# Windmill A motored blade cross batting balls through swing gates that springs hold shut. Space cuts the power, and friction slows it to a stop. Shows .compound bodies, drive(at:)/drive(to:), limits, and softenLimits. Run it with `swift run --package-path Examples Example-3D-Physics-Windmill`. ```swift import Ollin import OllinPhysics /// A windmill under power: a motored hinge spins the blade cross, balls drop /// into the sweep and get batted out through spring-shut swing gates. **Drag** /// a ball to feed the blades; **space** cuts the power and hinge friction /// coasts the mill to a stop. /// /// The motor showcase for `World3D`: the blade cross is one `.compound` body /// (hub plus four blades, posed as parts), `drive(at:)` turns it at a steady /// rate, each gate is a limited `.revolute` whose `drive(to: 0)` closer pulls /// it shut, `softenLimits` makes the stops bouncy, and `friction` is what /// winds the mill down when the power is off. @main final class Windmill: Sketch { let world = World3D() var mill: Joint3D? var balls: [Body3D] = [] var restFrames: [Int] = [] var powered = true @Param(0 ... 5, icon: "wind") var millSpeed = 2.5 let hubCenter = Vector3(0, 3, 0) let palette: [Color] = [ Color(hex: 0xE8632F), Color(hex: 0xF2A93B), Color(hex: 0xD94F70), Color(hex: 0x8D92E0), ] override func setup() { world.ground = 0 world.restitution = 0.3 buildMill() buildGate(at: 3.4) buildGate(at: -3.4) for index in 0 ..< 6 { dropBall(index) } } /// The tower behind the sweep, and the blade cross as ONE body: a compound /// of the hub cylinder and four blades, each a part posed in the cross's /// local space, hung on one motored hinge. Mass, balance, and inertia come /// from the whole assembly, and a part can never drift off its pose. func buildMill() { let tower = world.addBody(.box(width: 0.4, height: 2.9, depth: 0.3), at: Vector3(0, 1.45, -0.5), kind: .static) tower.userData = Color(hex: 0x6B5A48) // The hub faces the camera: a cylinder is upright, so lay its axis // along z with a quarter turn about x. Denser metal than the timber // blades, via the part's own density. var parts: [Collider3D.Part] = [ .part(.cylinder(height: 0.2, radius: 0.32), rotated: .pi / 2, axis: .unitX, density: 2), ] for arm in 0 ..< 4 { let angle = Double(arm) * .pi / 2 let direction = Vector3(cos(angle), sin(angle), 0) parts.append(.part(.box(width: 1.5, height: 0.26, depth: 0.08), at: direction * 1.11, rotated: angle, axis: .unitZ)) } let cross = world.addBody(.compound(parts), at: hubCenter) cross.userData = Color(hex: 0xE8DCC5) let hinge = world.connect(tower, cross, .revolute(at: hubCenter, axis: .unitZ)) hinge.friction = 80 // the brake that coasts it down when unpowered mill = hinge } /// A swing gate: a panel on a vertical limited hinge, pulled shut by a /// gentle position motor a rolling ball can still barge through. func buildGate(at x: Double) { let post = world.addBody(.box(width: 0.14, height: 1.15, depth: 0.14), at: Vector3(x, 0.575, -1.0), kind: .static) post.userData = Color(hex: 0x3A4150) let panel = world.addBody(.box(width: 0.08, height: 1.0, depth: 1.75), at: Vector3(x, 0.5, 0), friction: 0.3) panel.userData = Color(hex: 0x72BFB2) let gate = world.connect(post, panel, .revolute(at: Vector3(x, 0.5, -0.875), axis: .unitY, limits: -1.75 ... 1.75)) gate.softenLimits(frequency: 3, damping: 0.5) // bouncy end stops gate.friction = 0.5 gate.drive(to: 0, frequency: 1.2, strength: 60) // the door closer } /// Feed a ball into the sweep from above, staggered so they don't arrive /// as one clump. func dropBall(_ index: Int) { let ball = world.addBody(.sphere(radius: 0.21), at: Vector3(random(-1.2, 1.2), 5 + Double(index) * 0.9, random(-0.04, 0.04)), density: 1.5, friction: 0.4, restitution: 0.35) ball.userData = palette[index % palette.count] balls.append(ball) restFrames.append(0) } /// A ball batted past the gates comes back around for another pass. func recycle(_ ball: Body3D) { ball.position = Vector3(random(-1.2, 1.2), 5 + random(0, 1), random(-0.04, 0.04)) ball.velocity = .zero ball.angularVelocity = .zero } override func keyPressed() { if key == " " { powered.toggle() if !powered { mill?.stopMotor() } } } override func draw() { background(Color(hex: 0x0C0F15)) environment(.courtyard.lightingOnly()) lightingPreset(.studio) castShadows() perspective(eye: Vector3(-4.2, 3.8, 8.8), target: Vector3(0, 2.2, 0)) // Re-asserting the rate each frame keeps the parameter live and the mill // awake; the strength cap gives it a mechanical spin-up. if powered { mill?.drive(at: millSpeed, strength: 500) } dragBodies(in: world) world.advance(by: deltaTime) // Batted past the gates, or parked out of the blades' reach for a // couple of seconds: either way, back into the drop. for (index, ball) in balls.enumerated() { restFrames[index] = ball.velocity.length < 0.08 ? restFrames[index] + 1 : 0 if abs(ball.position.x) > 6.2 || abs(ball.position.z) > 4 || restFrames[index] > 150 { recycle(ball) restFrames[index] = 0 } } fill(Color(hex: 0x1A202A)) material(.dielectric(roughness: 0.85)) drawGround(size: 24) for body in world.bodies { withBody(body) { drawCollider(body.collider, tint: body.userData as? Color) } } } /// Draw a collider's matching mesh; a compound recurses into its parts at /// their local poses, so the drawn cross follows the one simulated body. func drawCollider(_ collider: Collider3D, tint: Color?) { switch collider { case .box(let w, let h, let d): fill(tint ?? .white) material(.dielectric(roughness: 0.6)) drawBox(width: w, height: h, depth: d) case .cylinder(let h, let r): fill(Color(hex: 0xB8C0CC)) material(.metal(roughness: 0.35)) drawCylinder(radius: r, height: h) case .sphere(let r): fill(tint ?? .white) material(.dielectric(roughness: 0.3)) drawSphere(radius: r) case .compound(let parts): for part in parts { withState { translate(part.position) if part.angle != 0 { rotate(part.angle, axis: part.axis) } drawCollider(part.collider, tint: tint) } } default: break } } } ```