[vtkusers] set certain point\'s scalar value

[xujf]

hello,vtk users:
    There is a Point A(x,y,z) in Polydata,and  I want to set the scalar value of this point. How to realize the function.
    I used Polydata->GetPointData()->SetScalars(faceColors); however this set all points\\\' scalar values of the ploydata,not certain point. 

Thanks in advance!
xujf