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Added prismatic joint example.
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‎physics/box2d_prismatic_joint/all.texture_profiles‎

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‎physics/box2d_prismatic_joint/box2d_v2.appmanifest‎

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‎physics/box2d_prismatic_joint/box2d_v3.appmanifest‎

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‎physics/box2d_prismatic_joint/example.md‎

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---
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tags: physics, box2d
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title: Box2D Prismatic Joint
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brief: Create a motorized Box2D prismatic joint with translation limits from script using Box2D.
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author: Defold Foundation
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scripts: box2d_prismatic_joint.script
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thumbnail: thumbnail.webp
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---
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This example creates a Box2D prismatic joint at runtime.
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The blue dynamic body (slider) is constrained to a diagonal rail,
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moves between two translation limits, and reverses its motor when it reaches either end.
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Click or tap the window to reverse the motor manually.
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There are no differences in scripting between Box2D V2 and V3 in this example.
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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 create a prismatic joint with `b2d.joint.create_prismatic()`
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- How `local_axis_a`, `lower_translation`, and `upper_translation` define the slide rail
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- How to control a prismatic joint motor with `b2d.joint.set_motor_speed()`
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## Setup
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The collection contains:
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- `controller` game object with the main script and labels with informations
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- `rail` game object with static collision object for frame reference for the prismatic joint and a sprite rotated by 45 degrees around Z axis to indicate the "rail"
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- `slider` game object with a dynamic collision object and a sprite to indicate where the slider is
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The `rail` and `slider` bodies start at the same world position in the middle.
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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.
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To test V2 locally after downloading the example, change `Native Extensions -> App Manifest` in `game.project` to `/box2d_v2.appmanifest`.
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## How It Works
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The script uses `b2d.get_body()` to fetch the Box2D bodies owned by the `rail` and `slider` collision objects.
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Then calls `b2d.joint.create_prismatic()` which creates the prismatic joint between them,
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with `local_axis_a` set to the same diagonal direction as the visible rail (rotated 45 degrees areound Z axis),
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enables limits from `-110` to `110` project units, and a motor, sets a maximum motor force, and starts the slider moving along the rail.
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The prismatic joint is a constraint solver - only defines *how* two bodies are allowed to move, not *why* they should move.
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So normally, the slider would fall in the gravity direction, but would be moving constrained like a slider along a rail.
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We enable the motor in the example to move in both direction along the rail.
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Motor applied on a prismatic joint moves the object along the joint axis at the given linear speed.
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During `update()`, the script reads `b2d.joint.get_joint_translation()`, which returns the position alongside the constraint,
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and reverses the motor when the translation reaches either limit.
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On input (touch or mouse click) the direction of the motor that applies force to the slider changes too.
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