HillLike/World.cs

using osu.Framework.Graphics;
using osu.Framework.Graphics.Containers;
using osu.Framework.Graphics.Primitives;
using osu.Framework.Graphics.Shapes;
using osu.Framework.Input.Events;
using osuTK;
using osuTK.Graphics;
using osuTK.Input;
using Triangle = osu.Framework.Graphics.Shapes.Triangle;

namespace HillLike;

public class World : CompositeDrawable
{
    private readonly Camera _camera = new();
    private readonly CarDrawable _carDrawable;
    private readonly TerrainDrawable _terrainDrawable;

    private readonly Container _worldLayer;
    public readonly SimpleCar Car = new();

    public World()
    {
        RelativeSizeAxes = Axes.Both;

        InternalChildren =
        [
            new Box
            {
                RelativeSizeAxes = Axes.Both,
                Colour = new Color4(150, 210, 255, 255)
            },
            _worldLayer = new Container
            {
                RelativeSizeAxes = Axes.Both,
                Children =
                [
                    _terrainDrawable = new TerrainDrawable(),
                    _carDrawable = new CarDrawable(Car)
                ]
            }
        ];

        Reset();
    }

    public sealed override Axes RelativeSizeAxes
    {
        get => base.RelativeSizeAxes;
        set => base.RelativeSizeAxes = value;
    }

    public void Reset()
    {
        Car.Reset(new Vector2(0, Terrain.HeightAt(0) + 3f));
        _camera.Reset(Car.Position);
    }

    public void Step(float dt)
    {
        dt = Math.Clamp(dt, 0, 1 / 20f);
        Car.Step(dt);
        _camera.Follow(dt, Car.Position);

        var pixelsPerUnit = _camera.PixelsPerUnit;
        var viewCenter = DrawSize / 2;

        _terrainDrawable.SetCamera(_camera, viewCenter);
        _carDrawable.SetCamera(_camera, viewCenter);
    }

    public void HandleKeyDown(KeyDownEvent e)
    {
        Car.HandleKeyDown(e);
        if (e.Key is Key.R)
            Reset();
    }

    public void HandleKeyUp(KeyUpEvent e)
    {
        Car.HandleKeyUp(e);
    }


    private class Camera
    {
        public readonly float PixelsPerUnit = 60f;
        public Vector2 Position;

        public void Reset(Vector2 pos)
        {
            Position = pos;
        }

        public void Follow(float dt, Vector2 target)
        {
            var desired = target + new Vector2(5f, 2f);
            var smooth = 1f - MathF.Exp(-dt * 4.5f);
            Position = Vector2.Lerp(Position, desired, smooth);
        }
    }

    private abstract class WorldDrawable : CompositeDrawable
    {
        protected Camera? Camera;
        protected Vector2 ViewCenter;

        public void SetCamera(Camera cam, Vector2 viewCenter)
        {
            Camera = cam;
            ViewCenter = viewCenter;
            UpdateFromWorld();
        }

        protected Vector2 ToScreen(Vector2 worldPos)
        {
            var rel = worldPos - Camera.Position;
            return new Vector2(
                ViewCenter.X + rel.X * Camera.PixelsPerUnit,
                ViewCenter.Y - rel.Y * Camera.PixelsPerUnit
            );
        }

        protected float ToScreen(float worldLen)
        {
            return worldLen * Camera.PixelsPerUnit;
        }

        protected abstract void UpdateFromWorld();
    }

    private class TerrainDrawable : WorldDrawable
    {
        private readonly Container _segments = new() { RelativeSizeAxes = Axes.Both };
        public TerrainDrawable()
        {
            RelativeSizeAxes = Axes.Both;
            AddInternal(_segments);
        }

        protected override void UpdateFromWorld()
        {
            if (Camera is null) return;

            _segments.Clear();

            var halfWidthWorld = DrawWidth / Camera.PixelsPerUnit;
            var left = Camera.Position.X - halfWidthWorld - 2f;
            var right = Camera.Position.X + halfWidthWorld + 2f;

            var step = 0.6f;
            var count = (int)Math.Ceiling((right - left) / step);

            for (var i = 0; i < count; i += 1)
            {
                var x0 = left + i * step;
                var x1 = x0 + step;

                var y0 = Terrain.HeightAt(x0);
                var y1 = Terrain.HeightAt(x1);
                var y = MathF.Min(y0, y1);

                var depth = 200f;
                var p = ToScreen(new Vector2(x0, y));
                var w = ToScreen(step);
                var h = ToScreen(depth);

                //_segments.Add(new Box
                //{
                //    Anchor = Anchor.TopLeft,
                //    Origin = Anchor.TopLeft,
                //    Position = p,
                //    Size = new Vector2(w + 1, h),
                //    Colour = new Color4(80, 200, 120, 255)
                //});

                _segments.Add(new Triangle
                {
                    Anchor = Anchor.TopLeft,
                    Origin = Anchor.BottomLeft,
                    Position = p,
                    Size = new Vector2(w + 1, -h),
                    Shear = new Vector2(x1 - x0, y1 - y0),
                    Colour = new Color4(80, 200, 120, 255)
                });
            }

            for (var i = 0; i < count; i += 1)
            {
                var x0 = left + i * step;
                var x1 = x0 + step;

                var y0 = Terrain.HeightAt(x0);
                var y1 = Terrain.HeightAt(x1);
                var y = (y0 + y1) / 2f;

                var p = ToScreen(new Vector2(x0, y));
                _segments.Add(new Box
                {
                    Anchor = Anchor.TopLeft,
                    Origin = Anchor.TopLeft,
                    Position = p,
                    Size = new Vector2(ToScreen(step) + 1, 3),
                    Colour = new Color4(20, 120, 60, 255)
                });
            }
        }
    }

    private class CarDrawable : WorldDrawable
    {
        private readonly Box _body;
        private readonly SimpleCar _car;
        private readonly Circle _wheelBack;
        private readonly Circle _wheelFront;

        public CarDrawable(SimpleCar car)
        {
            _car = car;

            RelativeSizeAxes = Axes.Both;

            InternalChildren = new Drawable[]
            {
                _wheelBack = new Circle { Colour = new Color4(25, 25, 25, 255) },
                _wheelFront = new Circle { Colour = new Color4(25, 25, 25, 255) },
                _body = new Box { Colour = new Color4(240, 70, 70, 255) }
            };
        }

        protected override void UpdateFromWorld()
        {
            if (Camera is null) return;

            var wheelR = SimpleCar.WheelRadius;
            var bodyW = 2.2f;
            var bodyH = 0.7f;

            var wb = ToScreen(_car.WheelBackPos);
            var wf = ToScreen(_car.WheelFrontPos);

            var rPx = ToScreen(wheelR);
            _wheelBack.Size = new Vector2(rPx * 2);
            _wheelBack.Position = wb - new Vector2(rPx);
            _wheelFront.Size = new Vector2(rPx * 2);
            _wheelFront.Position = wf - new Vector2(rPx);

            var bp = ToScreen(_car.Position);
            _body.Size = new Vector2(ToScreen(bodyW), ToScreen(bodyH));
            _body.Origin = Anchor.Centre;
            _body.Anchor = Anchor.TopLeft;
            _body.Position = bp;
            _body.Rotation = -_car.RotationDegrees;
        }
    }
}

public class Terrain
{
    public static float HeightAt(float x)
    {
        const float baseLine = 0f;

        var h =
            0.9f * MathF.Sin(x * 0.55f) +
            0.6f * MathF.Sin(x * 0.18f + 1.3f) +
            0.25f * MathF.Sin(x * 1.15f);

        var drift = 0.015f * x;

        return baseLine + h + drift;
    }

    public static float SlopeAt(float x)
    {
        const float eps = 0.02f;
        return (HeightAt(x + eps) - HeightAt(x - eps)) / (2 * eps);
    }

    public static Vector2 NormalAt(float x)
    {
        var slope = SlopeAt(x);
        var n = new Vector2(-slope, 1f);
        return n.Normalized();
    }
}