Use clipping mask to fill transparent shape in photoshop

Question

I have a rectangle in photoshop that is transparent with a black outline, and I'd like to make a frame-by-frame animation of the rectangle filling with red. I know there are a few ways to do this but right now I'm doing it by creating a clipping mask and slowly bringing the clipping mask upwards, saving every time, and then playing the images back quickly.

However, I have a bit of an issue that stems from the fact that the rectangle has a transparent background.

My setup looks something like this:

and the issue is that, even though the red rectangle has a solid red background, the only part of the black rectangle that I'm moving the clipping mask over that gets filled is the black border.

I'd like something like this instead: https://i.stack.imgur.com/EJOmU.jpg

but I can only achieve this effect when I actually fill in the background of the shape.

Any help here would be appreciated.

Thank you.

Answer 1

I don't think you need a clipping mask. In fact, I think the clipping mask is your problem entirely.

This simply requires a standard layer mask which is unlinked to the layer, so the mask moves without the layer contents moving.