Forget render times, Optimus Prime has 11,000+ parts to the model alone. It's gotten to the point where a person needs an engineering degree to create some of these effects.

Speaking of which, I can't imagine how much work went into the effects for 2012. Some of the shots in the trailers look unbelievable in terms of simulations required. Each and every car falling off an overpass has to be calculated, and there are tens of thousands. Not to mention the buildings falling, breaking apart, hitting each other. Wow.
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