[vtkusers] vtkimageactor's SetZSlice

[455535440]

merry christmas everybody.
 I met a problem of vtkimageactor,I used jpegreader to read 12 pictures ,then used vtkimageactor to show it,one of these jpegs was rendered.then i wanted to show another pictures ,So I called SetZSlice but it didn't work. I watch the souce of the method like below:
 void SetZSlice(int z) 
 {this->SetDisplayExtent(
    this->DisplayExtent[0], this->DisplayExtent[1],
    this->DisplayExtent[2], this->DisplayExtent[3], z, z);}
 then I debug the program,the vtkimageactor's DisplayExtent is an array of 6 int ,valus -1,0,0,0,0,0, this is strange .Then I watch the vtkImageData which vtkimageactoe setinput,there is an arry named extent in it values 0 383 0 383 0 11, I think this means the picture data is saved,but why the method didn't work?
 I google this problem and find some people met the same problem,but there is no solution. Anyone can help us?
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