How to achieve projectiveTexturing and save the projected color into the mesh's texture?

So,i need to project a picture from a projector(maybe a camera) on some meshes and save the project information into the mesh texture according to the mesh’s unfolded uv.It just like the light map which encode the light-info into the texture instead of the project-info.

The following pic is an example.I noticed blender actually has this function that allow you to draw a texture on to a mesh.But i have no idea on how to save those project color into the mesh’s texture.Anybody can help>?