How do we cast shadows on an invisible object in UE4?

I’m looking to make a game with prerendered backgrounds and my character will be walking on completely invisible collision boxes. The prerendered backgrounds will make them look like they’re walking in hallways, but I want them to cast shadows on the ground as they walk.

Here is an example from the Resident Evil 2002 Remake where the background is prerendered but the character still casts a shadow on the ground and wall:

To be clear, I’m not talking about an invisible object (like an invisible cube) casting a shadow on the ground like a ghost, I mean having your character cast a shadow under their feet on ground that is invisible.

Hello! I dont understand what do you mean. As far as I know If object (or it’s part) is not visible because of depth test (so it is hidden by other object closer to camera), it is not rendered this frame.

Take a look at the game Tharsis (2016) created in unity engine:

Notice how the dice rolls in this game are a separate 3D object rendered on the UI with the space station rendered normally in the background. The dice rolls have their own lighting and cast shadows from their own point light. The dice are not falling on top of any visible platform (invisible floor), yet the dice cast shadows onto this invisible floor (see video) and the shadows darken the scene behind them.

I’m trying to find out how to achieve this in unreal.

Its been a year since you asked this, but if you are still looking for a solution, I think what you’re looking for is the Composure plug-in. Composure allows for real-time compositing of a media plate (your pre-rendered backgrounds) with 3d objects and shadow catchers.