Virtualdub2 "merge layers" filter?

[Goggen240]

How does the "merge layers" filter work?

I've been fiddling around with VirtualDub2, and one of the new features (from VD 1.9.11 at least) is the "merge layers" filter.
The description within VirtualDub says "Blends two layers together using a third layer", but how?

It doesn't seem to come with a readme, and I haven't found any documentation for this in Help, the VirtualDub2 Wiki, this forum, the VideoHelp forum, or just googling it. (It's also a somewhat hopeless search term to start with.)


As far as I know, you would select layers the following way:
Add the "merge layers" filter
Right click
Add Input
Right click input ("No connection")
Choose between "No connection", "Source", or "Previous output".

That's two inputs, and only one of those could be considered a "layer".
The only options within the filter is a choice between the blending modes Lerp or Select. And it doesn't say what they actually are. I haven't found anywhere it says what Lerp actually means.
"Select" seems like an obvious place to select layers, but neither "Select" nor "Lerp" seem to actually do anything.
Having three other blended filters doesn't seem to make any difference.

This question also applies to the "blend layers" filter, which has two inputs, but the only two possible inputs seem to be "Source" and "Previous output", which seems like only one "layer".

How does the "merge layers" filter work?
Have I missed some obvious place where this filter and how to use it is described?