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# Glossary

This glossary presents definitions for words used in the documentation and in general in the Vortex® world.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

## A

### Accessory

Accessories are visual indicators or guides that appear in the 3D View to help locate or manipulate certain tool extensions. For example, Earthworks uses accessories extensively, such as for the Dynamics Bucket Extension, the Dynamics Soil Mass Sensor Extension, the Particle Spray Extension, etc.

Added mass (also known as virtual mass) is an effect resisting the acceleration of a dynamic rigid body inside a fluid.

### Ambient Occlusion

A shading and rendering technique used to determine the lack of light caused by indirect lighting.

### Anti-aliasing

A computer graphics technique used to remove jagged and pixelated edges from images.

### API

API stands for Application Programming Interface. It often has an associated document by the same name that describes the actual functions and services that are available. The interface exists in the code and is used by client-side applications to interface with the rest of Vortex.

### Articulated Bodies

These are collections of rigid bodies connected by joints or other Constraints.

### Artist

In the context of Vortex Studio, a 3D artist (also called a 3D modeler) is a worker who develops a mathematical representation of the surface of an object in three dimensions via Digital Content Creation software. The final product is called a 3D model; it can be displayed as a two-dimensional image through a process called 3D rendering or used in a computer simulation of physical phenomena.

In Vortex Studio, the 3D artist mostly uses the Graphics Gallery file type.

### Assembly

An assembly is a grouping of related parts, attachment points and constraints.

### Attachment Point

In the context of a cable system, an attachment point is at the end of a cable and can attach to a load or to a fixed part.

### Attachment Position

This is just an alias for Constraint Attachment.

### Attachment

An attachment is the union between two assemblies or two mechanisms.

### Axis Aligned Bounding Box

The axisalignedboundingbox, or AABB, that marks the area for which terrain data is requested by the paged terrain producer.

## B

### Bitmap

A bitmap is an image defined by a 2D array of colors.

A blade tool is a type of deformation tool provided as part of the Earthwork Systems. It represents the blade on a real-world Bulldozer vehicle.

### Boat

See Ship and Vessel.

### Bucket Tool

A bucket tool is a type of deformation tool provided as part of the Earthwork Systems. It represents the bucket on a real-world Excavator vehicle.

### Buoyancy

Buoyancy is an upward force opposed to the weight of a object immersed in fluid.

## C

A technique used by the infinite directional light that uses multiple shadow maps to enhance quality. The shadow map nearest the view camera is smaller and provides better quality, while the shadow map furthest from the view camera is very large and encompasses the visible parts of the world.

One split in the CSM for a viewer and infinite directional light pair.

### Chassis

A Chassis represents the main body of a vehicle (sometimes also called Hull). All vehicle components (suspension, engine, transmission, etc.) are attached to the chassis. The chassis is not part of a Vehicle Systems object per se: it has its own collision geometries and mass properties (center, inertia matrix, etc.).

### Coefficient

Multiplicative factor used in calculations. For example, a Drag Force uses a coefficient to proportionally increase the drag force effect on a rigid body moving through fluids.

### Collision

Collision can sometimes refer to Geometric Collision, or Physical Collision, depending on the context. Once Vortex determines there is a Geometric Collision between geometric objects, it simulates a physical collision between the physical objects that are interfering geometrically and applies the appropriate reaction for the Physical Collision.

### Collision Geometry

A CollisionGeometry defines a collision zone for rigid bodies in Vortex: that is, the shape of the collision geometry defines when collision occurs for a part.

### Compliance

Compliance is the inverse of a spring constant or stiffness. When it is 0, the position constraints are rigid. When it is large, the constraints become soft.

• Angular Compliance:
• Angular compliance has the same units as the inverse of a torsional spring constant, i.e., [time squared]/([mass][distance squared]). If $$h$$ is the time step and $$c$$ is the linear compliance, then the numerical value of $$\epsilon = c/h^2$$ is several orders of magnitude smaller than the inverse of the biggest mass in the system for constraints to behave rigidly. For instance, the default value of compliance is $$1.0E{^{ -12}}$$, with the assumption that a typical mass is 1 and the time step is 1/60. This gives $$\epsilon=3.6E{^{-9}}$$. If the user is working with masses of 1000 instead, the value of global compliance is decreased appropriately.
• Linear Compliance:
• Linear compliance has the same units as the inverse of a standard spring constant, i.e., [time squared]/[mass].

### Components

In the Editor, a component is any file-based Vortex object. Each type of component is created and modified in its own editor and can contain references to other component files, sometimes called sub-components.

There are only currently five components in Vortex, each with their own file format:

Component Name File Format/Extension Supported Sub-Component
Scene .vxscene Mechanism
Mechanism .vxmechanism Assembly
Assembly .vxassembly Part
Part .vxpart No supported sub-components
Graphics Gallery .vxgraphicgallery No supported sub-components
Note
Parts do not have a supported sub-component, but you can add Shape Geometries to parts, which are kind of like honorary sub-components.

### Composite Geometries

Compositegeometries allow the user to creation of concave shapes by fusing primitive geometries together. They are used to optimize collision detection when adding several collision shapes to a part. This speeds up the broad-phase collision detection and provides better control of the number of contacts created.

### Constraints

A constraint is a restriction applied to Rigid Bodies or Forces. Constraints generally fall into two categories: Kinematic constraints (restrictions on the motion of objects) and Dynamics (or effort) constraints (restrictions on the forces applied to the rigid body). In addition, constraints can be relaxed by setting compliance and damping parameters for position constraints, and kinetic loss for velocity constraints.

A freely moving rigid body has six Degrees of Freedom (DOF), and each constraint effectively removes one or more of these degrees of freedom. For example, a door attached to a frame with hinges has only one degree of freedom, namely, the opening angle. The other degrees of freedom have been removed, or constrained away. In the Constraint Types glossentry, each type of constraint available in Vortex is explained in terms of how many and which degrees of freedom are removed.

The most common forms of constraints handled by Vortex include the following:

• Joints:
• A joint is a pure kinematic equality constraint between two bodies, or between a single body and the global frame. Examples of joints include the ball-and-socket, hinge, slider, and cylindrical joints.
• Drivers:
• Constraints such as the hinge and the prismatic leave some degrees of freedom and it is possible to control these degrees of freedom further by imposing joint limits for constraining the range, drivers to impose a velocity, or locks to fix the degree of freedom at a given value.
• Contacts:
• A contact constraint prevents penetration between the geometric objects attached to rigid bodies. Contact constraints are generated automatically by the collision detection module at runtime.

### Constraint Attachment

A Constraint Attachment, or Attachment Position, is a coordinate system which includes a point and two axes (the third is implicit) that is rigidly attached to a part with a fixed offset and orientation, and is typically specified in world coordinates for the current (initial) part position. It is used to determine the relationship of that part to the other part(s) in the constraint.

### Constraint Coordinates

This is just an alias for Degrees of Freedom (DOF).

When you right-click on an element in the 3D View or any of the Panels (Explorer, Graphic Asset, Property, etc.), a context menu pops up, displaying choices (menu items) that are customized to the elements selected at that moment. For example, right-clicking on a Mesh-based Terrain gives you the option of adding an Earthwork Zone, while right-clicking on a Part in an Assembly gives you the option of inserting an Attachment Point or opening the Part file. In addition there are many common options presented for most objects (centering them in the 3D View, copying or cutting them, hiding or showing them, etc.).

### Control Type

The control types dictate how a part (in an assembly) interacts with other parts, as described by this table:

Control Type Has Velocity Affected by Force Example
Animated Yes No Windmill blades, conveyor belt, drawbridge
Dynamic Yes Yes Door on hinge, bouncing ball, articulated arm
Static No No Terrain, horizontal or vertical planes

### CSV File (Comma-separated Values)

A plain text file where each row represents a separate data record, and each field in a row is separated by a comma.

## D

### Damping

The Damping coefficient is the damper property that defines the relationship between speed violation and the corresponding reaction force.

### Decals

A decal is a texture that is rendered on top of the ocean mesh that follows the ocean waves.

### Deforms

A deformation is a change in the spatial relation of the particles that make up an object, such that the distance between the object's particles have changed. This kind of a change would be, for example, a dent in a ball. Deformations never happen to Rigid Bodies, which by definition do not deform.

### Degrees of Freedom (DOF)

The freedom to move along each of the X, Y and Z axes, and to rotate around the each of the X, Y and Z axes gives six degrees (three linear plus three angular), any of which can be constrained away.

Note
In some contexts, degrees of freedom is also called Constraint Coordinates or Joint Coordinates. The number of degrees of freedom of a system indicates the minimum number of independent parameters needed to describe the motion of a system. A point particle constrained to move on a line has one degree of freedom, a free rigid body has six. The relative orientation of two bodies constrained by a hinge has one degree of freedom, and that of two bodies constrained by a ball joint has three degrees of freedom.

Degrees of freedom can be mapped to coordinate systems locally but it is important to realise that it is not always possible to have a global Cartesian coordinate system in all cases. The most important counter example for this discussion is a ball joint, which has three rotational degrees of freedom. In this case, one can use three Euler angles (in any of the 24 possible conventions) to parameterize the orientation. However, the three angles are not independent of each other for all configurations. Indeed, each of the 24 conventions breaks down for some values of the three angles, something known as gimbal lock.

Nevertheless, for some constraints, there is a set of coordinates which are globally independent and orthogonal. A hinge joint has one degree of freedom which is an angle, a prismatic joint has one degree of freedom which is linear, a cylindrical joint has two independent degrees of freedom, one which is an angle, the other is a linear displacement, etc.

Vortex offers a unified interface to add control (that is, additional constraints) on the degrees of freedom, at least in cases where there exists an orthogonal coordinate system to parameterize the degrees of freedom.

See Drag Force.

### Dynamic

Anything that changes in time without reference to what is changing, or how. For example, kinematic constraints are restrictions on the motion of objects, whereas dynamics (or effort) constraints are restrictions on the forces applied to Rigid Bodies.

### Dynamics

Describes the motion of rigid bodies due to application of forces.

### Dynamic Materials

Dynamic Materials define how parts made of different textures and properties (such as rubber, cloth, metal) react differently when they collide. For example, friction, compliance, bounciness, etc. are all parameters which comprise a dynamics material in Vortex, from which the Vortex solver can produce realistic effects.

## E

### Earthwork Tool

Earthwork tools represent the entities interacting with the soil material in a zone to create deformation. The tool provides parameters that define the dynamic and graphic behavior. The following earth-moving tools are provided by CM Labs to match a real-world situation as much as possible:

### Earthwork Zones

Earthwork Zones represent areas of the terrain where soil materials can be dumped or removed. Earthwork zones can be diggable (deformable with a tool interaction) or not. A diggable zone is defined by its soil material properties and its area (the area on the terrain can be deformed). It can only be deformed using an Earthwork tool. All Earthwork zones can receive soil material (only identical material) from another zone (either diggable or not).

### Enhanced Subpixel Morphological Anti-aliasing (SMAA)

A post-processing technique, similar to but slower than FXAA while producing better results. SMAA is better than FXAA at handling sharp geometric features and diagonal shapes by exploiting local contrast features, along with more precise distance searches. However, it requires more system resources.

### Extensible

A user can extend a framework via selective overriding or by adding specialized code to provide functionality.

### Extension

An extension adds functionality to the scene or mechanism to which it is attached. It is the data model component of a plugin.

## F

### Fast Approximation Anti-aliasing (FXAA)

A post-processing method that does not require large amounts of computing power, trading quality for speed. It smooths jagged edges based on how they appear on-screen, rather than analyzing the 3D model itself. Thus, FXAA will smooth not only edges between triangles, but also edges inside alpha-blended textures or resulting from pixel shader effect. However, textures may not appear as sharp if they are included in the edge detection.

### Flexible Tracks

Flexible Tracks simulate tracks that contains wheels and suspensions (for example, a tank). All wheels that belong to a track component are geared and have gear ratios in order to simulate the track that joins them together.

### Forces

Forces are agents that apply changes to one or more objects in speed, direction, or space. Some examples of forces include:

• Global force fields, like gravity
• User-defined forces at given step(s)
• Contact force (arising from other parts or particles)
• Constraint force (arising from constraints attached to the part)
• Fluid drag forces and buoyancy

### FPSO

FPSO stands for Floating Production Storage and Offloading, which is a vessel used in the oil and gas industry to process raw materials and store the resulting oil near drilling platforms at sea.

## G

### Geometric Collision

Geometriccollision detection, which is sometimes abbreviated as collisiondetection or simply collision when the context is clear, refers to computation of interferences (overlap, contact, or penetration) between geometric objects.

### Geometry

This term refers both to the 3D graphics models used by the rendering engine to produce a graphical display and to the models used by the geometric collision-detection system. In the documentation, the distinction is made between the two meaning when the context is not clear.

### Graphics Materials

Graphics Material consist of a set of textures and parameters meant to be used together, with the one set of texture coordinates. The parameters include albedo and emission colors, and the reflection factor coefficient. This type of material defines a valid set of inputs for the lighting equation used in Vortex.

## H

### Hole Geometry

Hole Geometry refers to three special types of collision geometry primitives (Box Hole, Sphere Hole, and Cylinder Hole), which are entirely enclosed within a larger volume in order to produce a hollow geometry. For more information, see Parts and Geometries.

See Chassis.

### Hybrid Vehicles

Hybrid Vehicles are a mix of Wheeled Vehicles and Tracked Vehicles driveline.

### Hydrodynamics

Hydrodynamics is the study of fluid in motion.

## I

### ICD

ICD stands for Interface Control Document. This document outlines a high level description of the interface. It includes inputs and outputs used to interface with Vortex.

### Image

An image is a 2D abstract graphical layer. In the Vortex world, this is typically a bitmap loaded from a file. An image is defined by a width, height and has a format.

### Inversion of control

Inversion of control refers to an overall program's flow of control being dictated by the framework instead of by the caller. Usually, the caller in libraries and applications dictates the flow. Vortex Frameworks has an inversion of control.

### Inertia Tensor

When parts move through space, there is a resistance to changes in motion. This resistance happens when the object is translated from one position to another or rotated around an axis. The resistance to linear motion from one position to another is known as mass while the resistance to rotation change about a fixed axis is known as the rotational inertia or moment of inertia. The latter measures the difficulty in changing the angular motion of a part around a given axis.

There are an infinite number of axes around which a part can be rotated. Given a particular axis, represented as a unit vector, the inertia tensor provides a way to calculate the rotational inertia about that axis:

Iaa = aTIa

...where $$I$$ is a 3 x 3 matrix representing the inertia tensor and a is a unit 3-vector representing the axis direction. Here unit length implies that:

a.a = aTa = 1

## J

### Jacobian

The constraint jacobian is a transformation matrix which maps the relative velocity state of the connected bodies. Each constraint coordinate or constraint equation, as defined in the constraint class, adds a row in the Jacobian matrix.

### Joint Coordinates

Alias for Degrees of Freedom (DOF).

## K

### Kinematic

Pure geometric description of motion, without reference to the cause. For example, kinematic constraints are restrictions on the motion of objects, whereas dynamics (or effort) constraints are restrictions on the forces applied to Rigid Bodies.

### Kinematics

Describes the motion of objects that is created without the use of Dynamics.

## L

### Level Designer

In the context of Vortex Studio, a level designer is a worker who uses the Editor to create simulation environments and scenarios. Level design is both an artistic and technical process.

In Vortex Studio, the level designer mostly uses the Scene file type.

### Light Probe

A light probe is a means of capturing environmental lighting and objects in a scene and using that data to compute shading of an object using a PBR material.

### Lighting Equation

The Lighting Equation is a mathematical formula that generates color values for the fragments on the surface of objects.

 In the context of Cable Systems, loads are what the cables are hoisting.

### Luma

Luma represents the brightness in the black and white portion of an image (separate from chrominance, which conveys color information).

## M

### Materials

Vortex uses the concept of Materials in three different contexts:

• Dynamic Materials in the context of calculating the impact of collisions between objects made of different fabrics, metals, etc.
• Graphics Materials in the context of telling the renderer how to draw objects with different colors, reflection, etc.
• Soil Materials in the context of the Earthwork Systems add-on tool.

### Mechanical Engineer

In the context of Vortex Studio, a mechanical engineer is a worker who has been trained to apply engineering, physics, and materials science principles to design, analyze, manufacture, and maintain mechanical systems. They use the Editor to define simulated mechanisms by creating parts, constraints, contact materials, scripts, etc.

In Vortex Studio, the mechanical engineer mostly uses the Mechanism file type.

### Mechanism

Mechanisms are dynamic objects that interact with other objects. They are composed of collections of assemblies, parts, constraints and extensions, and make up the simulated objects in a scene.

### Model

The word model is used in many contexts, and can refer to any of the following:

• the 3D graphics geometries used for rendering (graphics models)
• the overall description of the physical system (physics model)
• specific aspects of the physical simulation such as friction models, and so on

In the documentation, the distinction is made between the meanings when the context is not clear.

### Module

Runtime plugin required to update parts and extensions.

### Multisample Anti-aliasing (MSAA)

A special case of supersampling, performed by the graphics card. This technique smooths jagged edges by analyzing the pixels of each frame and performing a smoothing/blending process. The 2x, 4x, 8x and 16x values indicate how many samples are used for each pixel to compute the average that is ultimately displayed. The higher the number, the better the quality of the anti-aliasing at the expense of speed. MSAA is good at smoothing edges and lines, but it isn't as effective at smoothing textures or color detail. Because of the way alpha testing works, the edges of transparent objects (or edges within textures) may be missed, though the quality will be no worse than without anti-aliasing. MSAA is multiple times more calculation-intensive than FXAA or SMAA, so be ready to reduce the sampling if framerate suffers.

## O

### Orthographic Projection

A way of representing three dimensional objects in two dimensions, where all the projection lines are perpendicular to the projection plane.

## P

### Paged Terrain Element

A set of related parts and collision geometries that the terrain source creates to represent some of the terrain. The terrain source identifies each element with a unique identitifer. The element may lie across multiple areas.

### Paged Terrain Interface

Description of the functionality a developer must provide to connect a terrain source with Vortex Dynamics. Same as Paged Terrain Source.

### Paged Terrain Producer

A terrain object that feeds the Vortex dynamics with parts and collision geometries, but limits its scope to the areas where collisions are possible. It may hold the logical representation of a very large terrain. However, it will allocate only the parts that are needed in a given simulation at a given time.

### Paged Terrain Source

Description of the functionality a developer must provide to connect a terrain source with Vortex Dynamics. Same as Paged Terrain Interface.

### Parts

A part represents rigid bodies: dynamic objects for which position, velocity and acceleration can be defined according to forces and/or impact computed during a step. Parts contain geometric (Geometric Collision) and state information.

• The Vortex Studio Editor guide for information about using parts in the Vortex Studio Editor.

### Perspective (graphical)

A way of representing a three dimensional image in two dimensions as it is seen by the eye. Objects that are further away from the viewing point appear smaller than those that are closer.

### Physical Collision

Physical collision refers to the actual collision that the Vortex core applies between the physical objects that are interfering geometrically.

### Physically Based Rendering (PBR)

PBR is a method of rendering graphics that can produce photo-realistic visuals by modeling the flow of light in a more accurate way.

### PID Controller

A PID controller is a feedback control system for continuous processes. A PID controller continuously (or every time step, in a discrete-time environment like Vortex Studio) calculates an error value as the difference between a desired setpoint (SP) and a measured process variable (PV), and provides a control output (CO) that is applied to correct the state of the system. The control output is determined by summing three terms based on the system error: proportional (P), integral (I), and derivative (D).

### Pipes

See Subsea Pipes.

### Plugin

In general terms, a plugin is a software component that adds funtionality to an existing application. In Vortex terms, there are two types of plugins:

• Extension, which is the data model component;
• Module, which is the runtime component.

### Powertrain

In Vortex, a Powertrain represents all the components that generate power and deliver it to the surface (for example the road). This includes the engine, torque converter, transmission, retarder, differential and final drive. As it is in the industry, the powertrain refers simply to everything between the engine and the transmission:

### Pulley

 A pulley is a wheel on an axle that is designed to support movement of a cable. In the context of the Cable Systems, the pulley is a cylinder with a hinge constraint (one degree of rotation). There is no freedom for the cable when in contact with the pulley. Only the entry and exit contact points change.

## R

### Real-time

In the Vortex documentation, real-time refers to a simulation that occurs at a rate that generally matches how long the process would take in the real world. However, simulations are not guaranteed to be truly real-time.

### Rigid Bodies

Rigid Bodies are 3-dimensional objects that do not deform. In Vortex Studio, rigid bodies are used in simulations to approximate real world objects.

### Rigid Tracks

Rigid Tracks simulate tracks without suspension or wheels (for example, an excavator). The interaction of the track with the ground is simulated by a part which has a box as its collision geometry.

### Rings

In the context of Cable Systems, rings allow cables to pass through an opening.

### ROV

ROV stands for Remotely Operated Vehicle, which are often simulated using Vortex, either as an underwater vehicle using a TMS (Tether Management System) or above ground on rocky and other rough surfaces, such as lunar terrain.

## S

### Scene

A scene is the highest-level element in the Vortex Studio hierarchy. A scene is where you position and connect the elements of the simulation (mechanisms, terrain, etc.) together.

### Scrub

Scrub refers to the action of navigating quickly to a specific location in the recording while using the vplayer.

### Seabed

A Seabed refers to the bottom of the ocean profile with imagery or texture wrapped on it. These are commonly used in Vortex subsea applications, such as your subsea runtime application.

### Shape File

A Shape File is a set of files (.shp, .shx, .dbf) which contain geometric locations and their attributes. These files are used in Vortex subsea applications as blueprints for creating assets such as pipelines, vessels, rigs, and any other set of polygons, edges and points that need to be created at a specific location in space. For more information, including a technical description of shape files, see http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf.

### Ship

A boat and a ship are both vessels just like a car and a truck are both automobiles.

In the days of sailing there was a set definition between ships and boats distinguishing between the two by the number of masts. In modern times ships are generally considered to be able to carry boats and boats are small enough to be carried by ships.

### Simulation

The software content that defines the objects being simulated.

### Simulator

The hardware on which a simulation is running.

### Soil Bin

A soil bin represents an area on a vehicle (such as a dump truck) where soil materials can be dumped. These areas are not deformable and can only receive soil material from an Earthwork tool such as an excavator bucket.

• Soil Bin (for more information about how it fits into the rest of the Earthwork Systems)

### Soil Materials

Soil material contains both dynamics and graphics behavior and representation. Alone, soil material has no behavior and cannot be affected by anything but an Earthwork tool (e.g., a standard rigid-bodies vehicle moves across sand dune without causing any deformation). The dynamics and graphics behavior of soil materials are driven by the Earthwork Systems tool interacting with it. Examples of soil materials include Clay, Gravel, Loam, or Sand.

### Sub-Components

A Sub-Component is a component that can be inserted into another component by means of a file reference. Only certain types of components can be sub-components. For details, see the Components glossary entry.

### Subsea Pipes

A Subsea Pipe can be any asset used in Vortex subsea applications, such as your subsea runtime application, that is linear. This includes pipelines, flow lines, risers, jumpers, mooring lines and umbilicals.

### Stiffness

The Stiffness coefficient is the spring property that relates position violation to the resulting reaction force.

### System Integrator

In the context of Vortex Studio, a system integrator is a worker who sets up, configures and troubleshoots simulators, either on a local machine or on a distributed system.

In Vortex Studio, the system integrator mostly uses the Setup file type.

## T

### Terrain

A terrain represents the land relief (physical geography) on which rigid bodies are interacting.

Terrains are built from mesh files. When working in the Editor, you can also use the Grid to mimic a terrain during design time.In Vortex, a terrain is static: rigid bodies are not able to modify its topology. However, you can use a plugin to enable dynamic interaction between the terrain and rigid bodies in a simulation:

• By using the Earthworks plugin
• By creating your own Paged Terrain Producer plugin using the SDK

### Terrain Producer

A terrain object that feeds the Vortex dynamics with parts and collision geometries. It is a superset of Paged Terrain Producer.

### Texel

Textures are represented by arrays of texture elements, also called texels.

### Texture

A texture is set of images sent together to the video card for rendering. The texture is configured so that the video can translate any 2D or 3D texture coordinate into a color value. This configuration includes a filtering mode and a wrap mode.

### TMS (Tether Management System)

TMS stands for Tether Management System, which is used with Remotely Operated Underwater Vehicles to ameliorate the effects of subsea forces on the cables tethering the vehicle.

### Tracked Vehicles

Tracked Vehicles run on continuous tracks, meaning that the vehicle is in contact with a larger surface area than a Wheeled Vehicles vehicle.

There are two types of tracks supported in Vortex: rigid and flexible.

## U

### UV Mapping

UV mapping is the process of projecting a 3D model's surface to a 2D texture. The letters U and V are equivalent to X and Y, but are used to avoid confusion with X and Y, which are used in describing 3D coordinates.

### UV Sets

There are multiple ways to put UV coordinates in correspondance with the 3D points on the surface of a 3D model. A UV set is one such mapping of 2D UV positions onto these 3D points.

## V

### Vessel

A vessel includes every description of watercraft, including non-displacement craft, WIG craft and seaplanes, used or capable of being used as a means of transportation on water.

### Virtual Mass

Wheeled Vehicles contain between 2 and N wheels, typically in pairs. Some or all of the wheels are powered and as such are called the drive wheels.