[vtkusers] I have a problem about change the scalar value of of some image points

[fengfenghsy]

hello ,everyone, 
     I am a fresh man of vtk user and vtk mailing list. I am now dealing with some 2D Dicom image , and I want to mark some point I have picked from the mouse leftbuttondown. Now, I have got the x and y value of the point. but I do not know how to change the scalar value or RGB value to mark the points out. So can you tell me which function and class I could use to make my function come true. 
     Sincerely
        Thanks a lot
 				

　　　　　　　　fengfenghsy
　　　　　　　　fengfenghsy at 163.com
　　　　　　　　　　2004-12-15