The triangle orientation of Constrain vtkDelaunay2D

Hi vtkers.
I made a polygon triangulation with Constrain vtkDelaunay2D.
But I found the orientation of resulting triangles were not the same.
The boundary triangles have orientations that consistent with boundary edges.
But the triangles inside do not.
Any mistakes I have made with vtkDelaunay2D?
Or how I can sort the resulting triangles?
Thank you.