.. _sprites: Sprites And Collisions ====================== Our games need support for handling objects that collide. Balls bouncing off paddles, laser beams hitting aliens, or our favorite character collecting a coin. All these examples require collision detection. The Arcade library has support for sprites. A sprite is a two-dimensional image that is part of the larger graphical scene. Typically a sprite will be some kind of object in the scene that will be interacted with like a car, frog, or little plumber guy. .. image:: sprite.png :width: 250px Originally, video game consoles had built-in hardware support for sprites. Now this specialized hardware support is no longer needed, but we still use the term "sprite." The `history of sprites`_ is interesting, if you want to read up more about it. .. _history of sprites: https://en.wikipedia.org/wiki/Sprite_(computer_graphics) Finding Images for Sprites -------------------------- There are several image formats that computers use: * ``.bmp`` - Bitmap. This is an uncompressed image that normally uses three bytes to represent each dot in the image. These files can be very large. Because there are so many better options, this format is not used often. It is, however, the simplest format to use. * ``.png`` - Great patent-free format for line art and clip art. Not great for photos. Can't hold animations. * ``.gif`` - Great format for line art and clip art. Has had issues with patents (now expired). Can do animations. * ``.jpg`` - Great file format for photos. Terrible for clip-art. Don't use for sprites. * ``.svg`` - File format for storing line-art images that can scale to any resolution. Not compatible with the "arcade" library. If you use Google's `advanced image search`_ you can find images that are "icon" sized, and either png or gif file format. .. _advanced image search: https://www.google.com/advanced_image_search There's also a great source for images from `kenney.nl`_. He has a lot of free and cheap game image assets. That's where the following images come from that we will use in our examples: .. _kenney.nl: http://kenney.nl/ .. figure:: character.png character.png .. figure:: coin_01.png coin_01.png Where to Save Images ^^^^^^^^^^^^^^^^^^^^ Where should you save them? If you load your sprite with the code below, the computer will look for the ``character.png`` image in the same directory as your Python file. Save the image anywhere else, and it won't be found. How to Reference Images ^^^^^^^^^^^^^^^^^^^^^^^ If you create your own game that you publish, you need to: * Create your own images * Hire someone to create your images * Buy your own images with a license to use them in your own game * Find images that are public domain or licensed for public use If you are just creating a game for class that won't be used in public, then right before you load the image leave a comment with the source. I'll show this in a bit. .. attention:: Do not list "Google" as a source. That's like using "The Library" as a source in your report. Find the source of the image that Google is pointing to. Basic Sprites and Collisions ---------------------------- Let's step through an example program that uses sprites. This example shows how to create a screen of sprites that are coins, and collect them using a sprite that is a character image controlled by the mouse as shown in the figure below. The program keeps "score" on how many coins have been collected. The code for this example may be found at: http://arcade.academy/examples/sprite_collect_coins.html In this chapter, we'll step through that example. .. figure:: collect_coins.gif Example Sprite Game Getting the Application Started ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The first few lines of our program start off like other games we've done. We import a couple libraries. Set a couple constants for the size of the screen, and a couple new constants that we will use to scale our graphics. The example below should have nothing new, it just creates a window and sets a background color. We'll add in the new stuff soon. .. literalinclude:: sprite_sample_start.py :caption: Sprite Sample Start :language: python :linenos: The Constructor ^^^^^^^^^^^^^^^ What's next? We need to add our attributes to the ``MyGame`` class. We add our attributes to the ``__init__`` method. Here is our code with the expanded ``__init__``: .. literalinclude:: sprite_sample_expanded_init.py :caption: Expanded Init :language: python :emphasize-lines: 23-34 :linenos: The variables we are creating: * ``player_list``: When working with sprites, we normally put them into lists. Other game engines might call these sprite groups, or sprite layers. Our game will have one list for the player, and one list for the coins. Even if there is only one sprite, we should still put it in a list because there is a lot of code in ``SpriteList`` to optimize drawing. * ``coin_list``: This is a list of all the coins. We will be checking if the player touches any sprite in this list. * ``player_sprite``: This points to our player's sprite. It is the sprite we will move. * ``score``: This keeps track of our score. We use a command built into the parent ``Window`` class called ``set_mouse_visible`` to make the mouse not visible. Finally we set the background color. The `setup` Function ^^^^^^^^^^^^^^^^^^^^ Next up, we will create a ``setup`` method. This will create our sprites and get our game set up. We do this in a different method than ``__init__`` so that if we ever want to restart the game, we can just call ``setup`` again. The ``setup`` method is not called automatically. Therefore in the example below, note we have added the code that calls the ``setup`` function near the end: ``window.setup()``. .. literalinclude:: sprite_sample_player.py :caption: Sprite Sample With Player :language: python :emphasize-lines: 36-51, 56-64, 70 :linenos: How does the code above work? First, we need some lists to hold our sprites. We could do use a list like this: .. code-block:: Python coin_list = [] But wait! ``coin_list`` is an instance variable that's part of our class. we need to prepend it with ``self.``. .. code-block:: Python self.coin_list = [] However, the Arcade library has a class especially for handling sprite lists. This class is called ``SpriteList``. For more information, check out the SpriteList_ documentation. So instead of creating an empty list with ``[]``, we will create a new instance of the ``SpriteList`` class: .. _SpriteList: http://arcade.academy/arcade.html#arcade.sprite.SpriteList .. code-block:: Python self.coin_list = SpriteList() Except that doesn't work. Why? ``SpriteList`` is in the Arcade library. We need to prepend any reference to things in the Arcade library with ``arcade`` of course, so now we have: .. code-block:: Python self.coin_list = arcade.SpriteList() We need a separate list for just coins. This list won't have the player. We also need to reset our score to 0. .. code-block:: Python self.coin_list = arcade.SpriteList() self.score = 0 Now we need to create our sprites. The name of the class that represents sprites is called ``Sprite``. You can read more about it by looking at the Sprite_ documentation. The Sprite constructor takes two parameters. A path to the image we will be using, and how big to scale it. For class, please source the image right before you load it. If you drew your own image, please note that as well. .. _Sprite: http://arcade.academy/arcade.html#arcade.sprite.Sprite .. code-block:: Python # Character image from kenney.nl self.player_sprite = arcade.Sprite("character.png", SPRITE_SCALING_PLAYER) How do we draw all our sprites? Really easy. Just call the ``draw`` method that exists for us in the ``SpriteList`` class. We just need to do this for each of our sprite lists. .. code-block:: Python def on_draw(self): arcade.start_render() # Draw all the sprite lists. self.coin_list.draw() self.player_list.draw() Wait. We don't have many sprites. There are no coins, and we have just the player. Let's add a ``for`` loop to our program and create a bunch of coins: .. literalinclude:: sprite_sample_coins.py :caption: Sprite Sample With Player And Coins :language: python :emphasize-lines: 54-65 :linenos: Drawing The Score ^^^^^^^^^^^^^^^^^ In addition to drawing the sprites, let's go ahead and put the score on the screen: .. code-block:: Python # Put the text on the screen. output = "Score: " + str(self.score) arcade.draw_text(output, 10, 20, arcade.color.WHITE, 14) Rather than do that ``"Score: " + str(self.score)`` it is possible to do print formatting if you are using Python 3.6 or later. We'll talk more about print formatting later, but that code would look like: .. code-block:: Python # Put the text on the screen. output = f"Score: {self.score}" arcade.draw_text(output, 10, 20, arcade.color.WHITE, 14) There are three standards for how to format strings in Python, so that whole subject is a bit confusing. The On Mouse Motion Method ^^^^^^^^^^^^^^^^^^^^^^^^^^ Moving the player sprite with the mouse is easy. All sprites have instance variables ``center_x`` and ``center_y``. Just change those values to the mouse's x and y location to move the sprite. .. code-block:: Python def on_mouse_motion(self, x, y, dx, dy): self.player_sprite.center_x = x self.player_sprite.center_y = y Now, our whole program looks like: .. literalinclude:: sprite_sample_with_mouse_motion.py :caption: Sprite Sample With Mouse Motion And Score :language: python :emphasize-lines: 73-75, 77-82 :linenos: The Update Method ^^^^^^^^^^^^^^^^^ Our ``update`` method needs to do three things: 1. Update each of the sprites 2. Check to see if the player is touching any coins 3. Remove any coins colliding with the player, and update the score. Each sprite has its own ``update`` method. This allows sprites to move and animate its images. Right now, our sprite does not have this method. But we will soon. Rather than call the ``update`` method of each sprite we have, there is an ``update`` method in each sprite list that will call ``update`` on each sprite in the list. Therefore, just calling ``update`` with our ``coin_list`` will cause all coin sprites to update. .. code-block:: Python self.coin_list.update() How do we detect what coins are touching the player? We call the ``check_for_collision_with_list`` method. Pass it in our player sprite, along with a list of all the coins. That function will return a list of all colliding sprites. If no sprites collide, the list will be empty. .. code-block:: Python # Generate a list of all sprites that collided with the player. coins_hit_list = arcade.check_for_collision_with_list(self.player_sprite, self.coin_list) What do we do with this ``hit_list`` we get back? We loop through it. We add one to the score for each sprite hit. We also need to get rid of the sprite. The sprite class has a method called ``remove_from_sprite_lists``. This method will remove the sprite from existence. .. code-block:: Python # Loop through each colliding sprite, remove it, and add to the score. for coin in coins_hit_list: coin.remove_from_sprite_lists() self.score += 1 Here's the whole ``update`` method put together: .. literalinclude:: sprite_sample_with_update.py :caption: Sprite Sample With Update Method :language: python :emphasize-lines: 84-98 :linenos: