vtk how to prevent the blur edge?

I want to display a color image and a gray image.

The code is about (python):


grayActor = ...
colorActor = ...

render.AddActor(grayActor)
render.AddActor(colorActor )

And the result is shown as the above picture.

The color image is actually a green color, but red edge appear.

How to prevent the blur red edge?

Any suggestion is appreciated~~~