So say I want to have around 10-15 creatures on the screen (at maximum) and a little bit of scenery in the battle instance, is 1-1.5k poly going to be alright? what would be a good ballpark number to try to stay under?
Well this totally depends on your target group. For any not totally aged PC 15 creatures with like 1k polies is piece of cake. If you target at old systems too (i.e. from 2002 or something) I'd stay between one and two thousand polies for each creature. If this isn't that important you easily can double the polycount and even more if you want to...
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