Fake tree shadow with mask?

Hello, I was wondering what would be the best way:

I want a shadow cast in a room, that looks like there are trees outside. I was thinking about a Plane with a masked tree texture in front of the window, but could not get any satisfying results so far.

So what would be the best way to achieve such a result and what settings should I use?

My Setup is a basic environment with a Stationary sun, skylight etc.

thx in advance

Plane with masked texture is actually fine. You could probably put an actual tree out there.
Alternatively, take a look at Light Function.