【视频编解码·学习笔记】13. 提取PPS信息程序

PPS结构解析
与之前解析SPS方式类似

一、定义PPS类：

在3.NAL Unit目录下，新建PicParamSet.cpp和PicParamSet.h，在这两个文件中写入类的定义和函数实现。

类定义写在PicParamSet.h文件中，定义待解析语法元素变量，并定义相应的setter函数，代码如下：

#ifndef _PICPARAM_SET_H
#define _PICPARAM_SET_H

class CPicParamSet
{
public:
    CPicParamSet();
    ~CPicParamSet();

    void Set_pps_id(UINT8 ppsID);
    void Set_sps_id(UINT8 spsID);
    void Set_num_slice_groups(UINT8 num_slice_grops);
    void Set_num_ref_idx(UINT8 l0, UINT8 l1);
    void Set_weighted_bipred_idc(UINT8 weighted_bipred_idc);
    void Set_pic_init_qp(int pic_init_qp);
    void Set_pic_init_qs(int pic_init_qs);
    void Set_chroma_qp_index_offset(int chroma_qp_index_offset);
    void Set_multiple_flags(UINT16 flags);

private:
    UINT8  m_pps_id;
    UINT8  m_sps_id;
    bool   m_entropy_coding_flag;
    bool   m_bottom_field_pic_order_in_frame_present_flag;
    UINT8  m_num_slice_groups;
    UINT8  m_num_ref_idx_l0_default_active;
    UINT8  m_num_ref_idx_l1_default_active;
    bool   m_weighted_pred_flag;
    UINT8  m_weighted_bipred_idc;
    int    m_pic_init_qp;
    int    m_pic_init_qs;
    int    m_chroma_qp_index_offset;
    bool   m_deblocking_filter_control_present_flag;
    bool   m_constrained_intra_pred_flag;
    bool   m_redundant_pic_cnt_present_flag;
    bool   m_transform_8x8_mode_flag;
};

#endif // !_PICPARAM_SET_H

setter函数具体实现写在PicParamSet.cpp中，均为简单的set赋值方法，所有的标志位仍按位存在一个flag中，并从中解析，代码如下：

#include "stdafx.h"
#include "PicParamSet.h"

CPicParamSet::CPicParamSet()
{
}

CPicParamSet::~CPicParamSet()
{
}

void CPicParamSet::Set_pps_id(UINT8 ppsID)
{
    m_pps_id = ppsID;
}

void CPicParamSet::Set_sps_id(UINT8 spsID)
{
    m_sps_id = spsID;
}

void CPicParamSet::Set_num_slice_groups(UINT8 num_slice_grops)
{
    m_num_slice_groups = num_slice_grops;
}

void CPicParamSet::Set_num_ref_idx(UINT8 l0, UINT8 l1)
{
    m_num_ref_idx_l0_default_active = l0;
    m_num_ref_idx_l1_default_active = l1;
}

void CPicParamSet::Set_weighted_bipred_idc(UINT8 weighted_bipred_idc)
{
    m_weighted_bipred_idc = weighted_bipred_idc;
}

void CPicParamSet::Set_pic_init_qp(int pic_init_qp)
{
    m_pic_init_qp = pic_init_qp;
}

void CPicParamSet::Set_pic_init_qs(int  pic_init_qs)
{
    m_pic_init_qs = pic_init_qs;
}

void CPicParamSet::Set_chroma_qp_index_offset(int chroma_qp_index_offset)
{
    m_chroma_qp_index_offset = chroma_qp_index_offset;
}

void CPicParamSet::Set_multiple_flags(UINT16 flags)
{
    m_entropy_coding_flag = flags & 1;
    m_bottom_field_pic_order_in_frame_present_flag = flags & (1 << 1);
    m_weighted_pred_flag = flags & (1 << 2);
    m_deblocking_filter_control_present_flag = flags & (1 << 3);
    m_constrained_intra_pred_flag = flags & (1 << 4);
    m_redundant_pic_cnt_present_flag = flags & (1 << 5);
}

二、解析NALUnit中PPS数据：

1. 添加解析有符号指数哥伦布编码函数：

由于PPS语法元素中包含有符号指数哥伦布编码的数据，这里新建一个函数Get_sev_code_num
无符号指数哥伦布编码(k)转为有符号的(n)公式：\(n = (-1)^{(k+1)} \times Ceil(k / 2)\)

int Get_sev_code_num(UINT8 * buf, UINT8 & bytePosition, UINT8 & bitPosition)
{
    int uev = Get_uev_code_num(buf, bytePosition, bitPosition);
    int sign = (uev % 2) ? 1 : -1;
    int sev = sign * ((uev + 1) >> 1);
    return sev;
}

2. 获取PPS中各个成员变量的值：

在NALUnit.h和NALUnit.cpp中添加函数，Parse_as_seq_param_set() 用于解析语法元素，代码如下。（均按照学习笔记12中官方文档顺序解析即可）

int CNalUnit::Parse_as_pic_param_set(CPicParamSet * pps)
{
    UINT8  pps_id = 0;
    UINT8  sps_id = 0;
    bool   entropy_coding_flag = 0;
    bool   bottom_field_pic_order_in_frame_present_flag = 0;
    UINT8  num_slice_groups = 0;
    UINT8  num_ref_idx_l0_default_active = 0;
    UINT8  num_ref_idx_l1_default_active = 0;
    bool   weighted_pred_flag = 0;
    UINT8  weighted_bipred_idc = 0;
    int    pic_init_qp = 0;
    int    pic_init_qs = 0;
    int    chroma_qp_index_offset = 0;
    bool   deblocking_filter_control_present_flag = 0;
    bool   constrained_intra_pred_flag = 0;
    bool   redundant_pic_cnt_present_flag = 0;


    UINT8 bitPosition = 0;
    UINT8 bytePosition = 0;
    UINT16 flags = 0;

    pps_id = Get_uev_code_num(m_pSODB, bytePosition, bitPosition);
    sps_id = Get_uev_code_num(m_pSODB, bytePosition, bitPosition);

    entropy_coding_flag = Get_bit_at_position(m_pSODB, bytePosition, bitPosition);
    flags |= entropy_coding_flag;
    bottom_field_pic_order_in_frame_present_flag = Get_bit_at_position(m_pSODB, bytePosition, bitPosition);
    flags |= bottom_field_pic_order_in_frame_present_flag << 1;

    num_slice_groups = Get_uev_code_num(m_pSODB, bytePosition, bitPosition) + 1;

    if (1 != num_slice_groups)
    {
        return -1;
    }

    num_ref_idx_l0_default_active = Get_uev_code_num(m_pSODB, bytePosition, bitPosition) + 1;
    num_ref_idx_l1_default_active = Get_uev_code_num(m_pSODB, bytePosition, bitPosition) + 1;

    weighted_pred_flag = Get_bit_at_position(m_pSODB, bytePosition, bitPosition);
    flags |= weighted_pred_flag << 2;

    // 这里是获取连续两个比特位数据——把第一次获取到的左移1位，再加上第二次获取到的
    weighted_bipred_idc = Get_bit_at_position(m_pSODB, bytePosition, bitPosition) << 1 + Get_bit_at_position(m_pSODB, bytePosition, bitPosition);

    pic_init_qp = Get_sev_code_num(m_pSODB, bytePosition, bitPosition) + 26;
    pic_init_qs = Get_sev_code_num(m_pSODB, bytePosition, bitPosition) + 26;
    chroma_qp_index_offset = Get_sev_code_num(m_pSODB, bytePosition, bitPosition);

    deblocking_filter_control_present_flag = Get_bit_at_position(m_pSODB, bytePosition, bitPosition);
    flags |= deblocking_filter_control_present_flag << 3;
    constrained_intra_pred_flag = Get_bit_at_position(m_pSODB, bytePosition, bitPosition);
    flags |= constrained_intra_pred_flag << 4;
    redundant_pic_cnt_present_flag = Get_bit_at_position(m_pSODB, bytePosition, bitPosition);
    flags |= redundant_pic_cnt_present_flag << 5;

    pps->Set_pps_id(pps_id);
    pps->Set_sps_id(sps_id);
    pps->Set_num_slice_groups(num_slice_groups);
    pps->Set_num_ref_idx(num_ref_idx_l0_default_active, num_ref_idx_l1_default_active);
    pps->Set_weighted_bipred_idc(weighted_bipred_idc);
    pps->Set_pic_init_qp(pic_init_qp);
    pps->Set_pic_init_qs(pic_init_qs);
    pps->Set_chroma_qp_index_offset(chroma_qp_index_offset);
    pps->Set_multiple_flags(flags);

    return 0;
}

三、添加调用部分：

回到Stream.cpp中，找到Parse_h264_bitstream() 函数中switch (nalType)条件分支，在后面添加解析序列参数集pps的部分：

case 8:
    // 解析PPS NAL 数据
    if (m_pps)
    {
        delete m_pps;
    }
    m_pps = new CPicParamSet;
    nalUint.Parse_as_pic_param_set(m_pps);
    break;