为什么以下代码的输出只是一个盒子？

Question

import vtk
import pickle
from numpy import *


data_matrix = Ilog
dataImporter = vtk.vtkImageImport()
# The preaviusly created array is converted to a string of chars and imported.
data_string = data_matrix.tostring()
dataImporter.CopyImportVoidPointer(data_string, len(data_string))
# The type of the newly imported data is set to unsigned char (uint8)
dataImporter.SetDataScalarTypeToUnsignedChar()

# must be told this is the case.
dataImporter.SetNumberOfScalarComponents(1)
# The following two functions describe how the data is stored and the dimensions of the array it is stored in. For this
# simple case, all axes are of length 75 and begins with the first element. For other data, this is probably not the case.
# I have to admit however, that I honestly don't know the difference between SetDataExtent() and SetWholeExtent() although
# VTK complains if not both are used.
dataImporter.SetDataExtent(0, 727, 0, 727, 0, 24)
dataImporter.SetWholeExtent(0, 727, 0, 727, 0, 24)

# The following class is used to store transparency-values for later retrieval. In our case, we want the value 0 to be
# completely opaque whereas the three different cubes are given different transparency-values to show how it works.
alphaChannelFunc = vtk.vtkPiecewiseFunction()
alphaChannelFunc.AddPoint(0, 1)

alphaChannelFunc.AddPoint(0.5, 0.9)
alphaChannelFunc.AddPoint(0.7, 0.9)
alphaChannelFunc.AddPoint(1, 0)


colorFunc = vtk.vtkColorTransferFunction()
colorFunc.AddRGBPoint(0, 1,1,1)

colorFunc.AddRGBPoint(0.2, 0.9, 0.9, 0.9)
colorFunc.AddRGBPoint(0.7, 0.2, 0.2, 0.2)
colorFunc.AddRGBPoint(1, 0, 0, 0)

# The preavius two classes stored properties. Because we want to apply these properties to the volume we want to render,
# we have to store them in a class that stores volume properties.
volumeProperty = vtk.vtkVolumeProperty()
volumeProperty.SetColor(colorFunc)
volumeProperty.SetScalarOpacity(alphaChannelFunc)

# This class describes how the volume is rendered (through ray tracing).   
compositeFunction = vtk.vtkVolumeRayCastCompositeFunction()
# We can finally create our volume. We also have to specify the data for it, as well as how the data will be rendered.
volumeMapper = vtk.vtkVolumeRayCastMapper()
volumeMapper.SetVolumeRayCastFunction(compositeFunction)
volumeMapper.SetInputConnection(dataImporter.GetOutputPort())

# The class vtkVolume is used to pair the previously declared volume as well as the properties to be used when rendering that volume.
volume = vtk.vtkVolume()
volume.SetMapper(volumeMapper)
volume.SetProperty(volumeProperty)

# With almost everything else ready, its time to initialize the renderer and window, as well as creating a method for exiting the application
renderer = vtk.vtkRenderer()

 renderWin = vtk.vtkRenderWindow()
 renderWin.AddRenderer(renderer)
 renderInteractor = vtk.vtkRenderWindowInteractor()
 renderInteractor.SetRenderWindow(renderWin)

 # We add the volume to the renderer ...
 renderer.AddVolume(volume)
 # ... set background color to white ...
 renderer.SetBackground(0.5,0.5,0.7)
 # ... and set window size.
 renderWin.SetSize(800,800)

 # A simple function to be called when the user decides to quit the application.
def exitCheck(obj, event):
    if obj.GetEventPending() != 0:
        obj.SetAbortRender(1)

# Tell the application to use the function as an exit check.
renderWin.AddObserver("AbortCheckEvent", exitCheck)

renderInteractor.Initialize()
# Because nothing will be rendered without any input, we order the first render manually before control is handed over to the main-loop.
renderWin.Render()
renderInteractor.Start()

Ilog是大小为728x728x25的逻辑矩阵，其横截面看起来像

在此图像中，红色表示值1，蓝色表示值0。

但是当编译上面的代码时，输​​出总是像

这样的框

矩阵包含的值仅为0和1。使用该逻辑，零值表示完全透明，零值表示完全不透明。