Asteroids game

We will try to make a clone of the game Asteroids that has been released in 1979.

Our version will look like this:

The project is quite complex. It uses a few things that were not covered by the beginner's course. I know that you will be able to look them up.

Spaceship

The first step is to program a spaceship that you can control by keyboard.

• An instance of the class Spaceship represents the spaceship.
• Every spaceship has two attributes, x and y (position), x_speed and y_speed, rotation, and sprite. (A sprite is a 2D object in Pyglet with position, speed, rotation, and image.)
• The spaceship has a method called tick that handles the spaceship mechanics – movement, rotation, and control.
• All objects that are in the game are stored in a global list, objects. It should contain only the spaceship for now.
• Store all pressed keys in a set (keyword set). It is a datatype similar to list but without order. It can contain each element only once. (Sets are like dictionaries without values.) You can use the sets cheatsheet, and the official Python documentation contains a tutorial and a reference. The spaceship uses the set as part of the processing in its tick method.
• You can use the image set, created by Kenney Vleugels. He made them public and free. Or you can draw your own images if you want!

• In the game, we will use a large number of Sprites. Drawing them one by one would take quite a long time. So add all the Sprites to the (pyglet.graphics.Batch)[https://pythonhosted.org/pyglet/api/pyglet.graphics.Batch-class.html] collection, which Pyglet can efficiently draw at once. Add arguments to "batch" by using Sprite() to create a sprite.delete(). For example:

    batch = pyglet.graphics.Batch()
    sprite1 = pyglet.sprite.Sprite(image, batch=batch)
    sprite2 = pyglet.sprite.Sprite(image, batch=batch)
  
    # and then you can draw all of them at once: 
    batch.draw()
  

  Create the batch collection, as well as the objects, as global variables.

• To move and rotate the objects relative to their center, it is good to set the "anchor" of the image to its center (otherwise, the anchor is in the lower left corner):

    image = pyglet.image.load(...)
    image.anchor_x = image.width // 2
    image.anchor_y = image.height // 2 
    self.sprite = pyglet.sprite.Sprite(image, batch=batch)
  

• You can use the arrow keys to move the rocket left, right, and straight. The arrows to the sides spin the rocket, the arrow forward accelerates the movement in the direction the rocket is turned.

  • The basic motion of the rocket is simple: the x-coordinate is added to the x-velocity times the elapsed time, and the same with the y-coordinate, and for the rotation angle:

      self.x = self.x + dt * self.x_speed
      self.y = self.y + dt * self.y_speed
      self.rotation = self.rotation + dt * rotation_speed
    

    The speed of rotation depends on the arrows (left or right). In one case, it is negative, in the other positive. Choose the appropriate value by experimenting
    • starting at 4 radians per second. All similar "magical values" should be defined as constants - i.e. variables that you set at the beginning and never change. It is a convention to name constants in capital letters and put them at the beginning of the file, right after the import:

      ROTATION_SPEED  =  4   # radians per second
      

  • Acceleration is a little more complicated: the x-axis speed is added to the cosine angle of rotation times elapsed time. The sinus is used with the y-axis.

      self.x_speed += dt * ACCELERATION * math.cos(self.rotation)
      self.y_speed += dt * ACCELERATION * math.sin(self.rotation)
    

    Notice the ACCELERATION constant example. Choose it again at your discretion.

  • If you have calculated the self.x, self.y, and self.rotation values, do not forget to project them into self.sprite, otherwise nothing interesting will happen.

    Beware that the math.sin and math.cos functions use radians, whereas the pyglet Sprite.rotation uses degrees. (Additionally, they start at different origins, and they rotate in opposite directions.) For a sprite, therefore, the angle needs to be converted:

      self.sprite.rotation = 90 - math.degrees(self.rotation)
      self.sprite.x = self.x
      self.sprite.y = self.y
    

  • When the rocket leaves the window, put it back into the game on the other side of the screen. (Check that it works on all four sides.)
• Bonus 1: Try to add a few rockets, each with a slightly different angle. Each individual Spaceship object maintains its own state, so it should not be difficult to create more (and to control all at once).

• Bonus 2 : You may have noticed a "jump" when a rocket escapes from the window and returns to the other side. This can be avoided by rendering the whole screen once more to the left, right, up and down.

  Pyglet has a special low-level feature that can tell "now draw everything moved by the X pixels to the left". Full explanation would be long, so just copy the code:

    from pyglet import gl
  
    def draw():
        window.clear()
  
        for x_offset in (-window.width, 0, window.width):
            for y_offset in (-window.height, 0, window.height):
                # Remember the current state 
                gl.glPushMatrix () 
                # Move everything drawn from now on by (x_offset, y_offset, 0)
                gl.glTranslatef(x_offset, y_offset, 0)
  
                # Draw 
                batch.draw()
  
                # Restore remembered state (this cancels the glTranslatef) 
                gl.glPopMatrix()
  

  For an overview, the documentation for the functions used here is: glPushMatrix, glPopMatrix, glTranslatef.

Have you succeeded? Can you fly through the universe?

Asteroids

Add a second type of space object: Asteroid.

• Asteroids and spaceships have many things in common: every space object will have its position, speed, rotation, and rules how it moves. So create a SpaceObject class, in which will be everything they have in common, and a Spaceship class, that inherits from SpaceObject, in which the spaceship-specific code remains (i.e., keyboard control, ship image, start from the middle of the screen).
• The part of the code for motion will be common to all space objects (e.g. code for the acceleration); another part will be specific to the rocket only (keypad control). Take advantage of the super() function (more in inheritance lesson).
• Write the Asteroid class, which is also inherited from SpaceObject, but has its own behaviour: it starts either at the left or bottom of the screen with a random speed, and each asteroid has a randomly assigned image. (In the Asteroids, the left and right edges are essentially the same, and the top and bottom too.)
• And then add some asteroids of different sizes to the game.

Have you succeeded? Do you have two types of objects?

Collisions

Our asteroids are still pretty harmless. Let's change that.

• In this section, your task will be to find out when the ship is hit by an asteroid. For simplicity, we replace each object with a circle and calculate if the circles collide. Each object will need to have a radius - the radius attribute.

• In order to see what the game "thinks" where and how big our objects are, draw a circle around each object. The best thing to do is to use pyglet.gl and a little math; for now, just copy the draw_circle function and call it for each object. After you got this part working, you won't need to highlight the radius any longer, and you can remove the draw_circle function again.

    def draw_circle(x, y, radius):
      iterations = 20
      s = math.sin(2 * math.pi / iterations)
      c = math.cos(2 * math.pi / iterations)
  
      dx, dy = radius, 0
  
      gl.glBegin(gl.GL_LINE_STRIP)
      for i in range(iterations + 1):
        gl.glVertex2f(x + dx, y + dy)
        dx, dy = (dx * c - dy * s), (dy * c + dx * s)
      gl.glEnd()
  

• When an asteroid crashes into the ship, the ship will explode and disappear. We'll leave the explosion for later, but it's important to remove the object from the game. Put it in the SpaceObject.delete method, because any object can be removed from the game. In this method, you must remove the object from the list of objects and then delete its Sprite so that it does not render within the batch.

• And how do you detect that collision? Within the Spaceship.tick, go through each object to see if the distance between the ship and the other object is less than the sum of their radiuses (they hit each other), and if so, call the object's hit_by_spaceship method.

  Finding a distance in a game where the objects that fly out of the screen return on the other side is not entirely straightforward, so copy the code for now:

    def distance(a, b, wrap_size):
        """Distance in one direction (x or y)"""
        result = abs(a - b)
        if result > wrap_size / 2:
            result = wrap_size - result
        return result
  
    def overlaps(a, b):
        """Returns true if and only if two objects overlap space"""
        distance_squared = (distance(a.x, b.x, window.width) ** 2 +
                            distance(a.s, b.y, window.height) ** 2)
        max_distance_squared = (a.radius +b.radius) ** 2
        return distance_squared < max_distance_squared
  

  Most other objects in the completed game (such as fire from the rocket, missile) will not do anything when the collision happens, so the SpaceObject.hit_by_spaceship should do nothing (the method only needs to exist). Only an asteroid will break the rocket, so redefine Asteroid.hit_by_spaceship to call delete ship.

  Because there could be more rockets in our game in general, the asteroid needs to know which rocket it broke. The hit_by_spaceship method should, therefore, have an argument.

Have you succeeded? Can you lose now?

Attack

Now try to break the asteroids.  

• The missile can fire a laser in 0.3 seconds. For each rocket save a number (as an attribute) which is set, after each shot, to 0.3 and then let this number drop by 1 per second in the tick method. If the number is negative user can fire again.
• When a player holds the space bar and has the possibility to fire, then the ship should fire. Reflect this in the game by adding an object of a new class, Laser. The laser starts at the rocket's coordinates, it has the rocket's rotation and rocket speed plus something extra in the direction of rotation.
• Each Laser object needs to "remember" how long it is in the game. In the beginning, set its lifetime to a number so the laser can fly little bit further than one screen. When its lifetime is over, the Laser disappears.
• In its tick method, the laser goes through all objects, and when its position overlaps with some of these objects, it calls their hit_by_laser method. For most objects, this method does nothing, only the asteroids will break.

• When the laser touches an asteroid, the asteroid divides into two smaller ones (or, if it's too small, it disappears completely).

  You can set the speeds of new asteroids how you want - it is important that every smaller asteroid flies elsewhere. Usually, new asteroids are faster than the original ones.

• And that's all! You have a functional game!

Have you succeeded? Can you also win?

Completion and extension

If you want to continue in the game, here are some ideas. You can do it in any order - or you can invent your own extension!

• Is the game too difficult?

  You can add lives: there are three at the beginning, and as long as there's one left, the rocket will appear again in the middle of the screen with zero speed after an
  asteroid hit it. The game should also ignore the keys that were held until the player presses them again (preferably use pressed_keys.clear ()).

  You can show the number of ships (lives) that are left with icons at the bottom of the screen.

  Bonus: A few seconds after the "restart", the rocket can be indestructible to have time to fly when there happens to be an asteroid in the middle of the screen.

• Is the game too easy?

  Add Levels: When the player shoots all the asteroids, they move to the next level where there are more the asteroids than in the previous level.

  You can display the level number using pyglet.text.Label.

• Is the background too black?

  In the set of pictures in the Backgrounds directory choose one background and paint the whole universe with it.

• Is the game too austere?

  Add fire and explosions! Like the Laser, only they don't destroy anything, they just change their colour depending on how long they are in the game.

  You can use the "Smoke particle assets" images drawn by Kenney Vleugels again. I recommend "White Puff". You can shrink them (e.g. sprite.scale = 1/10), change their colour (e.g. sprite.color = 255, 100, 0), or make them partially transparent (e.g. sprite.opacity = 100).

  I recommend to make a new batch for the effects and draw them before the main batch, so the effects can't overlap the game objects.

• Don't you know whether you lost or won?

  In the end, you can draw a big GAME OVER or WINNER sign.

• Are you bored?

  In the original game, UFOs sometimes appear, and sometimes they shoot at the rocket, so if the rocket stands still in one spot and it is just spinning around, the UFO will destroy it. You can try to complete the Ufo class and you can create ShipLaser and UfoLaser that inherit from the Laser class.

Have you succeeded? Does it look and behave professionally?