Since now i've succeded in creating a single texture file for each roof polygon, but as i said i'm requested to use one texture only for each 3D model.
I've tried to group the polygons by a field that is common for a number of buildings and apply a single image for the entire group. Clipping the image for the respective group was easy, but i can't apply that image to the roofs polygon.
The main problem is that the geometry of shape containing the group of buildings is not recognized by the ApperanceAdder transformer. If anybody had this problem please help me solve it.
I can't find neither the types of geometry that can be inputed via SURFACE port in ApparenceAdder transformer, because i could try to transform my shape in an apropiate format.