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Virtual World Structure
A Unigine-based virtual world is a scene that includes a set of different built-in objects with certain properties. Almost all of the objects can be seen in the virtual world. However, some of them are pseudo-objects, which cannot be displayed in a normal mode (such as physicals, sectors, occluders and so on).
In general, a world includes the following:
- Built-in objects called nodes, which, in turn, may refer to other nodes (*.node), meshes (*.mesh),
audio files (*.oga), paths (*.path) or scripts (*.h). A world can store both nodes themselves and/or references to nodes stored in external *.node files. A *.node file, in turn, can also store both nodes and references to other external *.node files.
- Materials and properties.
- Shaders and textures to which materials refer.
- General rendering, physics, sound and game settings.
Each world is stored on the disk in a separate file in the XML format with the .world extension.
- A world script has no implementation. In this case, the world content is created and managed via UnigineEditor.
- A .world file is empty. In this case, the world content is generated and managed via the world script.
Worlds can be as large as a level designer wishes as Unigine supports data streaming, and required resources — meshes, textures, animations, sounds — are dynamically loaded in run-time. Resources that are no longer needed are unloaded from memory.
Also, for common outdoor scenes, we recommend you to use the scale 1 unit = 1 meter, when you export objects from a 3D modeling software.
For convenience of a large world designing and editing, a world can be split into several parts.
The 3D space in Unigine is represented by the right-handed Cartesian coordinate system: X and Y axes form a horizontal plane, Z axis points up. When exporting an animation from 3D editors, Y is considered a forward direction.
Positive rotation angle sets the rotation counterclockwise. It corresponds to the right-hand rule: if you set right hand thumb along the axis, other fingers wrapped will show rotation direction.
Unigine provides a set of built-in objects allowing you to add to a world practically all of the objects existing in real life. For convenience, they are divided into several groups that are responsible for different kinds of operations.
A world can store both nodes themselves and/or references to nodes stored in external files with the .node extension.
Nodes of object-related types may have surfaces and physical bodies (including collision shapes connected with joints). Surfaces contain references to particular materials and properties. Such references are created automatically when you assign a material or a property to a surface.
- A part of the object that needs to have its own separate material.
- Different levels of details (LODs) (including reflection LODs and shadow LODs). In this case, they represent the same object or its part in more or less detailed variants (a high-polygonal and a low-polygonal model). Which of these surfaces is visible at any given time is determined by the surface properties.
Taking into account the information given above, the scheme of world's components can be extended as follows:
All nodes are organized into a hierarchy. Each node can have multiple children.
Usually coordinates and orientation of the first-level nodes are set in the global (world) coordinate system, while coordinates and orientation of their child nodes are in local coordinate systems of their parents (if not chosen otherwise). This allows easy transformation and rotation of the whole hierarchy tree branch only by transforming and rotating the root node.
The origin of the global (world) coordinate system is located at the scene center. The origin of the local coordinate system is at the pivot point of the parent node.
The number of surfaces depends on how a mesh was exported from a 3D editor and cannot be changed dynamically in the runtime. Each surface may add a separate DIP call to the GPU.
Surfaces also participate in culling of invisible parts of the object when it is hidden by some other objects on the foreground. Moreover, they are used to calculate collision detection with physically simulated objects.
Nodes share common options for their surfaces. Each surface can be enabled or disabled for rendering, as needed.
Physical Simulation of Objects
Besides being only a non-interactive environment, objects can take part in physical simulation. To be effective and at the same time keep the overall framerate high, physics is always a very rough approximation of the real world. Its main aim is to detect collisions between the objects.
For that, an object should be assigned a body that describes how it can behave and what physical properties it possesses. For example, whether it is a rigid body that always stays solid and undeformed or a cloth which can be folded and torn.
However, the type of the object's body is not enough for an object to start interacting. Its volume should be approximated using some kind of a physical primitive (or a set of them) — a shape. Unigine provides several types of such primitives: a box, a sphere, a capsule, a cylinder, a convex hull, or an arbitrary mesh shape. The shape is used to compute collisions between the objects.
A couple of shapes can be connected with one of the joints: fixed, ball, hinge, prismatic, cylindrical, wheel or suspension. They describe how two shapes can possibly move relative to each other.
Nodes themselves are rendered red in the scene. The information on how a node is to be correctly rendered is stored in the materials assigned to particular surfaces of the node. A material is a set of properties (states, options, parameters) and assets (2D, 3D textures), based on which surfaces are rendered.
Material Ball with No Material Assigned
Material Ball with mesh_base Material Assigned
To manage rendering of surfaces with no materials assigned, use the viewport mask.
In Unigine, there are 2 types of materials:
- Read-only base materials created by programmers and stored in *.basemat files. A Unigine-based project includes a set of default base materials.
- Editable user materials inherited from the base materials or from the other user materials and stored in *.mat files. Such materials are created by 3D artists and override properties of parent materials.
A set of base material properties may include:
- Options. The set of options is pre-defined and invariable for all materials. Each option either enables or disables some kind of behavior. Option values for base materials are hard-coded.
- States. They define conditions, basing on which Unigine selects appropriate shaders, textures, and parameters among all defined. For example, one can define a state turning on and off drawing of foam in water simulation and provide different shaders for both states (with and without foam).
- Textures. Usually, the base material has several textures defined. The common reasons for it are the following:
- A shader may need more than one texture for processing.
- Usually, different textures are used on different rendering passes.
- States also may influence the number of textures, for example, each state may require its own texture.
- Parameters. They are passed as arguments to shaders, which require them.
- Shaders. They are the very core of base materials. It is shaders which actually draw the material based on different conditions.
Each base material is usually equipped with several shaders, and the appropriate one is chosen according to the current rendering pass and state.Only base materials can refer to shaders.
A set of user material properties cannot differ from the base material ones: the user material inherited from the base material only overrides all its options, states, textures and parameters.
In Unigine, there are also manual materials. Such materials are created and edited manually: changes made via Materials Editor in run time won't be saved. All base materials (both the built-in and the custom ones) are manual. However, not every manual material is the base one: the user materials can be manual too.
In Unigine, materials are organized in hierarchy, like nodes, but are completely independent from the node hierarchy.
A parent material passes its properties to the inherited material so that they can be overridden.
The base materials cannot be organized in hierarchy: no base material can be inherited from another base material. However, if you need to modify properties of a base material, you can create a new user material by inheriting from the required base material: the base material will pass its properties to the inherited material and you can override them. Thus, the base materials are always on the top of the material hierarchy.
Materials hierarchy is based on GUIDs. For the base and manual materials, they are generated in run-time by using their names stored in materials files. For the user materials, they also generated in run-time on materials' creation and stored in the *.mat files. However, in materials files, references to the materials can be name-based or GUID-based.
When cloning or inheriting a material, each new material becomes internal until a name is assigned to it.
The loading order of materials doesn't matter: on the engine start-up, all materials of the project are loaded, the base materials are loaded first. And since each *.mat material refers to both the parent and the base materials, the base material is used as the parent one until the parent is loaded. So, when the application is loaded, all materials existing in the project are presented in the materials hierarchy.
Material Ball with mesh_base Material Assigned
Material Ball with a User Material Inherited from mesh_base
The left material ball is rendered using one of the built-in base materials. The right material ball inherits its material from that base and simply changes 2 textures (the albedo and normal textures). There is no need to set other properties, as they are inherited. However, if one needs, they may change any property, just like we've changed the texture for the material ball.
All material properties (except shaders) can easily be overridden in UnigineEditor. However, if you need to extend the set of existing properties or override shaders, you need to create a custom material. In this case, refer also to the Materials Files section, as it contains a thorough description of material declarations in XML.
For the object to be correctly integrated in the world, defining its position, inherent characteristics and outside appearance will be insufficient. The properties specify the way the object will behave and interact with other objects and the scene environment.
A property is a set of options, states and parameters:
- Options. The set of options is pre-defined and invariable for all properties. Each option either enables or disables some kind of behavior. Option values can be changed only for custom properties; options for base properties are hard-coded.
- States. The states define conditions, based on which parameters of the property becomes available/unavailabe for reading and editing.
- Parameters. Logic-related parameters.
For example, they specify if the object is interactive and whether it is a switch. For a character, properties can specify the character health points or gold amount. Thus, the properties make the nodes comply with the game/application logic.
A property hierarchy is represented by the hierarchically organized list of the properties, which can be assigned to the nodes used in the scene. A property hierarchy is independent of node and material hierarchies.
By default, Unigine provides 2 base properties which are at the top of hierarchy and cannot be changed. If you need to change a particular setting of the base property, you can create a new property by inheriting from the base one.
The hierarchy is easily edited in the desired way: the inheritance relations can be changed simply by dragging, properties can be cloned, assigned or removed with one button click. Apart from cloning the whole property, it is also possible to copy separately all its settings, choose another property, to which they will be applied, and merely paste them.
To be used in the world, a set of properties should be organized in libraries. Property libraries describe states and parameters of properties as well as parent-child relations. They are stored in the XML file with the .prop extension that should be declared in the .world file for library preloading.
If you add a property library via the Properties window of UnigineEditor, the reference to this library will be added to the .world file automatically.
By default, Unigine SDK includes a set of property libraries located in the data/core/properties directory.