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Details
3D car modeling is a very challenging task, which requires high level of skills, effort and a huge chunk of time. A car model has lots of little details, paying attention to all these little details and creating them, is what makes a model impressive and beautiful. Advanced rendering techniques play a part, but advanced rendering techniques can not make a sub-par model that lacks details, look impressive.
One needs a lot of references while tackling any new car project, starting with the basic plan of the car, although a good set of photographs also help as in case of very new models of the cars with no plans available or to modify a car, Maserati Trofeo model is a good example of this. I find most of the references from various sites on the net (but if the car is not too popular or one of those performance cars then things gets difficult) also magazines provide information and references.
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