Using my attached image as an example, imagine a game where you drive a little boat on this lake. In my (little and short) experience I would create as simple assets the trees, lake, and boat, but and those mountains?
I have no idea how to build those mountains, should I just get mountain assets, and put them as giant game objects far away from my camera (and maybe kill my game performance)?
Ps: sorry for the category where I’m posting this, no idea where to put so I tried here on World Building
There are various ways on how to approach this and it depends on you.
Should you want to use already available mountain assets (e.g. like this one), see if the ones you like come with low-poly versions of the mesh. Performance won’t necessary be a huge issue then (though that depends on which platform you want to target).
