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A Trainz Asset is a self-defining collection of mutually supporting more elementary data types that are usually used alone or in some cases, are used as an accessory in conjunction with other Assets to be rendered by the game software into a realistic 3D model of some sort and degree. The simplest assets are ones that reference only other assets or those referencing one or two accompanying files.
The kind consist a route builder or session writer might want in set outs or cuts in placing groups of cars in several locations or different sessions. It consists solely of a config.txt file and optionally may contain a thumbnail and texture.txt file. The config file is a consist container referencing traincar types by kuid, and whether it is facing forward or backwards, the rest is a kuid-table and the few data lines delineating, name, kuid, thumbnails and size, and might have other common TrainzBaseSpec tags
The kind texture(s) assets are nearly as simple, though modern Trainz implementations have added light scattering (diffuse maps) control images, 3D effects (normal map images) and an image for the main effect—an environmental (ground) texture is used to put a surface on the world's mesh, coloring it. Its cousin, the non-environmental texture is a misnomer—since most are used to generate lighting effects. Those do not need a bump or normal map, but do require an alpha mask image, the primary, and possibly a diffuse map. Each of these attributes will require not only the image files, but a control file, the texture.txt file.
A simple object would have a mesh and possibly the files needed in an animation.
When an asset is a sub-component (a wheel type making part of a bogey or truck), it is usually set off inside a container which will likely define other information about how the asset is used by the larger asset. Thus assets can greatly differ both in complexity and size, generality and specificity. Consider the many shapes on a steam locomotive and each part that moves independent of others. Each group will likely have component assets, and the complicated whole is made up of building blocks small asset by small asset. Each visible surface needs a definition in 3-space of a polygon mesh and each of these meshes need organized to attach to one another, animation files to generate motion effects, smokes to express the dynamism of the effluents issuing from the Iron Horse, and coloring or textures to hang on the meshes in a wrap if the aspect is viewable from multiple directions, or as a planar placement, if the surface is planar as with a box car exterior, CAB roof[note 1]
Further, Assets can range from a sound or HTML data file providing background (one as 'realism' or color, the other as a instruction in a game DriverSession), a texture used to fill in the surface color and surface effects of a polygon mesh, the mesh itself, or an aggregate of several or many sub-components, like the obvious kinds of rolling stock. But assets also include buildings, trackside related objects such as signals, relay boxes, junction switches (points) and switch motors or switch levers.
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↑Even curved roofs in mathematics are planar surfaces describable by a relatively simple vector equation to the mathematically cognizant. That the geometry and trigonometry are 3d-space variants in cylindrical or spherical co-ordinates is a more inclusive discipline than the more familiar 2D toolkit of Euclidian or plane geometry— which turn out to be a "degenerative case" under the more inclusive rules and analytic methods of the higher form of mathematics.