Glossary

Angular velocity:
Angular velocity gives the rate of change for an object orientation. In the True Axis Physics Wrapper, angular velocity is usually represented by an TA_Vec3. The direction of the vector gives the axis of rotation. The magnitude of the vector gives the rate of rotation in radians per second (rad/s).
Collision grid:
The True Axis Physics Wrapper can use either a collision grid, an octree or a ctree to find potential collisions between a large number of moving objects.

The True Axis Physics Wrapper uses the term collision grid to describe a 2D grid used to separate space. Objects are placed into grid segments based on there location. When an object is moved, only near by grid segments need to be checked for other objects that could cause a collision. This way, the simulation can easily cope with thousands of dynamic objects. A collision grid has the advantage of being very fast for level layouts that can be represented well by a 2D map.

Sometimes, a 2D grid may not do a good job of dividing the world. The collision grid solution has problems with dynamic objects that are larger than a grid segment. The collision grid copes best with a world containing evenly distributed objects. Large sparse areas with small pockets of tightly packet objects are less ideal. A 2D grid may not work well with a highly 3D environment. The octree and ctree methods provides an alternative solution that avoids these problems.

CTree:
The True Axis Physics Wrapper can use either a collision grid, an octree or a ctree to find potential collisions between a large number of moving objects.

A ctree is a spacial division structure. It is a new type of spacial division structure developed by True Axis. It is basically a modified version of an octree. It is called a c-tree because it was an attempt to make a compressed octree. In sparce environments, Octrees may contain a lot of nodes that only contain one Child. In CTree removes all these nodes by adding extra information to each node to specify its world location.

CTrees have the advantage of being more efficient in sparce environments then an octree. Also, the number of maximum internal nodes required for a ctree is equal to the number of objects minus one. This means that memory usage is predictable. Unlike octrees and collision grids, the user does not need to specify the amount of memory required.

CTrees may be more efficient in some situations then octrees. Some experimentation by the user may be necessary to find which suits them better.

Fast solver:
True Axis has two solvers for calculating physics response the slow solver and the fast solver. See Solvers
Impulse:
An impulse is similar to a force, but where a force has an effect over time, an impulse is instantaneous. The units are in kg.m/s. The True Axis Physics Wrapper deals mostly with impulses rather than forces.
Inertia tensor:
As used in the True Axis Physics Wrapper, an inertia tensor is a 3 by 3 matrix representation of rotational inertia. The inertia tensor of an object affects how that object's angular velocity responds to forces and impulses. The value controls how the collision response of a cube is different to that of a long thin pole.

The relationship between the inertia tensor and angular velocity is equivalent to the relationship between mass and linear velocity. The inertia tensor however, rotates with the object, where as mass is not affected by rotation.

Linear velocity:
Linear velocity gives the rate and direction of movement for an object's center of mass. In the True Axis Physics Wrapper, linear velocity is usually represented by an TA_Vec3. The magnitude of the vector gives the objects speed in meters per second (m/s).
Movement propagation:
Movement propagation is the name give to the method that the True Axis Physics Wrapper uses to manage the number of moving and resting objects. A list of moving objects is kept to save processing power. Only objects in this moving list are updated and tested for collisions. With movement propagation, objects are only added to the moving list when there is a high probability of movement due to an external collision.

By default, movement propagation is enabled. However, if movement propagation is disabled, then, if one object in a group of touching objects is moving, then the entire group will be added to the moving list. This will typically result in a large number objects in the moving list that that are not actually moving. This is because, although an object is set to moving, the sum of forces applied to it may not be enough to actually move it.

If movement propagation is on, only touching objects that have a high probability of being affected by moving objects are added to the moving list. This means that the number of moving objects can be kept much lower and larger numbers of touching objects can be handled more effectively. The cost of this is that movement propagation may occasionally cause some unrealistic results.

Movement propagation is most useful when the number of touching objects is far greater then the maximum number of moving objects. In this case, if movement propagation is off, the max number of moving objects will be met, after which point, the simulation will start to fail. With movement propagation on, this situation is handled more gracefully. Only the fastest moving objects and objects with the strongest collisions are added and kept in the moving list.

Octree:
The True Axis Physics Wrapper can use either a collision grid, an octree or a ctree to find potential collisions between a large number of moving objects.

An octree is a spacial division structure. Each node in the tree is represented by a cube. A nodes children are created by dividing the cube into eight equally sized pieces.

The octree implementation has a number of advantages over the collision grid implementation, but the collision grid method may often be faster. See collision grid for more information.

Reference Counting:
Reference counting if often used on objects in the True Axis Physics Wrapper. Reference counting allows multiple references to be taken to a pointer and automatically cleans up when the last reference is released.

After a reference counted object is created it will have a reference count of one. AddRef() and Release() can be called to increment and decrement the reference count. If the reference count is zero after a call to Release() the object will be automatically deleted. Reference counted objects should only be created using new.

Slow solver:
True Axis has two solvers for calculating physics response the slow solver and the fast solver. See Solvers
Swept collision testing:
Most collision detection implementations are only capable of testing if two objects are currently intersecting. An object is moved to its new position than at which point it may be intersecting something or have totally moved through something. The faster an object moves, the more problematic this method becomes. One solution is to decrease the time number of time steps for fast moving objects but this can be slow.

The True Axis Physics Wrapper performs swept collision testing. By this we mean that all collision that occur during an objects movement can be found. As an approximation of reality, the True Axis Physics Wrapper finds all of the collisions that will occur in a time step and applies them all at ones. Multiple collisions passes are performed in a single time step to help cope with sudden velocity changes due to collisions. A single pass of this method of collision testing is slower than intersection testing but less passes are needed for accuracy so it can be potentially faster overall. Because swept collision testing is not need when object are moving slowly, it is turned off in these cases to improve execution speed.

The end result of the use of swept collision testing is that the True Axis Physics Wrapper copes well with very fast moving objects where other simulations have problems or even fail totally.

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