Masks & Motion Blur

[martynd]

I have fast moving mask and it needs to be motion blurred. How do I apply motion blur just to the mask...I only manage to get the whole layer blurred?!? HEELP!!

[nordman]

Do a White Solid. Apply the Mask. Nest this Layer.
Apply Motion Blur. Define this layer as your alpha Matte.
Maybe you have to invert it. Tell me when it worked.

Martin