Pixel based refers to a behavior that usually only appears in a number of 2D games. The horizontal component of the FOV is directly tied to the number of horizontal pixels, and the vertical component of the FOV is directly tied to the number of vertical pixels. The larger your resolution, the more the game will display. For example, 1280x800 will show more, both horizontally and vertically, than 1024x768. But 1280x1024 will show more vertically than 1280x800 while showing the same horizontally. And 1440x900 will show more horizontally than 1280x1024, but less vertically. Windows behaves this way.