I assume that they are using some form of mipmapping, a technique by which you have several images at different sizes to represent different levels of detail. I

I assume that they are using some form of mipmapping, a technique by which you have several images at different sizes to represent different levels of detail. It is used a lot in 3D environments. https://en.wikipedia.org/wiki/Mipmap https://en.wikipedia.org/wiki/Mipmap