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A Box2D body is represented in JamJar by a Box2DBody.

Using a Box2D Body

A Box2D body can be set up by attaching Box2DBody and Transform components to an entity.

#include "entity/entity.hpp"
#include "geometry/vector_2d.hpp"
#include "standard/2d/transform/transform.hpp"
#include "standard/2d/box2d/box2d_body.hpp"
#include "geometry/polygon.hpp"
auto body = new JamJar::Entity(messageBus);
body->Add(new JamJar::Standard::_2D::Transform(JamJar::Vector2D(0, 30), JamJar::Vector2D(10, 10)));
body->Add(new JamJar::Standard::_2D::Box2DBody(
    JamJar::Polygon({-0.5, 0.5, 0.5, 0.5, 0.5, -0.5, -0.5, -0.5}),
    JamJar::Standard::_2D::Box2DBodyProperties({.density = 1.0f, .angularVelocity = 1.0f})));

This creates a body at position x: 0, y: 30 with a scale of x: 10, y: 10, the shape of the body uses the coordinates -0.5, 0.5, 0.5, 0.5, 0.5, -0.5, -0.5, -0.5 (this is a square) and some additional properties of the body are set using the Box2DBodyProperties (density is 1.0f, initial angular velocity is 1.0f).

Changing Box2D Body Position/Angle

Using a Box2DBody and a Transform the Box2DBody component takes precdence over the transform for determining the position, the position in the Transform should be treated as read only in this scenario, if you need to update the position of the entity use the Box2DBody.SetPosition function.

bodyComp->SetPosition(JamJar::Vector2D(3, 2));

This sets the entity's position to x: 3, y:2.

The same applies to changing the entities angle, use the Box2DBody.SetAngle function:


This sets the entity's angle to 2.

Scaling a Box2D Body

Due to the way Box2D handles shapes and scaling, you must adjust the entity's scale using the Box2DBody component by using the Box2DBody.SetScale function:

bodyComp->SetScale(JamJar::Vector2D(3, 2));

This will adjust the scale to x: 3, y: 2.

Scaling Behind the Scenes

Box2D does not support scaling an existing body, so when you call SetScale this adjusts an extra scaling value, and then marks the body as ready for regeneration using the regenerate property. Then in the next update the Box2DPhysicsSystem will create a copy of the body's fixture before deleting the old fixture and replacing it.

For more information around how Box2D works, check out the Box2D documentation.