Actually, there is no such thing as 'trilinear filtering', in SRB2 or any other game.
It's slang for "Bilinear w/ Mipmaps"
Mipmaps are pre-resized versions of a texture. Then, depending on how far the polygon is from the view, it decides which size texture to use. This is to increase rendering speed, as resizing a texture closer to the size it is going to be requires a whole lot less memory bandwidth.
What does this mean???
It means Trilinear is actually a lower image quality than Bilinear.
Oh horror of horrors.
Just like score in golf, higher numbers isn't always better.
Now shut up about trilinear being so much better. That's why the floor turns into a friggin' solid shade of green when it gets a certain distance from the viewpoint. Yeah, As I look out across my lawn, it becomes a solid shade of green, too.