talk is cheap show me the code —— Linus Torvalds
本章將結(jié)合開源項(xiàng)目IMBRA講解如何解析一個(gè)DCM文件
dcm文件結(jié)構(gòu).jpg
文件開頭會(huì)有128字節(jié)的導(dǎo)言备燃，這部分?jǐn)?shù)據(jù)沒(méi)有內(nèi)容。接著是4字節(jié)DICOM文件標(biāo)識(shí)展箱，存儲(chǔ)這"DICM"腹殿。然后緊接著就是dicom數(shù)據(jù)數(shù)據(jù)元素了
打開一個(gè)DCM文件.jpg
try
{
pStream->read(oldDicomSignature, 8);
}
catch(StreamEOFError&)
{
IMEBRA_THROW(CodecWrongFormatError, "detected a wrong format");
}
// Skip the first 128 bytes (8 already skipped)
///////////////////////////////////////////////////////////
pStream->seekForward(120);
// Read the DICOM signature (DICM)
///////////////////////////////////////////////////////////
std::uint8_t dicomSignature[4];
pStream->read(dicomSignature, 4);
// Check the DICM signature
///////////////////////////////////////////////////////////
const char* checkSignature="DICM";
if(::memcmp(dicomSignature, checkSignature, 4) != 0)
{
bFailed=true;
}
舊版本的DCM文件會(huì)在開頭8個(gè)字節(jié)驗(yàn)證簽名独悴，在讀完128字節(jié)后，讀取4個(gè)字節(jié)驗(yàn)證DICOM標(biāo)識(shí)锣尉。
pStream->read((std::uint8_t*)&tagId, sizeof(tagId));
//將轉(zhuǎn)化成大端格式 或者小端格式
pStream->adjustEndian((std::uint8_t*)&tagId, sizeof(tagId), endianType);
(*pReadSubItemLength) += (std::uint32_t)sizeof(tagId);
// Check for EOF
///////////////////////////////////////////////////////////
if(pStream->endReached())
{
break;
}
// Check the byte order
///////////////////////////////////////////////////////////
if(bFirstTag && tagId==0x0200)
{
// Reverse the last adjust
pStream->adjustEndian((std::uint8_t*)&tagId, sizeof(tagId), endianType);
// Fix the byte adjustment
endianType=streamController::highByteEndian;
// Redo the byte adjustment
pStream->adjustEndian((std::uint8_t*)&tagId, sizeof(tagId), endianType);
}
接下來(lái)開始讀取數(shù)據(jù)元素了刻炒，現(xiàn)讀取兩個(gè)字節(jié)的tagId，并且驗(yàn)證大小端自沧。
if(tagId!=0x0002 && bCheckTransferSyntax)
{
// Reverse the last adjust
pStream->adjustEndian((std::uint8_t*)&tagId, sizeof(tagId), endianType);
std::string transferSyntax = pDataSet->getString(
0x0002,
0x0,
0x0010,
0,
0,
endianType == streamController::lowByteEndian ? "1.2.840.10008.1.2.1" : "1.2.840.10008.1.2.2");
if(transferSyntax == "1.2.840.10008.1.2.2")
endianType = streamController::highByteEndian;
if(transferSyntax == "1.2.840.10008.1.2")
bExplicitDataType=false;
bCheckTransferSyntax=false;
// Redo the byte adjustment
pStream->adjustEndian((std::uint8_t*)&tagId, sizeof(tagId), endianType);
}
dicom的數(shù)據(jù)元素的tagId是從大到小的讀取的,而最小的tagId就是從0x0002坟奥，當(dāng)tagId不是0x0002說(shuō)明已經(jīng)讀取完所以關(guān)于0x0002的tagId。從中找出(0002,0010)的tag拇厢，這個(gè)tag設(shè)置當(dāng)前數(shù)據(jù)是否大端格式爱谁，顯示VR編碼還是隱式VR編碼。"1.2.840.10008.1.2.2"表示大端格式孝偎，1.2.840.10008.1.2表示隱式編碼访敌。
隱式編碼.jpeg
隱式編碼實(shí)例.jpeg
顯式編碼(用于 OB、OW衣盾、OF寺旺、SQ、UT势决、UN).jpeg
顯式編碼實(shí)例.jpeg
顯式編碼(用于 OB阻塑、OW、OF果复、SQ陈莽、UT、UN).jpeg
顯式編碼實(shí)例.jpeg
if(bExplicitDataType && tagId!=0xfffe)
{
// Get the tag's type
///////////////////////////////////////////////////////////
std::string tagTypeString((size_t)2, ' ');
pStream->read((std::uint8_t*)&(tagTypeString[0]), 2);
(*pReadSubItemLength) += 2;
// Get the tag's length
///////////////////////////////////////////////////////////
pStream->read((std::uint8_t*)&tagLengthWord, sizeof(tagLengthWord));
pStream->adjustEndian((std::uint8_t*)&tagLengthWord, sizeof(tagLengthWord), endianType);
(*pReadSubItemLength) += (std::uint32_t)sizeof(tagLengthWord);
// The data type is valid
///////////////////////////////////////////////////////////
try
{
tagType = dicomDictionary::getDicomDictionary()->stringDataTypeToEnum(tagTypeString);
tagLengthDWord=(std::uint32_t)tagLengthWord;
wordSize = dicomDictionary::getDicomDictionary()->getWordSize(tagType);
if(dicomDictionary::getDicomDictionary()->getLongLength(tagType))
{
pStream->read((std::uint8_t*)&tagLengthDWord, sizeof(tagLengthDWord));
pStream->adjustEndian((std::uint8_t*)&tagLengthDWord, sizeof(tagLengthDWord), endianType);
(*pReadSubItemLength) += (std::uint32_t)sizeof(tagLengthDWord);
}
}
catch(const DictionaryUnknownDataTypeError&)
{
// The data type is not valid. Switch to implicit data type
///////////////////////////////////////////////////////////
bExplicitDataType = false;
if(endianType == streamController::lowByteEndian)
tagLengthDWord=(((std::uint32_t)tagLengthWord)<<16) | ((std::uint32_t)tagTypeString[0]) | (((std::uint32_t)tagTypeString[1])<<8);
else
tagLengthDWord=(std::uint32_t)tagLengthWord | (((std::uint32_t)tagTypeString[0])<<24) | (((std::uint32_t)tagTypeString[1])<<16);
}
} // End of the explicit data type read block
顯示編碼解析 取先獲取VR 再獲取長(zhǎng)度 最后獲取值
if((!bExplicitDataType || tagId==0xfffe))
{
// Group length. Data type is always UL
///////////////////////////////////////////////////////////
if(tagSubId == 0)
{
tagType = tagVR_t::UL;
}
else
{
try
{
tagType = dicomDictionary::getDicomDictionary()->getTagType(tagId, tagSubId);
}
catch(const DictionaryUnknownTagError&)
{
tagType = tagVR_t::UN;
}
wordSize = dicomDictionary::getDicomDictionary()->getWordSize(tagType);
}
}
隱式編碼解析
