As a personal example, I am (still not finished) doing a model of a structure which is a concept 2D drawing by Feng Zhu. I have acknowledged this in my thread and have personally tried to contact him or a rep to ask permission to model it. I have not yet heard from them and will again try to contact them. I hope to soon have gained enough knowledge in lighting and get the thing finished. I have scaled etc the model from my observations etc from the 2D concept. I feel obliged to ask him for 2 reasons, its his concept and I would like to show off my work with his blessing...maybe even impress. As I am doing it purely for fun I probably dont have to do the above, for me it comes down to respect, without his concept, I wouldnt have a reference

cheers bullet