I’m not sure how many gamers, who aren’t 3D programmers, know what bilinear filtering means; however, I see it in the options of many games. It can be summarized by textures looking blurry up close and eliminating the “blocky pixel” effect which was so common back in the day of pure software-rendered 3D graphics engines. See one of my previous posts entitled, “How to Turn Off Bilinear Filtering in OpenGL” for a few screenshots.
In this post I’ll be explaining what “trilinear filtering” means and how it differs from bilinear. We see an option for trilinear filtering in virtually every game on the market today (although some call it by a different name), and depending on your 3D hardware, it may even be turned on by default. However, most gamers don’t know what trilinear filtering really is (unless they’re graphics programmers) or how it affects the visual characteristics of a game.
In a nutshell, just like bilinear filtering attempts to smooth out blocky pixels via interpolating between one texel and its surrounding four neighbors, trilinear filtering attempts to smooth out mipmapping gradients that are most commonly noticed when the camera is at a narrow angle relative to a wall (or any surface where the texture is very oblique to the point of view). Now, I’d really like to provide a screenshot of what this gradient looks like, but with the high resolution used in today’s games the effect becomes hard to see in a still screenshot. However, the effect is very noticeable when the camera is moving forward or backward.
The effect manifests itself by a sharp break between blurriness levels as a polygon recedes into the distance. At first, the polygon’s texture looks normal (the part closest to the camera). Then as you trace along the polygon’s surface all of sudden the texture loses half its resolution. If you continue tracing, you notice another break where again the resolution is cut in half. In most games this pattern is noticeable about four or five times before the polygon becomes too small to notice.
Trilinear filtering attempts to eliminate this sharp break by dynamically interpolating between different mipmap resolutions at every pixel. For example, instead of choosing between a mipmap resolution 1:1 or 1:2, it dynamically calculates a smooth gradient of mipmap levels between 1:1 and 1:2.
In the end trilinear filtering does a very good job eliminating abrupt changes in texture resolution. In some cases it is hard to tell mipmapping is even being used.
Sometime later in the week or next week, I’m going to cover something known as anisotropic filtering – which is meant to be a step better than trilinear filtering.
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-Greg Dolley
