You only need to mask an object once and it stays masked. It uses the brightness or level of either a sprite or a shape painter to determine what gets masked. The color white makes an image fully visible while darker colors or no color at all makes the masked parts of the image transparent.
Using the top sets of images creates the bottom masked images. Only the white part is visible.
Edit: I don’t know where the collision comes into play unless you want some form of interaction.