Tracking accurate mattes

I’m looking to track accurate mattes onto some car windows within a moving picture.
When I use the power matte node, it only tracks around the area, and the edges don’t track accurately as i thought it would be.
To picture it, the matte isn’t defined, and it just blurs around the b-spline.

Is there any way I can create an accurate matte?

Thank you.

Use the Roto node and track shapes onto the window.

Hi Marco,
Should I use roto instead of a power matte?
And how do I view a roto shape as a matte in silhouette?

Yes, for rotoing a window that is a defined shape, use Roto instead of Power Matte.

In the Roto node when view is set to Output, press the A key to view the alpha. When the Viewer is set to Foreground, press Shift-A to the view the alpha.