I searched for a while, but with 100+ pages of tutorials, I couldn't figure the key words to find the information I need.
The effect I want to pull off is a moving 'window' showing through to Layer X.
Example 1: A moving portal. The sprite (Curved rectangle) moves across the screen, and wherever it is, you see through to whatever is behind it.
Example 2: Two squares, one is red, and the other blue. Wherever they touch, instead of purple, you get invisible and see through to the background.
The reason I'd like to do this is because I have a fuel bar that is a rectangle and the width changes. I want to put an image on top that makes it invisible, except for the curved cutout through the middle. So that the fuel bar, looks curved as it goes up and down.
I'm guessing there is a simpler way to do a curved fuel bar, but I'm still learning stuff so I figure this would be more interesting to learn how to do.