[vtkusers] a problem when run TestAreaSelections.cxx

[455535440]

hello everybody~~
today I try the example :TestAreaSelections.cxx
here is a part of code in this file
renWin->Render();
   int rgba[4];
   renWin->GetColorBufferSizes(rgba);
   if (rgba[0] < 8 || rgba[1] < 8 || rgba[2] < 8)
     {
     cout <<"Color buffer depth must be atleast 8 bit. Currently: " 
       << rgba[0] << ", " << rgba[1] << ", " << rgba[2] << endl;
     return 0;
     }

when my programe run at this  i got the rgba[0] ，rgba[1] ， rgba[2] value 5,6,5,and it retrun wieh 0
the message is "Color buffer depth must be atleast 8 bit. Currently,"what did this mean?
i use the image data that suporrted in vtkdata, I don't know what the problem is,can any one give me help?  thx~~
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.vtk.org/pipermail/vtkusers/attachments/20110607/04e93b9f/attachment.htm>