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paweljaroszpaweljarosz
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Added revolute joint example (#202)
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‎physics/box2d_revolute_bridge/all.texture_profiles‎

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‎physics/box2d_revolute_bridge/box2d_v2.appmanifest‎

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‎physics/box2d_revolute_bridge/box2d_v3.appmanifest‎

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‎physics/box2d_revolute_bridge/example.md‎

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---
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tags: physics, box2d
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title: Box2D Revolute Bridge
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brief: Create a bridge from revolute joints between existing Defold collision objects.
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author: Defold Foundation
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scripts: box2d_revolute_bridge_v3.script, box2d_revolute_bridge_v2.script
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thumbnail: thumbnail.webp
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---
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This example builds a simple rope bridge from rectangular dynamic bodies connected with Box2D revolute joints.
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A ball drops onto the bridge so the hinge chain bends under load.
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Click or tap the window to reset the ball.
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## What You'll Learn
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- How to get Box2D bodies from Defold collision objects with `b2d.get_body()`
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- How to connect two bodies with a `revolute` type joint
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- How local joint anchors define hinge points on each connected body
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## Setup
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The collection contains these game objects:
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`controller`
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: Contains both backend scripts, `box2d_revolute_bridge_v2.script` and `box2d_revolute_bridge_v3.script`, plus the title and hint labels. Each script checks the active Box2D version and only one script runs.
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`left_anchor` and `right_anchor`
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: Static collision objects used as the fixed bridge supports.
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`segment_01` to `segment_10`
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: Dynamic rectangular collision objects. The active script creates revolute joints between neighboring segments.
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`ball`
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: A dynamic circle collision object that falls onto the bridge to show how the joints move.
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`floor`, `left_wall`, and `right_wall`
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: Static collision objects that keep the simulation in view after the ball falls through or off the bridge.
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The project uses `box2d_v3.appmanifest` by default, so it runs with Box2D V3.
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To use the older legacy V2 backend, switch the native extension app manifest in `game.project` to `/box2d_v2.appmanifest`.
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![setup](setup.png)
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## How It Works
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Both scripts use the same collection-authored bodies. `b2d.get_body()` returns the Box2D body owned by each collision object, and `b2d.joint.create_revolute()` creates the hinge constraints at runtime.
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Each bridge segment is 48 pixels wide. The script connects the right edge of one body to the left edge of the next body by using local anchors: `(24, 0, 0)` on the previous segment and `(-24, 0, 0)` on the current segment.
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The first and last joints connect to static anchors at the bridge supports.
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On reset, the script moves the ball up and drops it onto the bridge again. The joints are created once in `init()` and destroyed in `final()`.

‎physics/box2d_revolute_bridge/example/anchor.go‎

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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_gold\"\n"
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"material: \"/builtins/materials/sprite.material\"\n"
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"size {\n"
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" x: 22.0\n"
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" y: 22.0\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_STATIC\n"
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"mass: 0.0\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: 11.0\n"
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" data: 11.0\n"
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" data: 10.0\n"
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"}\n"
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""
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}

‎physics/box2d_revolute_bridge/example/ball.go‎

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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: \"ball\"\n"
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"material: \"/builtins/materials/sprite.material\"\n"
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"size {\n"
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" x: 254.0\n"
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" y: 254.0\n"
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"}\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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scale {
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x: 0.5
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y: 0.5
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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: 1.0\n"
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"friction: 0.35\n"
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"restitution: 0.2\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_SPHERE\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: 1\n"
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" }\n"
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" data: 32.0\n"
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"}\n"
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""
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}

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