Polygons are what the virtual world of Lara Croft is made of. In short a polygon is composed of at least three points in three dimensional space which form a triangle. A texture is then projected onto it, effects like lighting is applied and finally the result is displayed on screen.
All objects in the games are made out of polygons which can form a wire frame or mesh of enormous complexity, ever increasing throughout the series as the computational power of Video Consoles and PC hardware grew. For example, in the first Tomb Raider in 1996 Lara Croft was made out of only 350 polygons, while in the the 2008 Underworld it were about 10.000. The more polygons an object is made of the smoother its surface is perceived. In recent years additional techniques like Tessellation further increase the number of triangles.
In the Level Editor it is only possible to manipulate the basic building blocks (normally made out of six sided cubes, where each side is divided into two triangles) of the environment. More complex objects have to be build outside the editor with tools like Metasequoia and then imported into the WAD files using such tools as Wadmerger.
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