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agsbox2d

Build Status

AgsBox2D is a physics plugin for Adventure Game Studio that gives access to the Box2D library created by Erin Catto. Because I never used Box2D directly before, I tried to make the API similar to Love physics module.

AgsBox2D is still in early development.

In development warning

AgsBox2D is in development and you can reach uncommon bugs, like unstable load from regular ags save. This plugin does support regulas AGS Save and Load, but I can have forgotten something, so if you hit this, just open an issue here.

Still, if you want to experiment with it and report your findings, this README should prove itself useful and I appreciate any help in making this plugin work well with AGS.

Usage example

Below we will do a very simple example that creates a ground, and adds a ball and a box. The ball is controlled by keyboard arrows.

// room script file
World* world;
Overlay* ov;

struct Physical {
  Body* body;
  Shape* shape;
  Fixture* fixture;
};

Physical ground;
Physical ball;

function room_Load()
{
  if(world == null){
    AgsBox2D.SetMeter(32.0);
    world = AgsBox2D.CreateWorld(0.0, 9.8*AgsBox2D.GetMeter());
    
    ground.body = AgsBox2D.CreateBody(world, 160.0, 160.0, eBodyStatic);
    ground.shape = AgsBox2D.CreateRectangleShape(320.0, 40.0);
    ground.fixture = AgsBox2D.CreateFixture(ground.body, ground.shape);

    ball.body = AgsBox2D.CreateBody(world, 160.0, 40.0, eBodyDynamic);
    ball.shape = AgsBox2D.CreateCircleShape(20.0);
    ball.fixture = AgsBox2D.CreateFixture(ball.body, ball.shape, 1.0); 
    ball.fixture.Restitution = 0.5;
    
    AgsBox2D.CreateFixture(AgsBox2D.CreateBody(world, 80.0, 60.0, eBodyDynamic), 
                           AgsBox2D.CreateRectangleShape(30.0, 20.0), 5.0);
  }
}

function room_RepExec()
{
  if(IsKeyPressed(eKeyLeftArrow)) ball.body.ApplyForce(-500.0, 0.0);
  if(IsKeyPressed(eKeyRightArrow)) ball.body.ApplyForce(500.0, 0.0);
  if(IsKeyPressed(eKeyUpArrow) && ball.body.IsTouching(ground.body)){
    ball.body.ApplyForce(0.0, -6000.0);
    ball.body.SetLinearVelocity(0.0, 0.0);
  }
  
  if(ov!=null && ov.Valid) ov.Remove();  
  ov = Overlay.CreateGraphical(0, 0, world.GetDebugSprite(), true);
  
  world.Step(1.0/IntToFloat(GetGameSpeed()), 8, 3);
}

Check agsbox2d_demo AGS Game project by loading it on AGS!


Script API

AgsBox2D

void AgsBox2D.SetMeter(float meter)

Sets how many pixels equals to a meter. Default is 32 pixels per meter.

Do this only once, before using any other functions, it doesn't apply retroactively

You want the size of your moving objects roughly between 0.1 and 10 meters.

For the default 32px this enables objects between 3 and 320 pixels, so this usually needs to scale along with your game resolution and character size.

Internally, Box2D uses Meter, Kilograms and Seconds as it's main units.

float AgsBox2D.GetMeter()

Get's previously passed meter in pixels.

World* AgsBox2D.CreateWorld(float gravityX, float gravityY)

Creates a World object, this should be done before creating bodies.

A positive gravityY is directed to ground, and a negative gravityY is directed upwards.

Similarly, a positive gravityX is directed to right and a negative gravityX is directed to left.

Body* AgsBox2D.CreateBody(World* world, float x, float y, BodyType bodytype)

Creates a body object in the world, at specified x and y positions. These positions correspond to the center of the body.

The bodytype can be any of the types below:

  • eBodyStatic : An object that does not move under simulation, usually the ground in a platformer is a static body. It doesn't collide with other static or kinematic bodies. A static body has zero velocity.

  • eBodyDynamic : A fully simulated body, can collide with all body type, this body moves according to forces. It always has finite non-zero mass.

  • eBodyKinematic : A kinematic body moves according it's velocity, it doesn't move according to forces. A Kinematic body behaves as if it has infinite mass. It doesn't collide with other Kinematic bodies or with static bodies.

Any bodytype can be moved by user input, but you have to specific code the behavior in AGS.

You don't need to keep the returned pointer if you aren't going to need to access this body anymore, since the world will hold it, but you will be unable to destroy it unless the world is destroyed.

The specifics on a body form and mass are defined by using a Shape and Fixture.

void AgsBox2D.DestroyBody(World* world, Body* body)

Removes a body from the world, and marks it with the property IsDestroyed true.

Always remove Joints attached to a body before removing it, otherwise the joint may turn invalid and be accessed, causing a crash.

Shape* AgsBox2D.CreateRectangleShape(float w, float h, float x=0, float y=0)

Creates a RectangleShape with Width w and Height h, and returns a Shape object.

You can also change it's relative center which will be mapped to the body center. An x of 0.0 and y of 0.0, which are defaults, maps to the shape center.

Shape* AgsBox2D.CreateCircleShape(float radius, float x=0, float y=0)

Creates a Circle shape, and similar to RectangleShape, you can also translate it's center.

Fixture* AgsBox2D.CreateFixture(Body* body, Shape* shape, float density=0)

Creates a Fixture, and attachs a body a shape, and specifies a density.

You should always pass finite non-zero densities for dynamic bodies.

You don't need to keep the pointer to the shape attached to a body through a fixture, since the body will hold a copy of the shape. Similarly, you also don't need to keep a pointer to the fixture, because the body will hold it too.

Joint* AgsBox2D.CreateDistanceJoint(Body* bodyA, Body* bodyB, float a_x, float a_y, float b_x, float b_y, bool collideConnected = 0)

Create Distance Joint, pass anchors on bodies A and B using world coordinates. The two bodies are assumed to be in place when this joint is created.

This joint constrains the distance between two points on two bodies to be constant. The first anchor point is connected to the first body and the second to the second body, and the points define the length of the distance joint.

Joint* AgsBox2D.CreateMotorJoint(Body* bodyA, Body* bodyB, float correction_factor, bool collideConnected = 0)

Create Motor Joint. This is a joint between two bodies which controls the relative motion between them.

Position and rotation offsets can be specified once the MotorJoint has been created, as well as the maximum motor force and torque that will be be applied to reach the target offsets.

Joint* AgsBox2D.CreateMouseJoint(Body* bodyA, float x, float y)

Create Mouse Joint between body and a target point in the world. To make it follow the mouse, the fixed point must be updated every time-step.

The advantage of using a MouseJoint instead of just changing a body position directly is that collisions and reactions to other joints are handled by the physics engine.

Joint* AgsBox2D.CreatePulleyJoint(Body* bodyA, Body* bodyB, PointF* groundAnchorA, PointF* groundAnchorB, PointF* localAnchorA, PointF* localAnchorB, float ratio, bool collideConnected = 0)

Creates a PulleyJoint to join two bodies to each other and the ground.

The pulley joint simulates a pulley with an optional block and tackle. If the ratio parameter has a value different from one, then the simulated rope extends faster on one side than the other. In a pulley joint the total length of the simulated rope is the constant length1 + ratio * length2, which is set when the pulley joint is created.

Pulley joints can behave unpredictably if one side is fully extended. It is recommended that the method setMaxLengths be used to constrain the maximum lengths each side can attain.

void AgsBox2D.AgsBox2D.DestroyJoint(World* world, Joint* body)

Removes a joint from the world, it should no longer return true to isValid.

PointF

PointF* PointF.Create(float x, float y)

Creates a PointF object with x and y values.

float PointF.X

The X coordinate property of a PointF.

float PointF.Y

The Y coordinate property of a PointF.

float PointF.Length()

Returns distance from point (X,Y) coordinates to origin (0,0).

float PointF.SquaredLength()

Returns squared distance from point (X,Y) coordinates to origin (0,0). Slightly faster.

PointF* PointF.Add(PointF* pointF)

Returns a new point with the sum of this with pointF.

PointF* PointF.Sub(PointF* pointF)

Returns a new point with the subtraction of pointF from this.

PointF* PointF.Scale(float scale)

Returns a new point that is a copy of this point multiplied by a scalar.

PointF* PointF.Rotate(float angle, float pivot_x = 0, float pivot_y = 0)

Returns a new point with this point treated as a vector to a pivot point, rotated by an angle in radians. If you don't specify, pivot is origin (0,0).

Point* PointF.ToPoint()

Rounds this point as integer and returns a standard AGS Point object.

Body

int Body.X

The X position property of a body as integer.

Avoid setting this property directly. Bodies coordinates are actually float values in the simulation, this is provided as convenience.

int Body.Y

The Y position property of a body as integer.

Avoid setting this property directly. Bodies coordinates are actually float values in the simulation, this is provided as convenience.

float Body.fX

The X position property of a body as float.

float Body.fY

The Y position property of a body as float.

float Body.Angle

The Body Angle property. AGS can't easily rotate Sprites so avoid using angles with bodies that you expect to map directly in someway to screen sprites.

bool Body.FixedRotation

By default, bodies are created with FixedRotation set to true.

A body with FixedRotation set to true does not rotate, causing it's rotational inertia and it's inverse to be set to zero.

bool Body.Bullet

By default, bodies are created with Bullet set to false.

Set bullet to true when the body has a small shape and moves really fast, this will prevent the body from having wrong collisions with thin bodies.

readonly bool Body.IsDestroyed

Returns true if it's destroyed by AgsBox2D.DestroyBody().

float Body.LinearDamping

The LinearDamping property of a body. Damping occurs independently from contact and is different than friction. Normally the value for damping is between 0.0 and 0.1.

float Body.AngularDamping

The AngularDamping property of a body, the angular drag, also happens independently from contact.

float Body.AngularVelocity

The AngularVelocity property of a body.

float Body.Inertia

Rotational Inertia, body's resistance to changes in angular velocity.

readonly float Body.LinearVelocityX

Gets the X vector from the body's Linear Velocity.

readonly float Body.LinearVelocityY

Gets the Y vector from the body's Linear Velocity.

void Body.SetLinearVelocity(float fx, float fy)

Set's the body LinearVelocity vector.

void Body.ApplyForce(float fx, float fy)

Applies a force on a body from it's center 0.0, 0.0 to the specified fx, fy direction.

void Body.ApplyAngularImpulse(float impulseIntensity)

Applies an angular impulse on the body.

void Body.ApplyLinearImpulse(float intensity_x, float intensity_y)

Applies an impulse from the body center with the specified vector.

void Body.ApplyTorque(float torque)

Applies a torque on the body. Positive values are counter clockwise.

bool Body.IsTouching(Body* otherBody)

Returns true when a body is in contact (being touched) by other body. This function only evaluates at the current time, so prefer using it for resting states.

World

The world holds all the information needed for the physics simulation. Once a world is destroyed, the previous pointers (Bodies, Fixtures, ...) will be of no use and you will need to recreate any objects you need in the new world.

void World.Step(float dt, int velocityIteractions=8, int positionIteractions=3)

Advances a step in the World physics simulation of dt seconds.

Because AGS uses fixed game steps, a good value is dt = 1.0/IntToFloat(GetGameSpeed()).

velocityIteractions and positionIteractions relates to how Box2D simulates the world, so for information on these values I recommend looking into Box2D own documentation.

int World.GetDebugSprite(int camera_x=0, int camera_y=0)

Returns a sprite of the size of the screen with the objects in the world drawn on it.

A common usage is to create a GUI of the size of the screen and set the background graphic of it with the sprite this function outputs.

You can pass a camera x and y value to scroll the camera on the world.

FixtureArray* World.BoundingBoxQuery(float lower_x, float lower_y, float upper_x, float upper_y)

Returns array of fixtures which their bounding boxes are overlapped by the supplied box.

A fixture bounding box is the non rotated smallest rectangle that contains it's shape, this means a rotate rectangle or a circle, a empty area is part of the bounding box. This is usually good enough for a first stage of a detection, but may require additional steps.

RaycastResult* World.Raycast(float x0, float y0, float x1, float y1, RaycastType rc_type = 0, FixtureArray* stopping_fixtures = 0)

Returns RaycastResult with fixtures hit by a line, along with the hit normals. The raycast goes through all fixtures on the line if you supply eRaycastPassthrough (this is the default).

You can use eRaycastUntilHit for it to stop at the first fixture hit, or additionally supply an array of target fixtures so that the raycast only stops if hit any fixture on the array.

readonly int World.ContactCount

How many contacts are available. Use it to know the range to access World.Contacts[].

readonly Contact* World.Contacts[]

Gets the contacts in the world by index. These only contain fixtures in contact right now.

Shape

ShapeRectangle* Shape.AsRectangle

If the shape is a ShapeRectangle, it returns it. Otherwise, it returns null. You should not hold pointers to it, and instead access it directly like myshape.AsRectangle.Width as needed.

ShapeCircle* Shape.AsCircle

If the shape is a ShapeCircle, it returns it. Otherwise, it returns null. You should not hold pointers to it, and instead access it directly like myshape.AsCircle.Radius as needed.

Fixture

Fixtures are used when linking a shape to an object and assigning it's density.

float Fixture.Density

Density is used to compute the mass of the linked body. It's preferable to use similar densities to all your fixtures, because this will improve the simulation.

float Fixture.Friction

Friction is used to make objects slide along each other realistically.

It's usually a value between 0.0 and 1.0, but can be any non-negative value.

Box2D uses the square root of the multiplication of two contacting fixtures to calculate the contact friction. This means if one fixture has 0.0 friction, the contact will have no friction.

float Fixture.Restitution

Restitution is used to make objects bounce, and is usually a value between 0.0 and 1.0. A value of 0.0 means the object won't bounce, and a value of 1.0 means the object velocity will be exactly reflected.

readonly Body* Fixture.Body

Returns Body if it's defined for this fixture, otherwise null.

int Fixture.GroupIndex

Group the fixture belongs to, from -32768 to 32767. Fixtures with the same group will always collide if group is positive or never collide if it's negative. Zero means no group, and is default.

int Fixture.CategoryBits

Category of this fixture, from 16 possible categories encoded as 16-bit integer (1, 2, 4, 8, ... 32768). 65535 means all categories.

int Fixture.MaskBits

Mask of this fixture, encoded as 16-bit integer. Categories selected will collide with this fixture (ex: 5, means category 1 and 4 will collide). Default is 65535 - collide with all categories.

bool Fixture.TestPoint(float x, float y)

Returns true if a point is inside the shape of the fixture.

bool Fixture.IsSensor

Whether this fixture is a sensor. Sensors do not cause collision responses, but generate begin-contact and end-contact events.

Joint

JointDistance* Joint.AsDistance

If this joint is a distance joint, returns the JointDistance interface; otherwise null.

JointMotor* Joint.AsMotor

If this joint is a motor joint, returns the JointMotor interface; otherwise null.

JointMouse* Joint.AsMouse

If this joint is a mouse joint, returns the JointMouse interface; otherwise null.

JointPulley* Joint.AsPulley

If this joint is a pulley joint, returns the JointPulley interface; otherwise null.

bool Joint.IsValid

If this joint is valid, returns true.

bool Joint.IsActive

If this joint is active, returns true.

Body* Joint.BodyA

Returns Body A if it's defined, otherwise null, for this joint.

Body* Joint.BodyB

Returns Body B if it's defined, otherwise null, for this joint.

JointType Joint.Type

Returns this joint type.

JointDistance

float JointDistance.Length

The equilibrium distance between the two Bodies.

float JointDistance.DampingRatio

The damping ratio, typically between 0 and 1. At 1, the damping is critical.

float JointDistance.Frequency

The frequency of a harmonic oscillator. Should be smaller than half the frame rate.

JointMotor

void JointMotor.SetLinearOffset(float fx, float fy)

Sets the target linear offset between the two bodies the joint is attached to.

float JointMotor.LinearOffsetX

The target linear offset X axis between the two bodies the joint is attached to.

float JointMotor.LinearOffsetY

The target linear offset Y axis between the two bodies the joint is attached to.

float JointMotor.AngularOffset

The target angular offset between the two bodies the joint is attached to.

float JointMotor.MaxForce

The Maximum Force applied to reach target position.

float JointMotor.MaxTorque

The Maximum Torque applied to reach target rotation.

JointMouse

void JointMouse.SetTarget(float fx, float fy)

Sets the target point.

float JointMouse.TargetX

The target point X axis.

float JointMouse.TargetY

The target point Y axis.

float JointMouse.DampingRatio

The damping ratio, typically between 0 and 1. At 1, the damping is critical.

float JointMouse.Frequency

The frequency of a harmonic oscillator. Should be smaller than half the frame rate.

float JointMouse.MaxForce

The Maximum Force applied to reach target position.

JointPulley

float JointPulley.LengthA

The current length of the segment attached to the first body.

float JointPulley.LengthB

The current length of the segment attached to the second body.

float JointPulley.Ratio

The pulley ratio.

Contact

readonly bool Contact.IsValid

Whether the Contact is still valid.

readonly PointF* Contact.Normal

The normal vector between two shapes that are in contact.

readonly PointF* Contact.Positions[]

The contact points of the two colliding fixture, use PositionsCount to find out how many. Index starts at 0.

readonly int Contact.PositionsCount

How many position of contact are available.

readonly Fixture* Contact.FixtureA

One of the Fixtures that hold the shapes in contact.

readonly Fixture* Contact.FixtureB

The other of the Fixtures that hold the shapes in contact.

bool Contact.Enabled

Whether the contact is enabled.

float Contact.Restitution

The restitution between two shapes that are in contact.

float Contact.Friction

The friction between two shapes that are in contact.


Download AgsBox2D

This plugin is available as agsbox2d.dll under assets, in the latest release, for usage with Windows and the AGS Editor. You can also find it there built for Linux as libagsbox2d.so and for MacOS as libagsbox2d.dylib.

Building agsbox2d

AgsBox2D both Makefile and the VS Solution file, expects to find Adventure Game Studio source code in a folder ../ags/. After you do this, you need to clone this source code.

  ~/git/ags/
  ~/git/agsbox2d/

Navigate then to the directory where you cloned this repository.

On Windows, you can load the solution on the root of it's directory and load it on Visual Studio. It should work with VS 2015, 2017 and 2019. You will need v140 tools (VS should promptly warn you to install if you don't have it). The dll provided by Release Win32 builds is the one you should build to use with an AGS Game at the time of this writing.

On Linux and MacOS, navigate to agsbox2d/ inside the directory and type make.

License and Author

AgsBox2D is made by Érico Vieira Porto provided with Z-Lib LICENSE.

Box2D itself is made by Erin Catto and is provided with a Z-Lib LICENSE too.

Sprites used here includes the ones included with the Sierra template.

Magic Cliffs background used on the demo is made by ansimuz and is available as CC-BY 3.0.

Room 5 and 6 sprites are made by Buch, with the CC0 license.