Receiving shadows by transparent floor

Hey there!

I want to create a 2.5D game in which all locations is be represented by prerendered images and a 3D character walks through them. I've rendered a location and added a simple character in Unity 3D just to try what it's going to be like.

One of the problems I can't deal with is floor and shadows that the character casts onto it. If I get it right I have to make a plane which will represent the floor and use a shader to set it up to be invisible but to cast shadows. How can I do it?

I've tried a solution from here but it makes my floor simply black.

Or, if it's impossible to make a transparent floor, is there any way to project the backgroung image onto objects in the scene using camera mapping?,I want to create a 2.5D game in which all locations is be represented by prerendered images and a 3D character walks through them. I've rendered a location and added a simple character in Unity 3D just to try what it's going to be like.

One of the problems I can't deal with is floor and shadows that the character casts onto it. If I get it right I have to make a plane which will represent the floor and use a shader to set it up to be invisible but to cast shadows. How can I do it?

I've tried a solution from here but it makes my floor simply black.

What should I do? Or, if it's impossible to make a transparent floor, is there any way to project an image onto the objects in the scene using camera mapping?