decrease the quality when the player isn't near to the models

I'm creating maps larger and start to have performance issues and this problem may also arise in a variety of users who will be playing my game.
I would like to be that when the player is far from a 3D model (or an entire zone) then the model loses quality (texture) or it disappears and does not load as long as the player does not come close again to that model.
A bit the same method that is kept here in modern video games.
How can I accomplish this? there is already a script pre-filled?

 

Thank you, but there is a way to "disappear" a model instead of replacing it when the camera arrives at the "culled"?

 

instead as regards the textures?
I wish that when the model is far away, it remains the same (already in low poly) but the texture quality decreases.
Is possible to achieve this with the same method?

 

That already happens automatically because of mipmapping.

--Eric

 

I do not think I fully understand the operation ....
If active "generate Mip map - In line space and border Mip Map" does not change anything.
The texture changes of quality only if active "fadeout mip map" but in this case the texture becomes completely gray and not very aesthetic ....
I something wrong?

 

All you need is to have mipmaps active for the texture. Then it automatically uses progressively lower quality versions of the texture as the object gets farther away from the camera. It shouldn't be particularly noticeable in most cases, though it's more obvious with some textures than others. If you use trilinear filtering then it blends between mipmap levels, which makes the mipmap transitions much smoother, though trilinear is naturally more expensive than bilinear.

--Eric