Suppose you have multiple photos of the same object (from slightly different angles), each have some obstacles in foreground.

You added an alpha channel to each picture and "erased" the obstacle (leaving "holes" in each picture).

You want to combine them and obtain a picrure without obstacles or holes.

What [open source] software should I use to do this? I tried ALE, but it does not preserve alpha channels. Will Hugin or some Gimp plugin do this?
Update Tried Hugin, got this:

It looks like what I want, hovewer...
- It asks for some "lense" while this is not a photo at all. I tried entering "infinity" as focal length (meaning "don't distort anything"), but it just failed in that mode.
- It failed to align the size of images until I manually adjusted the "lens" parameter of one of the images. Can it use control points to scale image in addition to moving?
- I don't know how to turn off those "photometrics".