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paweljaroszpaweljarosz
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212 box2d dynamic scale (#221)
* Added more comments in the weld joints example. * Added dynamic scale example.
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‎physics/box2d_dynamic_scale/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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os: OS_ID_GENERIC
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formats {
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format: TEXTURE_FORMAT_RGBA
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compression_level: BEST
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‎physics/box2d_dynamic_scale/box2d_v2.appmanifest‎

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‎physics/box2d_dynamic_scale/box2d_v3.appmanifest‎

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‎physics/box2d_dynamic_scale/example.md‎

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---
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tags: physics, box2d
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title: Box2D dynamic shape scale and mass
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brief: Resize dynamic Box2D collision shapes at runtime and recalculate body mass.
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author: Defold Foundation
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scripts: box2d_dynamic_scale_v3.script, box2d_dynamic_scale_v2.script
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thumbnail: thumbnail.webp
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---
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This example shows how to change bodies shapes and mass in runtime.
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It includes 4 independent dynamic Box2D bodies.
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Click or tap to give every square body a new random size,
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update its collision shape, recalculate its mass, and apply a random impulse.
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## What You'll Learn
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- How to get a Box2D body from a Defold collision object
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- How to detect the active Box2D version with `b2d.get_version()`
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- How to resize collision geometry through the V2 fixture API
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- How to resize collision geometry through the V3 shape API
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- How `update_mass = true` makes body mass follow the new collision size
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- How to apply a dynamic impulse to the body
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## Setup
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The collection contains four instances of `block.go` game object.
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The `block.go` prototype contains:
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- `script_v3`, which runs only with the Box2D V3 backend
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- `script_v2`, which runs only with the Box2D V2 backend
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- a square sprite
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- a dynamic box collision object with base half-extents of `40.0`
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The collection also contains `walls` game object with:
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- one static collision object with four box shapes around the play area
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- a label that explains the click or tap interaction.
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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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Both scripts read `b2d.get_version()` in `init()` and are only active if a matching version is used -
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`box2d_dynamic_scale_v2.script` only continues when the major version is 2,
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while `box2d_dynamic_scale_v3.script` only continues when the major version is 3.
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The active script acquires input focus and initialized a random number generator.
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When the built-in `touch` action is pressed, each block picks a random uniform scale from `0.5` to `1.5`.
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The script multiplies the base sprite size and base collision half-extent by that scale,
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so the visible square and physics box stay aligned.
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The mass changes because `update_mass = true` makes Box2D recalculate the body mass from the resized shape.
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In Box2D V2, collision geometry is attached to a body through fixtures.
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The V2 script reads the first fixture with `b2d.body.get_fixtures()`
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and resizes it with `b2d.fixture.set_shape(body, fixture.index, shape, true)`.
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In Box2D V3, collision geometry is attached through shapes.
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The V3 script reads the first shape with `b2d.body.get_shapes()`
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and resizes it with `b2d.shape.set_shape(shape.shape_id, shape, true)`.
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After resizing, the active script applies a random linear impulse at the body's world center.
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Since each body has just recalculated its mass from its new collision size,
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the resulting movement makes the size and mass changes visible.

‎physics/box2d_dynamic_scale/example/block.go‎

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components {
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id: "script_v3"
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component: "/example/box2d_dynamic_scale_v3.script"
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}
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components {
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id: "script_v2"
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component: "/example/box2d_dynamic_scale_v2.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: \"body_soft\"\n"
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"material: \"/builtins/materials/sprite.material\"\n"
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"size {\n"
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" x: 80.0\n"
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" y: 80.0\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: 1.0\n"
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"friction: 0.2\n"
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"restitution: 0.2\n"
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"group: \"block\"\n"
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"mask: \"block\"\n"
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"mask: \"wall\"\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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" id: \"box\"\n"
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" }\n"
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" data: 40.0\n"
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" data: 40.0\n"
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" data: 10.0\n"
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"}\n"
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"linear_damping: 0.2\n"
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"angular_damping: 0.1\n"
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""
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}

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