删除画布游戏中的背景

我正在尝试学习JavaScript，并找到了本教程：http://www.lostdecadegames.com/how-to-make-a-simple-html5-canvas-game/

但是我不知道如何从游戏中去除背景。如有任何关于删除背景的帮助，我们将不胜感激！：）

影响后台的代码：

// Background image
var bgReady = false;
var bgImage = new Image();
bgImage.onload = function () {
bgReady = true;
};
bgImage.src = "images/background.png";
// Draw everything
var render = function () {
if (bgReady) {
    ctx.drawImage(bgImage, 0, 0);
}

整个JavaScript文件：http://www.lostdecadegames.com/demos/simple_canvas_game/js/game.js

// Create the canvas
var canvas = document.createElement("canvas");
var ctx = canvas.getContext("2d");
canvas.width = 512;
canvas.height = 480;
document.body.appendChild(canvas);
// Background image
var bgReady = false;
var bgImage = new Image();
bgImage.onload = function () {
  bgReady = true;
};
bgImage.src = "images/background.png";
// Hero image
var heroReady = false;
var heroImage = new Image();
heroImage.onload = function () {
  heroReady = true;
};
heroImage.src = "images/hero.png";
// Monster image
var monsterReady = false;
var monsterImage = new Image();
monsterImage.onload = function () {
  monsterReady = true;
};
monsterImage.src = "images/monster.png";
// Game objects
var hero = {
  speed: 256 // movement in pixels per second
};
var monster = {};
var monstersCaught = 0;
// Handle keyboard controls
var keysDown = {};
addEventListener("keydown", function (e) {
  keysDown[e.keyCode] = true;
}, false);
addEventListener("keyup", function (e) {
  delete keysDown[e.keyCode];
}, false);
// Reset the game when the player catches a monster
var reset = function () {
  hero.x = canvas.width / 2;
  hero.y = canvas.height / 2;
  // Throw the monster somewhere on the screen randomly
  monster.x = 32 + (Math.random() * (canvas.width - 64));
  monster.y = 32 + (Math.random() * (canvas.height - 64));
};
// Update game objects
var update = function (modifier) {
  if (38 in keysDown) { // Player holding up
    hero.y -= hero.speed * modifier;
  }
  if (40 in keysDown) { // Player holding down
    hero.y += hero.speed * modifier;
  }
  if (37 in keysDown) { // Player holding left
    hero.x -= hero.speed * modifier;
  }
  if (39 in keysDown) { // Player holding right
    hero.x += hero.speed * modifier;
  }
  // Are they touching?
  if (
    hero.x <= (monster.x + 32)
    && monster.x <= (hero.x + 32)
    && hero.y <= (monster.y + 32)
    && monster.y <= (hero.y + 32)
  ) {
    ++monstersCaught;
    reset();
  }
};
// Draw everything
var render = function () {
  if (bgReady) {
    ctx.drawImage(bgImage, 0, 0);
  }
  if (heroReady) {
    ctx.drawImage(heroImage, hero.x, hero.y);
  }
  if (monsterReady) {
    ctx.drawImage(monsterImage, monster.x, monster.y);
  }
  // Score
  ctx.fillStyle = "rgb(250, 250, 250)";
  ctx.font = "24px Helvetica";
  ctx.textAlign = "left";
  ctx.textBaseline = "top";
  ctx.fillText("Goblins caught: " + monstersCaught, 32, 32);
};
// The main game loop
var main = function () {
  var now = Date.now();
  var delta = now - then;
  update(delta / 1000);
  render();
  then = now;
  // Request to do this again ASAP
  requestAnimationFrame(main);
};
// Cross-browser support for requestAnimationFrame
var w = window;
requestAnimationFrame = w.requestAnimationFrame || w.webkitRequestAnimationFrame || w.msRequestAnimationFrame || w.mozRequestAnimationFrame;
// Let's play this game!
var then = Date.now();
reset();
main();