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paweljaroszpaweljarosz
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Added motor joint example for Box2D v2 and v3. (#197)
* Added joints examples for Box2D v2 and v3. * Changed joints to motor joint.
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‎physics/box2d_motor_joint/all.texture_profiles‎

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path_settings {
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path: "**"
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profile: "Default"
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}
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profiles {
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name: "Default"
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platforms {
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formats {
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format: TEXTURE_FORMAT_RGBA
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‎physics/box2d_motor_joint/box2D_V2.appmanifest‎

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‎physics/box2d_motor_joint/box2D_V3.appmanifest‎

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‎physics/box2d_motor_joint/example.md‎

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---
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tags: physics, box2d
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title: Box2D Motor Joint
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brief: Create and control a motorized Box2D joint from script using Box2D V2 and V3.
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author: Defold Foundation
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scripts: box2d_motor_joint_v3.script, box2d_motor_joint_v2.script
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thumbnail: thumbnail.webp
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---
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This example creates a motorized Box2D joint at runtime. It works with both Box2D V2 and V3 by attaching one script for each backend. Each script checks `b2d.get_version()` during `init()` and becomes a no-op when the other backend is active.
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Click or tap the window to reverse the motor direction.
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## What You'll Learn
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- How to get Box2D body handles from Defold collision objects
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- How to detect the active Box2D version with `b2d.get_version()`
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- How to create a pivoted motor joint with `b2d.joint.create_revolute()`
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- How to control joint motor speed with `b2d.joint.set_motor_speed()`
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- How to tune the Box2D V3 joint solver with `b2d.world.set_joint_tuning()`
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## Setup
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The collection contains a static `pivot` game object, one dynamic `arm` game object with a particle effect, and a controller game object.
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The `pivot` object marks the world point the arm should rotate around. The `arm` object is the dynamic body driven by the joint. The example uses a revolute joint with its motor enabled, because the revolute joint provides a stable hinge pivot while the motor drives the rotation.
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The controller has both backend scripts attached. Each script checks `b2d.get_version()` and only runs when the selected app manifest matches its Box2D backend.
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![setup](setup.png)
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The `game.project` of this example is configured to build with `/box2D_V3.appmanifest` by default. To test V2 locally after downloading the example, change `Native Extensions -> App Manifest` in `game.project` to `/box2D_V2.appmanifest`.
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![game_project](game_project.png)
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## How It Works
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Both scripts read `b2d.get_version()` once. `box2d_motor_joint_v2.script` only continues when the major version is 2, while `box2d_motor_joint_v3.script` only continues when the major version is 3.
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`b2d.get_body()` returns the Box2D bodies owned by the `pivot` and `arm` collision objects. The active script then creates a revolute joint between those bodies with `b2d.joint.create_revolute()`.
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The joint definition uses:
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- `local_anchor_a` on the pivot body
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- `local_anchor_b` on the arm body
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- `enable_motor` to turn on the joint motor
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- `max_motor_torque` to limit how strongly the motor can rotate the arm
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- `motor_speed` to set the current motor direction and speed
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The important part is the arm anchor. The pivot is placed in the collection as a visible reference point. The scripts convert that world position into the arm's local space, so the arm rotates around the same visible pivot instead of rotating around its center.
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Click or tap to reverse the motor with `b2d.joint.set_motor_speed()`. The V3 script also calls `b2d.world.set_joint_tuning()` to adjust the joint solver used by the Box2D V3 backend.

‎physics/box2d_motor_joint/example/arm.go‎

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components {
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id: "trail"
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component: "/particlefx/trail.particlefx"
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position {
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x: 90.0
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z: -0.1
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}
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}
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components {
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id: "play"
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component: "/particlefx/play.script"
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}
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embedded_components {
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id: "sprite"
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type: "sprite"
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data: "default_animation: \"pixel_blue\"\n"
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"material: \"/builtins/materials/sprite.material\"\n"
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"size {\n"
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" x: 200.0\n"
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" y: 40.0\n"
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"}\n"
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"size_mode: SIZE_MODE_MANUAL\n"
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"textures {\n"
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" sampler: \"texture_sampler\"\n"
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" texture: \"/textures/textures.atlas\"\n"
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"}\n"
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""
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}
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embedded_components {
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id: "collisionobject"
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type: "collisionobject"
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data: "type: COLLISION_OBJECT_TYPE_DYNAMIC\n"
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"mass: 3.0\n"
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"friction: 0.4\n"
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"restitution: 0.0\n"
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"group: \"default\"\n"
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"mask: \"default\"\n"
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"embedded_collision_shape {\n"
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" shapes {\n"
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" shape_type: TYPE_BOX\n"
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" position {\n"
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" }\n"
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" rotation {\n"
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" }\n"
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" index: 0\n"
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" count: 3\n"
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" }\n"
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" data: 100.0\n"
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" data: 20.0\n"
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" data: 10.0\n"
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"}\n"
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"linear_damping: 0.05\n"
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"angular_damping: 0.2\n"
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""
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}

‎physics/box2d_motor_joint/example/box2d_motor_joint.collection‎

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name: "box2d_motor_joint"
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instances {
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id: "pivot"
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prototype: "/example/pivot.go"
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position {
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x: 360.0
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y: 360.0
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z: 0.5
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}
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}
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instances {
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id: "arm"
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prototype: "/example/arm.go"
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position {
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x: 480.0
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y: 360.0
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}
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}
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scale_along_z: 0
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embedded_instances {
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id: "controller"
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data: "components {\n"
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" id: \"box2d_v2\"\n"
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" component: \"/example/box2d_motor_joint_v2.script\"\n"
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"}\n"
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"components {\n"
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" id: \"box2d_v3\"\n"
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" component: \"/example/box2d_motor_joint_v3.script\"\n"
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"}\n"
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"embedded_components {\n"
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" id: \"label\"\n"
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" type: \"label\"\n"
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" data: \"size {\\n"
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" x: 640.0\\n"
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" y: 96.0\\n"
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"}\\n"
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"text: \\\"Box2D motor joint\\\"\\n"
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"font: \\\"/fonts/text28.font\\\"\\n"
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"material: \\\"/builtins/fonts/label-df.material\\\"\\n"
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"\"\n"
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" position {\n"
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" x: 360.0\n"
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" y: 60.0\n"
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" }\n"
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"}\n"
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""
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}

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