6. DICOM图像显示-DCMTK-像素数据获取的三种方式和源码分析

  本篇文章对应的DICOM原理部分，在专栏从零讲解DICOM协议-成像协议的文章DICOM图像CT值计算和DICOM图像CT值转RGB，建议先了解原理，有助于理清代码思路。

  dcmimgle模块提供处理DICOM 单色图像的类。模块dcmimage提供对彩色图像的支持。dcmjpeg模块提供了对 JPEG 压缩图像的支持。因此如果要获取压缩图像的像素数据，需要先用dcmjpeg模块解压，再用dcmimgle模块读取像素数据。
  要显示DICOM图像，首先要正确获取像素数据，DCMTK有三种方式获取像素数据：

1. dcmdata模块的getUint8Array方法和getUint16Array方法
2. dcmimgle模块的getInterData方法
3. dcmimgle模块的getOutputData方法

  原理部分讲过，要将12位灰阶的CT值转换为普通显示器能显示的8位灰阶的灰度值，要进行如下几步：

1. 获取图像的像素值，即PixelData元素的Value值
2. 根据（0028|1052）rescale intercept和（0028|1053）rescale slope计算CT值
3. 通过窗宽窗位和CT值计算灰度值

以上三个dcmtk方法，分别对应三种像素计算方式。
通过手动计算，原点像素的像素值为0，CT值为-1028，灰度值为63。

下面进行详细分析：
dcmtk对getOutputData和getInterData函数描述如下

 /** render pixel data and return pointer to internal memory buffer.
     *  apply VOI/PLUT transformation and (visible) overlay planes.
     *  internal memory buffer will be delete for the next getBitmap/Output operation.
     *  output data is always padded to 8, 16, 32, ... bits (bits allocated).
     *  Supported output color models: Monochrome 2 for monochrome images and RGB
     *  (or YCbCr_Full if flag CIF_KeepYCbCrColorModel is set) for color images.
     *  The rendered pixel data is always unsigned.
     *
     ** @param  bits    number of bits per sample used to render the pixel data
     *                  (image depth, 1..MAX_BITS, 0 means 'bits stored' in the image)
     *                  (MI_PastelColor = -1 for true color pastel mode, EXPERIMENTAL)
     *  @param  frame   number of frame to be rendered (0..n-1)
     *  @param  planar  0 = color-by-pixel (R1G1B1...R2G2B2...R3G3B3...),
     *                  1 = color-by-plane (R1R2R3...G1G2G3...B1B2B3...)
     *                  (only applicable to multi-planar/color images, otherwise ignored)
     *
     ** @return pointer to internal memory buffer containing rendered pixel data
     *          (if successful, NULL otherwise)
     */
    inline const void *getOutputData(const int bits = 0,
                                     const unsigned long frame = 0,
                                     const int planar = 0)

 /** get intermediate pixel data representation (read-only).
     *  This function allows to access the pixel data after they have been extracted
     *  from the DICOM data element and the modality transformation has been applied
     *  (if present and not disabled).  Please note that for monochrome images the
     *  internal representation might be signed whereas color images are automatically
     *  converted to unsigned RGB format.  Pixels are aligned to 8, 16 or 32 bits.
     *  Always use DiPixel::getRepresentation() in order to determine the integer
     *  representation of the internally stored pixel data.
     *
     ** @return pointer to intermediate pixel data representation
     */
    inline const DiPixel *getInterData() 

  getInterData函数描述较清晰，说明了输出数据是基于PixelData和转换函数转换后的数据，且monochrome images（灰阶图像，如CT）为有符号或无符号数据， color images(彩色图像ÿ