Physics

Unlike many other engines using third-party solutions, Unigine features its own built-in physics module.

The advantages of using a built-in module are as follows:

• Memory efficiency: a single instance of the world is used (in case if external solution is used we have to create and store another copy of our world)
• Performance: calls to external functions, excessive data conversion and transmission between integrated systems are avoided

It should be clear that it is not a high-precision simulation of real world physics! The simulation is just an approximation based on simplified calculations and decreased accuracy providing realistic look within the strict limits of real-time mode. Achieving this always involves a trade-off between accuracy and performance.

Though in general Unigine uses a simplified Newtonian physics with an impulse-based approach, the integration of collision detection, gravity, friction, buoyancy, joints and external physical forces provides realistic simulation of physical bodies and complex interactions between them.

Unigine physics module is quite limited, however, in a number of cases it helps programmers and designers to make their work easier. Here is the list of the use cases when it is better to use built-in physics rather than hard-coding or bone-based animation:

• Collision detection (prevention of moving through walls, interpenetration of solid bodies, etc.)
• Simulation of perfectly elastic collisions (redistribution of kinetic energy)
• Simulation of various joints, motors and springs
• Simulation of basic physical phenomena: gravity, friction (static and sliding), buoyancy (for relatively calm water without big waves)
• Simulation of external forces (wind, force field)
• Procedural destruction of meshes
• Simulation of deformable cloth and ropes
• Rag doll simulation

It should be understood that the scope of simulation using built-in physics is limited to the cases listed above. Thus, there are a large number of cases that cannot be solved this way, including the following:

• High-precision physics simulation
• Flight dynamics simulation
• Fluid dynamics simulation
• Simulation of gravitational fields
• Simulation of inelastic collisions
• Digging of the ground
• Physical destruction of complex objects

Though some tasks involving a large number of objects can be solved using built-in physics, it may significantly reduce performance. In this case, as in the cases listed above, it is recommended to use alternative ways described in the next chapter.

When a problem cannot be solved by means of the built-in physics module or takes too much time, several other approaches can be used: bone-based animation, hard coding or using an external physics engine.

Bone-based Animation

Code-based Approach

External Physics Engines

Combined Approach

While the majority of node types in the scene serves as auxiliary means or as decorations, the Object type can have physical properties and interact with other objects and the environment. For these properties to be assigned, the object should have a "body " (a physical approximation of the object), and the body should have at least one "shape " (a volume in space occupied by a body). To connect bodies and restrict their movement relative to each other the "joints " are used. The picture below illustrates relationships between these entities.

Body

Shape

Joint

To restrict interpenetration of solid bodies, prevent moving through walls, and make things look more natural and familiar to the eye, the collision detection is used.

There are two types of collision detection implemented in Unigine:

• Discrete collision detection is performed in certain intervals of time and each frame is treated separately from others. In general, discretization improves performance. However, when a project framerate is already low, a small fast-moving object is likely to teleport from one point to another instead of moving there smoothly and collision will not be detected.
• Continuous collision detection does not suffer this problem as the moving body is extruded along its trajectory (between two adjacent frames). In cases when something gets into this volume and a collision is detected, the body is taken back in time to correct the collision reaction.
  Notice

  Continuous collision detection is available for sphere and capsule shapes only.

Collision detection is a very expensive operation, and since our scene may have hundreds of objects, a lot of effort is put into optimization.

• The first parameter contributing to the cost is collision shape complexity. As a rule, most 3d objects are represented by a complex and detailed visible mesh and an invisible simplified shape used by a physics engine for collision detection. Types of shapes available are listed above.
  Notice

  To reduce computational load it is strongly recommended to use simple collision shapes instead of complex ones when it is possible.
• The second parameter is a number of checks to be performed – the worst case would be all-against-all. To exclude unnecessary checks (e.g. two objects hardly collide if they are far from each other) the scene is to be split up into sections.

The process of collision detection is divided into two phases:

• Broad phase detection is a computationally low cost operation that quickly answers the question: Which objects have a strong possibility of colliding?
• Narrow phase is a computationally intense mesh on mesh collision detection, and thus cannot be performed on every pair of objects each frame. It answers the question: What part of object A collided with object B?

There is another important step which follows collision detection – collision response, or the result of collision (e.g. two balls bounce off of each other). Friction, restitution and other parameters are taken into account in calculation of collision response.

In case when some object's surfaces participate in collisions, and others don't, collision detection can be enabled on a per-surface basis. For example, a dashboard inside a cockpit, as it is covered by other surfaces.

Intersection detection lies at the heart of collision detection, but a particular case of ray intersection (ray casting) serves a little different purposes. Calculation of intersection between a ray cast from a certain point in a certain direction and a surface is fast and inexpensive and therefore sometimes can be used instead of computing collision. For example, calculation of collisions of car wheels with the ground takes time and decreases framerate. In this case we can use intersection of rays cast from the bottom of the car with the ground instead of precise collision detection using collision shapes, thus reducing computational costs and increasing performance.

To make collision and intersection detection flexible and selective, and to reduce calculation costs, the mechanism of bit masking is used. For example, we have an object, which does not participate in interactions with others, but we want it to lie on the ground. Matching the collision bit masks of this object and the ground (at least one bit in the masks should match) provides the necessary effect. One object can participate in several collision and intersection checks as only one bit in the mask is required to match for a pair of objects.

Simulation of physics is performed in a looped sequence of the following stages on a per-frame basis:

There are two modes of physics simulation:

• Single-threaded, when all stages go one after another
• Multi-threaded, when some of the operations are performed in parallel

Physics is simulated with its own fixed FPS, which does not depend on rendering framerate, a variable FPS cannot be used here because it makes calculation results unstable.

Calculation of physics for all dynamic objects on the scene takes too much time. To ensure consistent frame rate and exclude unnecessary calculations some optimization approaches are used:

• When a body is moving slower than a specified minimum linear or angular velocity for a specified period (freeze frames), it is assumed that it has come to a halt. During this period of inactivity, there is actually no need to simulate it. If this happens, the object remains in a resting state until it is affected by another object or a force. Therefore, such object is excluded from all steps of the simulation except collision detection. This state is called freezing and it allows to save a great deal of computational resources.

  Frozen blue and unfrozen red boxes. The impulse applied to the pyramid of boxes unfroze all but one.

• If interacting objects are far away from the viewer they do not make a noticeable contribution to the whole image and thus can be neglected. Simulation distance setting specifies the distance from the viewer when physical interactions are no longer calculated.

Global physics settings (FPS, gravity, simulation distance, freeze frames and velocities, penetration factor, penetration tolerance) can be accessed via the "Physics settings" panel of the editor.