufferBitStream::GetBit() {
//    if(index >= len) return 0;
    unsigned char* b = (unsigned char *)Buffer;
    unsigned char ret = b[index]>>(7-BitIndex++)&1;
    if(BitIndex==8) {
        index++;
        BitIndex = 0;
    }
    return ret;
}

bool TimeBufferBitStream::IsEnd() {
    return index>=len;
}

unsigned char TimeBufferBitStream::GetUChar() {
    if(len < 1 ) return 0;
    if(index==len && BitIndex>0) return 0;
    unsigned char* b = (unsigned char *)Buffer;
    // 如果上次读取当前字节的一点bit  则这次读取的Char为上次没读取完的Char
    if(BitIndex) {
        BitIndex = 0;
    }
    return b[index++];
}

具体Box的定义：

// BaseBox.h  

// ...
// 其他 Box 的定义

class TimeAVCCBox : public BaseBox{
public:
    unsigned char configuration_version = 0; // 1 保留位默认0x01
    unsigned char avc_profile_indication = 0; // 1 保留位默认0x01 0x64即100表示Baseline 0x4D即77表示Main
    unsigned char profile_compatibility = 0; // 1
    unsigned char avc_level_indication = 0; // 1 H.264编码级别
    unsigned char reserved = 0; // 6bit 默认填1
    unsigned char length_size_minus_one = 0; // 2bit 读出的每个packet的前几字节代表数据大小
    unsigned char reserved2 = 0; // 3bit 默认填1
    unsigned char number_of_sps = 0; // 5bit sps的个数，一般情况是1
    unsigned short sps_length = 0; // 2 sps 长度
    void * sps = nullptr; // sps实际内容
    unsigned char reserved3 = 0; // 3bit 默认填1
    unsigned char number_of_pps = 0; // 5bit sps的个数，一般情况是1
    unsigned short pps_length = 0; // 2 sps 长度
    void * pps = nullptr; // sps实际内容

    TimeAVCCBox(BoxHeader h);
    TimeAVCCBox(BoxHeader h, Timebyte * d): BaseBox(h, d){};
    void PrintDataInfo() override;
};

实现

TimeAVCCBox::TimeAVCCBox(BoxHeader h) : BaseBox(h) {
    if (h.GetDataSize()) {
        data = new Timebyte[h.GetDataSize()];
    }
}

void TimeAVCCBox::PrintDataInfo() {
    printf("===========================\n");
    h.to_string();
    TimeBufferBitStream timeBufferBitStream(data, h.GetSize() - 8);
    configuration_version = timeBufferBitStream.GetUChar();
    avc_profile_indication = timeBufferBitStream.GetUChar();
    profile_compatibility = timeBufferBitStream.GetUChar();
    avc_level_indication = timeBufferBitStream.GetUChar();
    reserved = timeBufferBitStream.GetUChar();
    reserved2 = timeBufferBitStream.GetUChar();
    sps_length = timeBufferBitStream.GetUChar() << 8 | timeBufferBitStream.GetUChar();
    printf("sps: ");
    for (int i = 0; i < sps_length; ++i) {
        printf("%d ", timeBufferBitStream.GetUChar());
    }
    printf("\n");
    reserved3 = timeBufferBitStream.GetUChar();
    pps_length = timeBufferBitStream.GetUChar() << 8 | timeBufferBitStream.GetUChar();
    printf("pps: ");
    for (int i = 0; i < pps_length; ++i) {
        printf("%d ", timeBufferBitStream.GetUChar());
    }
    printf("\n");
}

完善一下Avc1 Box

// TimeAvc1.cpp

// ...

void TimeAvc1::AnalyzeBoxHeader(BoxHeader header, size_t offset) {
    // 如果是avc/h.264编码则构造Avcc Box
    // 如果是MPEG-4编码则构造Esds Box
    // 如果是MPEG-4编码则构造Hvcc Box

    if (header.GetType() == AVCC) {
        TimeAVCCBox avccBox(header, data + offset + 8);
        avccBox.PrintDataInfo();
    }
}

运行结果