【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:

3.NAL Unit目录下,新建PicParamSet.cppPicParamSet.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;
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