Dicom学习之一：大尾和小尾LittleEndian/BigEndian
是否进行交换的条件：是本地的编码方式是否和dicom文件的编码方式不同，如是，就需要进行交换。
交换的原则：
按照格式化入dicom数据集流中的基本数据单元而定的，如是ss us，那么，就需要按照两个字节进行交换，用swap2Bytes函数；如是ul sl，那么就按照四个字节进行交换，利用函数swap4Bytes来进行。
首先是Tag，默认tag是分成两步格式化进dicom文件二进制流中，是Group Tag和Element Tag的两个部分进行。所以读取的过程中，就是要分成两步，分别把两个字节读取出来后，最后在进行数据的交换。
但是对于VR值，由于它是一个字符串，写入的时候，最基本单元是一个byte，所以不用进行大小尾的交换。
同理，字符长度也需要进行交换。
再同理，数据的值也是根据写入的最基本数据单元进行的。
以下是dcmtk中的函数：
//其中，newByteOrder：EBO_LittleEndian// oldByteOrder: EBO_BigEndian OFCondition swapIfNecessary(const E_ByteOrder newByteOrder, const E_ByteOrder oldByteOrder, void * value, const Uint32 byteLength, const size_t valWidth) /* * This function swaps byteLength bytes in value if newByteOrder and oldByteOrder * differ from each other. In case bytes have to be swapped, these bytes are seperated * in valWidth elements which will be swapped seperately. * * Parameters: * newByteOrder - [in] The new byte ordering (little or big endian). * oldByteOrder - [in] The current old byte ordering (little or big endian). * value - [in] Array that contains the actual bytes which might have to be swapped. * byteLength - [in] Length of the above array. * valWidth - [in] Specifies how many bytes shall be treated together as one element. */{ /* if the two byte orderings are unknown this is an illegal call */ if(oldByteOrder != EBO_unknown && newByteOrder != EBO_unknown ) { /* and if they differ from each other and valWidth is not 1 */if (oldByteOrder != newByteOrder && valWidth != 1){ /* in case the array length equals valWidth and only 2 or 4 bytes have to be swapped */ /* we can swiftly swap these bytes by calling the corresponding functions. If this is */ /* not the case we have to call a more sophisticated function. */ if (byteLength == valWidth) {if (valWidth == 2) swap2Bytes(OFstatic_cast(Uint8 *, value));else if (valWidth == 4) swap4Bytes(OFstatic_cast(Uint8 *, value));else swapBytes(value, byteLength, valWidth); } elseswapBytes(value, byteLength, valWidth);}return EC_Normal; } return EC_IllegalCall;}inline void swap2Bytes(Uint8 * toSwap)// swaps [byte0][byte1] to [byte1][byte0]{ Uint8 tmp = toSwap[0]; toSwap[0] = toSwap[1]; toSwap[1] = tmp;}inline void swap4Bytes(Uint8 * toSwap)// swaps [byte0][byte1][byte2][byte3] to [byte3][byte2][byte1][byte0]{ Uint8 tmp = toSwap[0]; toSwap[0] = toSwap[3]; toSwap[3] = tmp; tmp = toSwap[1]; toSwap[1] = toSwap[2]; toSwap[2] = tmp;}