more stuff, ok?

js/Level.js

 		this.ctx = canvas.getContext("2d");
 		this.lastTime = null;
 		this.raf = null;
-		this.minFPS = 10;
+		this.minPhysFPS = 30;
+		this.minFPS = 5;
 		this.backgroundColor = "#87CEFA";
 
 		this.entities = [];
 		if (this.lastTime != null) {
 			let dt = (time - this.lastTime) / 1000;
 
-			if (1 / dt > this.minFPS) {
-				this.physics(dt);
-			} else {
+			if (1 / dt < this.minFPS) {
 				console.log(
 					"Too long between updates ("+dt.toFixed(2)+"s, "+
 					(1 / dt).toFixed(2)+"FPS). Skipping.");
+			} else {
+
+				// We accept really low FPSes,
+				// but if the FPS gets too low, we want to run multiple
+				// physics steps in the frame.
+				let nSteps = 1;
+				while (1 / dt < this.minPhysFPS) {
+					dt /= 2;
+					nSteps *= 2;
+				}
+
+				if (nSteps != 1)
+					console.log(nSteps);
+
+				for (let i = 0; i < nSteps; ++i)
+					this.physics(dt);
 			}
 
 			this.draw();

js/Tile.js

 	}
 }
 
-Tile.createLine = function(width, name) {
+Tile.createLine = function(width, name, x = 0, y = 0) {
 	if (width <= 1) {
-		return [ new Tile(0, 0, name+"-lr") ];
+		return [ new Tile(x, y, name+"-lr") ];
 	} else {
 		return [
-			new Tile(0, 0, name+"-l"),
+			new Tile(x, y, name+"-l"),
 			Array.from({ length: width - 2 }, (_, i) =>
-				new Tile(i + 1, 0, name)),
-			new Tile(width - 1, 0, name+"-r"),
+				new Tile(x + i + 1, y, name)),
+			new Tile(x + width - 1, y, name+"-r"),
+		];
+	}
+}
+
+Tile.createBox = function(width, height, nameTop, nameMid, nameBottom) {
+	if (height <= 1) {
+		return Tile.createLine(width, nameTop);
+	} else if (height <= 2) {
+		return [
+			Tile.createLine(width, nameTop),
+			Tile.createLine(width, nameBottom),
+		];
+	} else {
+		return [
+			Tile.createLine(width, nameTop),
+			Array.from({ length: height - 2 }, (_, i) =>
+				Tile.createLine(width, nameMid, 0, i + 1)),
+			Tile.createLine(width, nameBottom, 0, height - 1),
 		];
 	}
 }

js/assets.js

 export default {
 	tiles: new SpriteSheet("assets/tiles.png", 32, 32, 1)
 		.defineTile("ground", 0, 0)
+		.defineTile("ground-l", 1, 0)
+		.defineTile("ground-r", 2, 0)
+		.defineTile("ground-lr", 3, 0)
 
-		.defineTile("grass", 0, 1)
-		.defineTile("grass-l", 1, 1)
-		.defineTile("grass-r", 2, 1)
-		.defineTile("grass-lr", 3, 1)
+		.defineTile("ground-top", 0, 1)
+		.defineTile("ground-top-l", 1, 1)
+		.defineTile("ground-top-r", 2, 1)
+		.defineTile("ground-top-lr", 3, 1)
 
 		.defineTile("platform", 0, 2)
 		.defineTile("platform-l", 1, 2)

js/entities/FallingPlatform.js

+import Entity from "../Entity.js";
+import Texture from "../Texture.js";
+import Tile from "../Tile.js";
+import assets from "../assets.js";
+
+import TPlatform from "../traits/TPlatform.js";
+import TCollider from "../traits/TCollider.js";
+import TPhysics from "../traits/TPhysics.js";
+
+export default class Platform extends Entity {
+	constructor(level, width = 5) {
+		super(level);
+
+		this.bounds.size.set(width, 0.5);
+		this.addTrait(new TCollider(this));
+		this.addTrait(new TPlatform(this));
+		this.addTrait(new TPhysics(this));
+
+		this.texture = new Texture(assets.tiles,
+			Tile.createLine(this.bounds.size.x, "platform"));
+	}
+
+	draw(ctx) {
+		this.texture.drawAt(ctx, this.bounds.pos);
+	}
+}

js/entities/Platform.js → js/entities/FloatingPlatform.js

 import TPhysics from "../traits/TPhysics.js";
 
 export default class Platform extends Entity {
-	constructor(level) {
+	constructor(level, width = 5) {
 		super(level);
 
-		this.bounds.size.set(5, 0.5);
+		this.bounds.size.set(width, 0.5);
 		this.addTrait(new TCollider(this));
 		this.addTrait(new TPlatform(this));
 		this.addTrait(new TPhysics(this));
 		this.targetSpeed = 3;
 		this.accel = 5;
 		this.dir = 1;
+		this.movement = 5;
 
 		this.texture = new Texture(assets.tiles,
 			Tile.createLine(this.bounds.size.x, "platform"));
 
 	init() {
 		this.targetTop = this.bounds.pos.y;
-		this.targetBottom = this.targetTop + 5;
+		this.targetBottom = this.targetTop + this.movement;
 
 		this.t.physics.gravity = 0;
 		this.t.physics.velocity.y = 0;

js/main.js

 import Level from "./Level.js";
+import Vec2 from "./Vec2.js";
+
 import Player from "./entities/Player.js";
-import Platform from "./entities/Platform.js";
+import FloatingPlatform from "./entities/FloatingPlatform.js";
+import FallingPlatform from "./entities/FallingPlatform.js";
 import structures from "./structures.js";
-import Vec2 from "./Vec2.js";
 
 let canvas = document.getElementById("canvas");
 
 let level = new Level(canvas);
 
 level.spawnEntity(new Player(level), 10, 1);
-level.spawnEntity(new Platform(level), 16, 4);
-level.spawnStructure(structures.floor(8), 4, 4);
+level.spawnEntity(new FloatingPlatform(level), 16, 4);
+level.spawnEntity(new FallingPlatform(level), 20, 1);
+level.spawnStructure(structures.floor(8, 6), 4, 4);
 
 level.start();
 

js/structures.js

 import Tile from "./Tile.js";
 
 export default {
-	floor: width => new Structure(
+	floor: (width, height = 1)=> new Structure(
 		[ "wall" ],
-		Tile.createLine(width, "grass")),
+		Tile.createBox(width, height, "ground-top", "ground", "ground")),
 };