Android Question Changing polygon on a image

I would like to create a (filled) polygon (triangle) that (partly) covers an image being displayed (background).
The polygon would change every few instances, so the part of the image being covered/not covered by the polygon would change.
I also would like to avoid any flickering while the polygon is being redrawn.

What would be the best approach to do so (it should be drawn very fast so fully reloading the image seems nog option) ? Something like an overlay ?

Thanks