Box2D.NET Physics

A 2D simulation run by Box2D.NET rather than by Stride's own physics, with Stride reduced to drawing the result. That split is the whole lesson: the physics world steps on a fixed timestep of its own, and entity transforms are copied from body poses afterwards, so nothing in the scene graph is allowed to move a body directly. The same pattern applies to any external simulation you want to drive a Stride scene with.

The Program.cs file shows how to:

Hosting an external physics world alongside Stride
Stepping a simulation on a fixed timestep, independent of frame rate
Creating dynamic and static bodies in Box2D
Copying body poses onto entity transforms each frame
Why the scene graph must not write back to the simulation
Requires the Box2D.NET NuGet package

Note

This example references Stride.CommunityToolkit.Box2D, which is not on NuGet yet. Run it from a clone of the repository, where the package is a project reference; a copy of the project on its own will not build.

Box2D.NET Physics

View on GitHub.

using E06_Box2D.Helpers;
using Stride.CommunityToolkit.Box2D;
using Stride.CommunityToolkit.Engine;
using Stride.CommunityToolkit.Shapes;
using Stride.Core.Mathematics;
using Stride.Engine;
using Stride.Games;

// Box2D.NET in Stride: a 2D simulation with shapes, collisions and interactive controls

// Per-monitor DPI awareness, so a 150% display gets a sharp window rather than a stretched one;
// it has to happen before the window exists
WindowsDpiManager.EnablePerMonitorV2();

Box2DSimulation? simulation = null;
SceneManager? sceneManager = null;

using var game = new Game();

var sceneSetup = new SceneSetup2D
{
    ClearColor = new Color(0.2f),
    Ground = null,
    Profiler = true
};

game.Run(sceneSetup, start: Start, update: Update);

void Start(Scene rootScene)
{
    game.Window.AllowUserResizing = true;
    game.Window.Title = "Box2D Physics Example - Stride Community Toolkit";

    // Shapes render through the toolkit's Box2D debug draw - testbed-style fill and pixel-constant
    // borders from an SDF shader, replacing the old per-mesh outline render features
    var shapeBatch = game.AddShapeBatch();
    shapeBatch.BorderWidth = 1.5f;
    shapeBatch.Fill.Alpha = 0.4f;
    // Neon tubes: a bloom outside every border in the border's own colour, adding light rather than
    // covering. A third of the strength at the edge is what reads as light around the stroke; full
    // strength would just look like a fatter border
    shapeBatch.Glow.Set(10f);
    shapeBatch.Glow.Strength = 0.35f;
    shapeBatch.Glow.Additive = true;

    simulation = new Box2DSimulation();
    ConfigurePhysicsWorld();

    // Left mouse picks any shape up and throws it; a click on empty space still spawns one
    game.GetCameraEntity().Add(new Grabber2DScript { Simulation = simulation });

    sceneManager = new SceneManager(game, rootScene, simulation);
    sceneManager.Initialize();
}

void Update(Scene rootScene, GameTime time)
{
    simulation?.Update(time.Elapsed);

    sceneManager?.Update(time);
}

void ConfigurePhysicsWorld()
{
    // Negative Y is down
    simulation.Gravity = new Vector2(0f, GameConfig.Gravity);

    simulation.EnableContactEvents = true;
    simulation.EnableSensorEvents = true;

    simulation.TimeScale = 1.0f;
    simulation.MaxStepsPerFrame = 3;
}

Table of Contents

Box2D.NET Physics

Note