Hi Dorin,
I think editing a roof after it has been placed could be a plus value. A possible way to implement this could be to encode the generating perimeter along with any user supplied parameter within a custom property attached to the roof object (custom properties can be added with the standard API, see setProperty and getProperty methods in com.eteks.sweethome3d.model.HomeObject), so that you can rebuild the perimeter later even if the user deletes the generating room. Since properties can be made only of strings, all the required information have to be serialized/unserialized in order to be stores/retrieved.
I'm not completely sure about the fact you have to code only the user interface, since from my understanding the library only generates 2D coordinates, so you also have to figure out a way to compute the vertical position of the points. Another open point is the texture mapping, however this could be solved later.
That said, I'm going to put my hands on
