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### 4.8.1 Components of a World

This section describes the objects used to build a world. This is useful to understand the format of the Crystal Space map file (see section Format of Map File (XML)). To understand this you should know the basics of the Portal Engine. This section describes the components of the world independent from the file format, so it is also useful if you want to build a world from within your program.

The world is built up from the following objects:

#### Sectors

(`iSector`). For technical details, see the portal engine discussion (see section Portal Engine). Basically sectors define an area that can contain several geometrical objects. As opposed to earlier versions of Crystal Space, a sector does not itself represent geometry. Instead all geometry in a sector is made from Mesh Objects. Theoretically a sector is infinite although in practice it is usually logically bounded by sector walls or a sky object. You can create sectors using the engine (`iEngine`) or they can be created in a map file.

#### Mesh Factories

(`iMeshFactoryWrapper`, `iMeshObjectFactory`). See the mesh object section for more specific information about mesh factories (see section Mesh Object Plug-In System). Mesh object factories are objects that can be used to create mesh objects that inherit characteristics from their mesh factory. Basically a mesh object factory is like a blue-print for creating a mesh object. Mesh factories can also be hierarchical like mesh objects. In most cases (i.e. like genmesh (see section Genmesh Mesh Object and thing (see section Thing Mesh Object)) the factory defines the actual geometry of an object. You can then create multiple mesh objects from those factories. They will all share the same geometry.

#### Mesh Objects

(`iMeshWrapper`, `iMeshObject`). See the mesh object section for more specific information about mesh objects (see section Mesh Object Plug-In System). Mesh objects represent geometry in some sector. Everything you can see (walls, actors, particle systems, ...) is represented by some kind of mesh object. This is a very important concept in the Crystal Space engine. Mesh objects can also be hierarchical. A mesh object can have several children which in turn can also have children and so on. The children of a mesh object are positioned relative to the parent. So when you move the parent the children will automatically move with the parent. Children of a mesh can be meshes or lights.

#### Textures

(`iTextureWrapper`). See the material section for more information about textures. (see section Materials and Textures). Basically a texture represents an actual image that can be used in materials. Textures are used to give your mesh objects a real appearance. Textures are never used alone but always in a material.

#### Materials

(`iMaterialWrapper`). See the material section for more information about materials. (see section Materials and Textures). Basically a material represents a surface appearance for a polygon or triangle. A material is most typically made out of a single texture but it is also possible to make materials out of a combination of textures (using a shader) or even without any textures at all (for example a solid color).

#### Shaders

(`iShader`). See the shader section for more information about shaders (see section Shaders). A shader is a piece of code that is typically run on the 3D card. It is this code that is responsible for actually rendering the surface attributes using the material data. A material and a shader work closely together.

#### Renderloops

(`iRenderLoop`). See the renderloop section for more information about renderloops (see section Renderloops). A renderloop is an engine structure that tells the engine how to render the objects in a given sector. Basically it tells the engine the steps required to do the rendering of the mesh objects and also the steps required to do the lighting. You can make your own renderloops but typically you would use one of the standard provided renderloops from the ``data/shader'` directory.

#### Portals

(`iPortal`, `iPortalContainer`). See the portal discussion (see section Portal Engine). The idea is that sectors are normally totally independent areas unless you connect them through portals. A portal is basically a 3D polygon that has no material but instead represents a view of the destination sector.

#### Lights

(`iLight`). A light is needed to illuminate your level. Lights can also be part of a hierarchical object (like mesh objects). There are different ways how lighting is applied to objects. See section Lighting.

#### Scene Nodes

(`iSceneNode`). A scene node is a generic representation of an object in a hierarchy. Currently mesh objects and lights are scene nodes.

#### Collection Objects

(`iCollection`). These are simply groups of other objects. Do not confuse them with hierarchical objects or with regions. Crystal Space itself does not use them but you can use them for your own purposes in your application.

#### Regions

(`iRegion`). A region is also a collection of objects but the purpose is different (as opposed to collections). A region can contain textures, materials, mesh objects, mesh factories, lights, collections, sectors, sequences, triggers, shared variables, .... Regions are useful when you want to load/unload big parts of a world dynamically. This can be used in cases where the world is too big to fit in memory and you want to use dynamic loading. It is important to realize that the concept of a region is not geometrical. A region is typically a collection of objects that are located close to each other in 3D but it doesn't have to be that way. Regions can be organized in any way you want. Some examples:

- One region represents a geometrical part of the world, including sectors, meshes, lights, materials, ...) used in that region. This is probably the most common usage.
- One region represents objects of a given type. For example, all materials are in one region. Meshes in another.
- One region represents the static objects of your world. The other region represents the dynamic objects (moving objects).

#### Key/Value pairs

(`iKeyValuePair`). A key/value pair is a {key,value} tuple describing some game specific concept. Key/value tuples are represented by a pair of strings (or one name and several named value strings). If you have built levels for Quake before, you are probably familiar with this concept from Quake's Entities. Almost all objects in the Crystal Space engine support these key/value pairs (this is done through the `iObject` system) so you can attach this kind of information to anything. Crystal Space itself will not use this information but your game can query it and perform the appropriate actions. In theory it is up to your program as to how these key-value tuples are interpreted. However, Crystal Space has set some standards to make life easier for level designers and also to make levels as portable as possible between different Crystal Space games. Note that key value pairs actually allow multiple named values for a single key. See the section about game data for more information on this subject. (see section Managing Game Specific Data).

#### Nodes

(`iMapNode`). A node represents a point in space. The engine itself will not use nodes but a game can use them to represent game specific data. See the section about game data for more information on this subject. (see section Managing Game Specific Data).

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