MovementSystem

The MovementSystem is the physics integrator. It applies velocity to position over time. It is the fundamental engine that makes things move in the game world.

📦 Required Components

Entities processed by this system must have: 1. Transform: The current position $(x, y)$. 2. Velocity: The direction and speed vector $(v_x, v_y)$.

⚙️ Logic & Algorithm

The system uses explicit Euler integration. It also applies a global ScaleFactor retrieved from the GameEngine to ensure movement speed remains consistent regardless of window resizing or resolution scaling.

Formula

$$ P_{new} = P_{old} + (V \times dt \times S) $$

Code reference

src/game/src/systems/MovementSystem.cpp

void MovementSystem::onUpdate(float dt) {
    auto& positions = _engine.getComponents<Transform>();
    auto& velocities = _engine.getComponents<Velocity>();
    float scaleFactor = _engine.getScaleFactor();
    for (size_t e : _entities) {
        if (!positions[e] || !velocities[e]) continue;
        auto& pos = positions[e].value();
        auto& vel = velocities[e].value();
        pos.x += vel.vx * scaleFactor * dt;
        pos.y += vel.vy * scaleFactor * dt;
    }
}

Where: * $P$: Position (Transform.x, Transform.y) * $V$: Velocity (Velocity.vx, Velocity.vy) * $dt$: Delta Time (Time elapsed since last frame) * $S$: Scale Factor (Global window scaling)

Usage

Simply add Velocity to any entity with a Transform.

// Example: Moving a bullet
Entity bullet = gameEngine.createEntity("Bullet");
gameEngine.addComponent(bullet, Transform(x, y, BULLET_ROT, BULLET_SCALE));
// Move 500 pixels/sec to the right
gameEngine.addComponent(bullet, Velocity(0.f, 0.f));