Physics behavior

The Physics behavior simulates realistic object physics. It is powered by Box2D. For examples of how the behavior works, search for Physics in the Start dialog.

The Physics behavior is relatively complex. The following tutorials are recommended to gain a basic understanding of how to use the Physics behavior and some important points to know before beginning to use it:
Physics in Construct 2: The basics
Physics in Construct 2: Forces, impulses, torque and joints
This manual section will not repeat the information in these tutorials. Instead it will simply describe each feature of the Physics behavior. The tutorials describe how Physics engines work, what the different types of joints are, the difference between impulses and forces, and so on in case you're not already familiar with them.

Using Physics in Construct 2

The Physics behavior simulates physics separately to the Construct 2 layout. Construct 2 will try to keep the Physics and Construct 2 "worlds" synchronised if one changes but not the other, but this can be unpredictable. For example, setting an object's position or angle will cause Construct 2 to teleport the corresponding object in the physics simulation to the object's new position, which does not always properly take in to account collisions. The same is true of using other Construct 2 behaviors at the same time as Physics.

Therefore it is highly recommended to control Physics objects entirely via the Physics behavior (by setting forces, impulses, torques etc.), rather than trying to manipulate objects by Set position, Set angle etc.

Another consequence is Physics won't respond to objects with the Solid or Jumpthru behaviors. These behaviors are totally redundant when using Physics and have no effect. Instead, use the Immovable property.

Physics properties

Immovable
If Yes, simulate the object having infinite mass. Its density is ignored and it will never move.

Collision mask
Use collision mask uses the object's collision polygon from the Image/Animations editor for physics collisions. If it doesn't have a collision polygon it will use the object's bounding box.
Bounding box ignores the object's collision polygon if any, and for the purposes of Physics collisions considers the object to be a rectangle.
Circle ignores the object's collision polygon if any, and for the purposes of Physics collisions considers the object to be a circle. This allows objects to smoothly roll along (like for example barrels). This is especially useful since object's collision polygons is made out of straight lines, so a smooth circle cannot be created that way.

Prevent rotation
Lock the object's angle so physics collisions do not cause it to rotate.

Density
The density of the physics object. Only used if Immovable is No. The object mass is calculated as its density multiplied by the area of its collision mask. The exact density values used are not important and have no specific units - only the relative density is significant (i.e. an object with density 6 will be twice as dense as an object with density 3).

Friction
The friction coefficient of the physics object from 0 (no friction) to 1 (maximum friction). This adjusts how easily objects move against each other while touching.

Elasticity
The elasticity (also known as restitution or bounciness) of the physics object, from 0 (inelastic, like a rock) to 1 (maximum elasticity, like a rubber ball). This affects how high objects bounce when hitting the floor.

Linear damping
The rate the object slows down over time while moving, from 0 (no slowdown at all) to 1 (maximum slowdown).

Angular damping
The rate the object slows down over time while spinning, from 0 (no slowdown at all) to 1 (maximum slowdown).

Bullet
Enable enhanced collision detection for fast-moving objects. This can affect performance, so do not enable it unless the object moves so fast the physics engine's standard collision detection is unreliable.

Initial state
Whether the physics simulation is initially enabled or disabled. If disabled, no physics is processed for the object, and other physics objects can pass through the object as if it were empty space.

Physics conditions

Compare angular velocity
Compare the current angular velocity of the physics body, in degrees per second. A positive value indicates clockwise rotation and a negative value indicates anticlockwise rotation.

Compare mass
Compare the mass of the physics body. This is determined by multiplying the Density by the area of the object's collision polygon.

Compare velocity
Compare the current velocity (speed) of the physics body, in pixels per second. The velocity can be compared on an individual axis, such as just the X axis to compare the horizontal motion, or the overall velocity can be used.

Is enabled
True if the physics behavior is currently enabled. When disabled the physics body is completely removed from the physics simulation, so other physics objects will pass through the object.

Is sleeping
True if the object has been at rest and not moved or been disturbed for a while, so that the physics engine can stop processing it. Note objects can still be moving imperceptibly which can prevent them from being asleep even when they appear to be stopped.

Physics actions: Forces

Apply force
Apply force at angle
Apply force towards position
Apply a force on the object, either at an angle, towards a position, or with custom X and Y axis forces. Applying a force causes the object to accelerate in the direction of the force. Forces can be applied from an image point or the object's origin.

Apply impulse
Apply impulse at angle
Apply impulse towards position
Apply an impulse on the object, either at an angle, towards a position, or with custom X and Y axis impulses. Applying an impulse simulates the object being struck, e.g. hit by a bat. Impulses can be applied from an image point or the object's origin.

Set velocity
Set the object's current velocity directly, providing a speed in pixels per second for the X and Y axes.

Physics actions: Global settings

These actions affect Physics behaviors in general, not just the one it was set for.

Enable/disable collisions
By default, all Physics objects collide with each other. You can disable collisions between the object and another Physics object so they pass through each other. This affects all instances of both object types. Note: enabling collisions again when objects are overlapping can cause instability in the simulation.

Set stepping iterations
Set the number of velocity iterations and position iterations used in the physics engine. The default is 8 and 3 respectively. Lower values run faster but are less accurate, and higher values can reduce performance but provide a more realistic simulation.

Set stepping mode
Choose whether the Physics time step uses dt (delta time, for Framerate independent) or a fixed value. By default it uses a fixed time step to eliminate any randomness or instability coming from dt, ensuring simulations are exactly reproducible every time. Set to Framerate independent to adjust the time step according to the framerate, which can cause the same simulation to have different results if run twice. Note even when in this mode, the behavior clamps the maximum time step to 1/30 (about 33ms, equivalent to 30 FPS) to prevent the instability that can result from large time steps. For more information see Delta-time and framerate independence.

Set world gravity
Set the force of gravity affecting all Physics objects. By default this is a force of 10 downwards.

Physics actions: Joints

Create distance joint
Fix two physics objects at a given distance apart, as if connected by a pole. An image point can be specified to connect to a specific part of the object. Note that an image point of 0 specifies the center of gravity of the object - if you intend to connect to the object origin, use -1.

Create revolute joint
Create limited revolute joint
Hinge two physics objects together, so they can rotate freely as if connected by a pin. Limited revolute joints only allow rotation through a certain range of angles, like the clapper of a bell. An image point can also be specified to connect to a specific part of the object. Note that an image point of 0 specifies the center of gravity of the object - if you intend to connect to the object origin, use -1.

Remove all joints
Remove all joints from the object. Any objects this object was attached to via joints is also affected. Note some joints automatically disable collisions between the objects, so you may want to manually disable collisions again after removing joints otherwise overlapping objects will "teleport" apart (as the physics engine will try to prevent them overlapping).

Physics actions: Object settings

These set the corresponding properties. For more information, see Physics properties.

Physics actions: Torque

Apply torque
Apply torque towards angle
Apply torque towards position
Apply a torque (rotational acceleration) to the object, either directly, or towards an angle or position.

Set angular velocity
Set the angular velocity (rate of rotation) directly, in degrees per second.

Physics expressions

AngularVelocity
The current angular velocity (rate of rotation) of the physics object, in degrees per second.

CenterOfMassX
CenterOfMassY
The position of the center of mass of the physics object, as calculated by the physics engine. This depends on the collision mask property, and is not necessarily in the middle of the object.

Mass
The mass of the physics object, as calculated by the physics engine. This is the area of the object's collision mask multiplied by its density.

VelocityX
VelocityY
The current speed of the physics object, in pixels per second.

AngularDamping
Density
Elasticity
Friction
LinearDamping
These return the corresponding properties. For more information, see Physics properties.

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