HEVC学习（十七） —— NAL unit 的解码过程之一

HEVCHMNAL解码过程

下图为官方标准中NAL层的句法元素，且以伪代码的形式给出了解码过程：



在HM中由TAppDecTop::decode()调用byteStreamNALUnit(bytestream, nalUnit, stats)实现如上伪代码：

[cpp]
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/**
* Parse an AVC AnnexB Bytestream bs to extract a single nalUnit
* while accumulating bytestream statistics into stats.
*
* Returns false if EOF was reached (NB, nalunit data may be valid),
* otherwise true.
*/
Bool
byteStreamNALUnit(
InputByteStream& bs,
vector<uint8_t>& nalUnit,
AnnexBStats& stats)
{
Bool eof = false;
try
{
_byteStreamNALUnit(bs, nalUnit, stats); //!< 实际完成NAL解析工作的函数

}
catch (...) //!< 捕获所有异常

{
eof = true;
}
stats.m_numBytesInNALUnit = UInt(nalUnit.size());
return eof;
}

/**
* Parse an AVC AnnexB Bytestream bs to extract a single nalUnit
* while accumulating bytestream statistics into stats.
*
* Returns false if EOF was reached (NB, nalunit data may be valid),
*         otherwise true.
*/
Bool
byteStreamNALUnit(
InputByteStream& bs,
vector<uint8_t>& nalUnit,
AnnexBStats& stats)
{
Bool eof = false;
try
{
_byteStreamNALUnit(bs, nalUnit, stats); //!< 实际完成NAL解析工作的函数
}
catch (...) //!< 捕获所有异常
{
eof = true;
}
stats.m_numBytesInNALUnit = UInt(nalUnit.size());
return eof;
}

在分析NAL解析过程之前，先介绍几个会被调用到的子函数，以便更好地理解解析过程。
1. Bool eofBeforeNBytes(UInt n)
如果在读码流的接下来的n字节的过程中遇到了文件结束符，则该函数返回true，否则返回false。

[cpp]
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/**
* returns true if an EOF will be encountered within the next
* n bytes.
*/
Bool eofBeforeNBytes(UInt n)
{
assert(n <= 4);
if (m_NumFutureBytes >= n) //!< m_NumFutureBytes大于等于n只会在该函数被调用2次及2次以上的情况下发生，满足该条件时无须继续读多余的字节，故返回false

return false;

n -= m_NumFutureBytes; //!< n先减去m_NumFutureBytes的目的是防止被函数peekBytes调用时再读入接下来的n字节数据

try
{
for (UInt i = 0; i < n; i++)
{
m_FutureBytes = (m_FutureBytes << 8) | m_Input.get(); //!< 每次读入一个字节，循环结束后，m_FutureBytes存放的是读入的n个字节的数据

m_NumFutureBytes++;
}
}
catch (...) //!< 出现异常即读到文件结尾，返回true

{
return true;
}
return false;
}

/**
* returns true if an EOF will be encountered within the next
* n bytes.
*/
Bool eofBeforeNBytes(UInt n)
{
assert(n <= 4);
if (m_NumFutureBytes >= n) //!< m_NumFutureBytes大于等于n只会在该函数被调用2次及2次以上的情况下发生，满足该条件时无须继续读多余的字节，故返回false
return false;

n -= m_NumFutureBytes; //!< n先减去m_NumFutureBytes的目的是防止被函数peekBytes调用时再读入接下来的n字节数据
try
{
for (UInt i = 0; i < n; i++)
{
m_FutureBytes = (m_FutureBytes << 8) | m_Input.get(); //!< 每次读入一个字节，循环结束后，m_FutureBytes存放的是读入的n个字节的数据
m_NumFutureBytes++;
}
}
catch (...) //!< 出现异常即读到文件结尾，返回true
{
return true;
}
return false;
}

2. uint32_t peekBytes(UInt n)
该函数在不移动文件指针的前提下返回文件中接下来的n字节。实现的即是伪代码中的next_bits(n)的功能。

[cpp]
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/**
* return the next n bytes in the stream without advancing
* the stream pointer.
*
* Returns: an unsigned integer representing an n byte bigendian
* word.
*
* If an attempt is made to read past EOF, an n-byte word is
* returned, but the portion that required input bytes beyond EOF
* is undefined.
*
*/
uint32_t peekBytes(UInt n)
{
eofBeforeNBytes(n);
return m_FutureBytes >> 8*(m_NumFutureBytes - n); //!< 若m_NumFutureBytes=4, n=3，则返回m_FutureBytes左移8位后（即有效数据位为3字节）的数据

}

/**
* return the next n bytes in the stream without advancing
* the stream pointer.
*
* Returns: an unsigned integer representing an n byte bigendian
* word.
*
* If an attempt is made to read past EOF, an n-byte word is
* returned, but the portion that required input bytes beyond EOF
* is undefined.
*
*/
uint32_t peekBytes(UInt n)
{
eofBeforeNBytes(n);
return m_FutureBytes >> 8*(m_NumFutureBytes - n); //!< 若m_NumFutureBytes=4, n=3，则返回m_FutureBytes左移8位后（即有效数据位为3字节）的数据
}

3. uint8_t readByte()
该函数读文件的一个字节并返回。

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/**
* consume and return one byte from the input.
*
* If bytestream is already at EOF prior to a call to readByte(),
* an exception std::ios_base::failure is thrown.
*/
uint8_t readByte()
{
if (!m_NumFutureBytes) //!< m_FutureBytes为NULL，则从文件中读入一个字节并返回

{
uint8_t byte = m_Input.get();
return byte;
}//! m_FutureBytes非NULL，则从它当中取出一个字节出来

m_NumFutureBytes--; //!< 计数值减1
uint8_t wanted_byte = m_FutureBytes >> 8*m_NumFutureBytes; //!< m_FutureBytes为4字节，取出有效数据中的最高字节

m_FutureBytes &= ~(0xff << 8*m_NumFutureBytes); //!< 对应位置的数据清零

return wanted_byte;
}

/**
* consume and return one byte from the input.
*
* If bytestream is already at EOF prior to a call to readByte(),
* an exception std::ios_base::failure is thrown.
*/
uint8_t readByte()
{
if (!m_NumFutureBytes) //!< m_FutureBytes为NULL，则从文件中读入一个字节并返回
{
uint8_t byte = m_Input.get();
return byte;
}//! m_FutureBytes非NULL，则从它当中取出一个字节出来
m_NumFutureBytes--; //!< 计数值减1
uint8_t wanted_byte = m_FutureBytes >> 8*m_NumFutureBytes; //!< m_FutureBytes为4字节，取出有效数据中的最高字节
m_FutureBytes &= ~(0xff << 8*m_NumFutureBytes); //!< 对应位置的数据清零
return wanted_byte;
}

4. uint32_t readBytes(UInt n)
该函数读文件的n个字节并返回。

[cpp]
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/**
* consume and return n bytes from the input. n bytes from
* bytestream are interpreted as bigendian when assembling
* the return value.
*/
uint32_t readBytes(UInt n)
{
uint32_t val = 0;
for (UInt i = 0; i < n; i++)
val = (val << 8) | readByte(); //!< 每次调用readByte()读入一个字节，通过对val左移8位且与输入值进行或运算实现将n个字节存储到val这个变量中

return val;
}

/**
* consume and return n bytes from the input.  n bytes from
* bytestream are interpreted as bigendian when assembling
* the return value.
*/
uint32_t readBytes(UInt n)
{
uint32_t val = 0;
for (UInt i = 0; i < n; i++)
val = (val << 8) | readByte(); //!< 每次调用readByte()读入一个字节，通过对val左移8位且与输入值进行或运算实现将n个字节存储到val这个变量中
return val;
}

（http://blog.csdn.net/hevc_cjl/article/details/8461660#comments）