Graphics and Animation

The pygame.draw.line() Function

33. # draw some blue lines onto the surface
34. pygame.draw.line(windowSurface, BLUE, (60, 60), (120, 60), 4)
35. pygame.draw.line(windowSurface, BLUE, (12 0, 60), (60, 120) )
36. pygame.draw.line(windowSurface, BLUE, (60, 120), (12 0, 120), 4)

The pygame.draw.line() function will draw a line on the Surface object that you provide. Notice that the last parameter (the width of the line) is optional. If you pass 4 for the width, the line will be four pixels thick. If you do not specify the width parameter it will take on the default value of 1.

The pygame.draw.circle() Function

38. # draw a blue circle onto the surface
39. pygame.draw.circle(windowSurface, BLUE, (300, 50), 20, 0)

The pygame.draw.circle() function will draw a circle on the Surface object you provide. The third parameter is for the X and Y coordinates of the center of the circle as a tuple of two ints. The fourth parameter is an int for the radius (that is, size) of the circle in pixels. A width of 0 means that the circle will be filled in.

The pygame.draw.ellipse() Function

41. # draw a red ellipse onto the surface
42. pygame.draw.ellipse(windowSurface, RED, (300, 250, 40, 80), 1)

The pygame.draw.ellipse() function will draw an ellipse. It is similar to the pygame.draw.circle() function, except that instead of specifying the center of the circle, a tuple of four ints is passed for the left, top, width, and height of the ellipse.

The pygame.draw.rect() Function

44. # draw the text's background rectangle onto the surface
45. pygame.draw.rect(windowSurface, RED, (textRect.left - 20, textRect.top - 20, textRect.width + 40, textRect.height +
40) )

The pygame.draw.rect() function will draw a rectangle. The third parameter is a tuple of four ints for the left, top, width, and height of the rectangle. Instead of a tuple of four ints for the third parameter, you can also pass a Rect object. In line 45, we want the rectangle we draw to be 20 pixels around all the sides of the text. This is why we want the drawn rectangle's left and top to be the left and top of textRect minus 20. (Remember, we subtract because coordinates decrease as you go left and up.) And the width and height will be equal to the width and height of the textRect plus 40 (because the left and top were moved back 20 pixels, so we need to make up for that space).

The pygame. Pixel Array Data Type

47. # get a pixel array of the surface
48. pixArray = pygame.PixelArray(windowSurface)
49. pixArray[480] [380] = BLACK

On line 48 we create a pygame.PixelArray object (which we will just call a PixelArray object for short). The PixelArray object is a list of lists of color tuples that represents the Surface object you passed it. We passed windowSurface object when we called the PixelArray() constructor function on line 48, so assigning BLACK to pixArray[480][380] will change the pixel at the coordinates (480, 380) to be a black pixel. Pygame will automatically modify the windowSurface object with this change.

The first index in the PixelArray object is for the X-coordinate. The second index is for the Y-coordinate. PixelArray objects make it easy to set individual pixels on a PixelArray object to a specific color.

50. del pixArray

Creating a PixelArray object from a Surface object will lock that Surface object. Locked means that no blit() function calls (described next) can be made on that Surface object. To unlock the Surface object, you must delete the PixelArray object with the del operator. If you forget to delete the Surface object, you will get an error message that says pygame.error: Surfaces must not be locked during blit.

The blit() Method for Surface Objects

52. # draw the text onto the surface
53. windowSurface.blit(text, textRect)

The blit() method will draw the contents of one Surface object onto another Surface object. Line 54 will draw the "Hello world!" text (which was drawn on the Surface object stored in the text variable) and draws it to the Surface object stored in the windowSurface variable.

Remember that the text object had the "Hello world!" text drawn on it on line 22 by the render() method. Surface objects are just stored in the computer's memory (like any other variable) and not drawn on the screen. The Surface object in windowSurface is drawn on the screen (when we call the pygame.display.update() function on line 56 below) because this was the Surface object created by the pygame.display.set_mode() function.

The second parameter to blit() specifies where on the windowSurface surface the text surface should be drawn. We will just pass the Rect object we got from calling text.get_rect() (which was stored in textRect on line 23).

The pygame.display.update() Function

55. # draw the window onto the screen
56. pygame.display.update()

In Pygame, nothing is drawn to the screen until the pygame.display.update() function is called. This is done because drawing to the screen is a slow operation for the computer compared to drawing on the Surface objects while they are in memory. You do not want to draw to the screen after each drawing function is called, but only draw the screen once after all the drawing functions have been called.

You will need to call pygame.display.update() each time you want to update the screen to display the contents of the Surface object returned by pygame.display.set_mode(). (In this program, that object is the one stored in windowSurface.) This will become more important in our next program which covers animation.

Events and the Game Loop

In our previous games, all of the programs print out everything immediately until they reach a input() function call. At that point, the program stops and waits for the user to type something in and press Enter. Pygame programs do not work this way. Instead, Pygame programs are constantly running through a loop called the game loop. (In this program, we execute all the lines of code in the game loop about one hundred times a second.)

The game loop is a loop that constantly checks for new events, updates the state of the window, and draws the window on the screen. Events are values of the pygame.event.Event data type that are generated by Pygame whenever the user presses a key, clicks or moves the mouse, or makes some other event occur. Calling pygame.event.get() retrieves any new pygame.event.Event objects that have been generated since the last call to pygame.event.get().

58. # run the game loop
59. while True:

This is the start of our game loop. The condition for the while statement is set to True so that we loop forever. The only time we exit the loop is if an event causes the program to terminate.

The pygame.event.get() Function

60.      for event in pygame.event.get():
61.          if event.type == QUIT:

The pygame.event.get() function returns a list of pygame.event.Event objects. This list has every single event that has occurred since the last time pygame.event.get() was called. All pygame.event.Event objects have an attribute called type which tell us what type of event it is. (A list of event types is given in the next chapter. In this chapter we only deal with the QUIT event.)

Pygame comes supplied with its own constant variables in the pygame.locals module. Remember that we have imported the pygame.locals module with the line from pygame.locals import *, which means we do not have to type pygame.locals in front of the variables and functions in that module.

On line 60 we set up a for loop to check each pygame.event.Event object in the list returned by pygame.event.get(). If the type attribute of the event is equal to the value of the constant variable QUIT (which is provided by the pygame.locals module), then we know the user has closed the window and wants to terminate the program.

Pygame generates the QUIT event when the user clicks on the X button at the top right of the program's window. It is also generated if the computer is shutting down and tries to terminate all the programs running. For whatever reason the QUIT event was generated, we know that we should run any code that we want to happen to stop the program. You could choose to ignore the QUIT event entirely, but that may cause the program to be confusing to the user.

The pygame.quit() Function

62.                                pygame.quit()
63.                                sys.exit()

If the QUIT event has been generated, then we can know that the user has tried to close the window. In that case, we should call the exit functions for both Pygame (pygame.quit() ) and Python (sys.exit() ).

This has been the simple "Hello world!" program from Pygame. We've covered many new topics that we didn't have to deal with in our previous games. Even though they are more complicated, the Pygame programs can also be much more fun and engaging than our previous text games. Let's learn how to create games with animated graphics that move.

Animation

In this program we have several different blocks bouncing off of the edges of the window. The blocks are different colors and sizes and move only in diagonal directions. In order to animate the blocks (that is, make them look like they are moving) we will move the blocks a few pixels over on each iteration through the game loop. By drawing new blocks that are located a little bit differently then the blocks before, we can make it look like the blocks are moving around the screen.

The Animation Program's Source Code

Type the following program into the file editor and save it as animation.py. You can also download this source code from http://inventwithpython.com/chapter17.

animation.py

This code can be downloaded from http://inventwithpython.com/animation.py

If you get errors after typing this code in, compare it to the book's code with the online

diff tool at http://inventwithpython.com/diff or email the author at

al@inventwithpython.com

1.  import pygame, sys, time
2.  from pygame.locals import * 3 .
4. # set up pygame
5. pygame.init() 6 .
7. # set up the window
8. WINDOWWIDTH = 400
9. WINDOWHEIGHT = 400
10. windowSurface = pygame.display.set_mode((WINDOWWIDTH, WINDOWHEIGHT), 0, 32)
11. pygame.display.set_caption('Animation') 
12.
13. # set up direction variables
14. DOWNLEFT = 1
15. DOWNRIGHT = 3
16. UPLEFT = 7
17. UPRIGHT = 9 18.
19. MOVESPEED = 4 20.
21. # set up the colors
22. BLACK = (0, 0, 0)
23. RED = (255, 0, 0)
24. GREEN = (0, 255, 0)
25. BLUE = (0, 0, 255) 
26.
27. # set up the block data structure
28. b1 = {'rect':pygame.Rect(300, 80, 50, 100), 'color':RED, 'dir':UPRIGHT}
29. b2 = {'rect':pygame.Rect(200, 200, 20, 20), 'color':GREEN, 'dir':UPLEFT}
30. b3 = {'rect':pygame.Rect(100, 150, 60, 60), 'color':BLUE, 'dir':DOWNLEFT}
31. blocks = [b1, b2, b3] 32.
33. # run the game loop
34. while True:
35.     # check for the QUIT event
36.     for event in pygame.event.get():
37.         if event.type == QUIT:
38.             pygame.quit()
39.             sys.exit() 40.
41.     # draw the black background onto the surface
42.     windowSurface.fill(BLACK) 
43.
44.     for b in blocks:
45.         # move the block data structure
46.         if b['dir'] == DOWNLEFT:
47.             b['rect'].left -= MOVESPEED
48.             b['rect'].top += MOVESPEED
49.         if b['dir'] == DOWNRIGHT:
50.             b['rect'].left += MOVESPEED
51.             b['rect'].top += MOVESPEED
52.         if b['dir'] == UPLEFT:
53.             b['rect'].left -= MOVESPEED
54.             b['rect'].top -= MOVESPEED
55.         if b['dir'] == UPRIGHT:
56.             b['rect'].left += MOVESPEED
57.             b['rect'].top -= MOVESPEED 
58.
59.         # check if the block has move out of the window
60.          if b ['rect'] .top < 0:
61.             # block has moved past the top
62.              if b['dir'] == UPLEFT:
63.                 b['dir'] = DOWNLEFT
64.              if b['dir'] == UPRIGHT:
65.                 b['dir'] = DOWNRIGHT
66.          if b [ 'rect'] .bottom > WINDOWHEIGHT:
67.             # block has moved past the bottom
68.              if b['dir'] == DOWNLEFT:
69.                 b['dir'] = UPLEFT
70.              if b['dir'] == DOWNRIGHT:
71.                 b['dir'] = UPRIGHT
72.            if b ['rect'] .left < 0:
73 .                                    # block has moved past the left side
74.              if b['dir'] == DOWNLEFT:
75.                 b['dir'] = DOWNRIGHT
76.              if b['dir'] == UPLEFT:
77.                 b['dir'] = UPRIGHT
78.          if b [ 'rect'] .right > WINDOWWIDTH:
79.             # block has moved past the right side
80.              if b['dir'] == DOWNRIGHT:
81.                 b['dir'] = DOWNLEFT
82.              if b['dir'] == UPRIGHT:
83.                 b['dir'] = UPLEFT 84 .
85.         # draw the block onto the surface
86.         pygame.draw.rect(windowSurface, b['color'], b ['rect' ])
87 .
88.     # draw the window onto the screen
89.     pygame.display.update()
90.     time.sleep(0.02)

How the Animation Program Works

In this program, we will have three different colored blocks moving around and bouncing off the walls. In order to do this, we need to first consider exactly how we want the blocks to move.

Moving and Bouncing the Blocks

Each block will move in one of four diagonal directions: down and left, down and right, up and left, or up and right. When the block hits the side of the window, we want it to "bounce" off the wall and move in a new diagonal direction. The blocks will bounce as shown in this picture:

The new direction that a block moves after it bounces depends on two things: which direction it was moving before the bounce and which wall it bounced off of. There are a total of eight possible ways a block can bounce: two different ways for each of the four walls. For example, if a block is moving down and right, and then bounces off of the bottom edge of the window, we want the block's new direction to be up and right.

We can represent the blocks with a Rect object to represent the position and size of the block, a tuple of three ints to represent the color of the block, and an integer to represent which of the four diagonal directions the block is currently moving. On each iteration in the game loop, we will adjust the X and Y position of the block in the Rect object. Also in each iteration we will draw all the blocks on the screen at their current position. As the program execution loops through the game loop, the blocks will gradually move across the screen so that it looks like they are smoothly moving and bouncing around on their own.

Creating and Setting Up Pygame and the Main Window

1. import pygame, sys, time 

In this program, we also want to import the time module.

7. # set up the window
8. WINDOWWIDTH = 400
9. WINDOWHEIGHT = 400 10. windowSurface = pygame.display.set_mode((WINDOWWIDTH,
WINDOWHEIGHT), 0, 32)

In this program the size of the window's width and height is used for more than just the call to set_mode(). We will use a constant variables to make the program more readable. Remember, readability is for the benefit of the programmer, not the computer. If we ever want to change the size of the window, we only have to change lines 8 and 9.

If we did not use the constant variable, we would have to change ever occurance of the int value 400. If any unrelated values in the program were also 400, we might think it was for the width or height and also accidentally change it too. This would put a bug in our program. Since the window width and height never change during the program's execution, a constant variable is a good idea.

11. pygame.display.set_caption('Animation')

For this program, we will set the caption at the top of the window to 'Animation' with a call to pygame.display.set_caption().

13. Setting Up Constant Variables for Direction
14. # set up direction variables
15. DOWNLEFT = 1
16. DOWNRIGHT = 3
17. UPLEFT = 7
18. UPRIGHT = 9

We will use the keys on the number pad of the keyboard to remind us which belongs to which direction. This will be similar to our Tic Tac Toe game. 1 is down and left, 3 is down and right, 7 is up and left, and 'Animation' 9 is up and right. However, it may be hard to remember this, so instead we will use constant variables instead of these integer values.

We could use any values we wanted to for these directions, as long as we had different values for each direction. For example, we could use the string 'downleft' to represent the down and left diagonal direction. However, if we ever mistype the 'downleft' string (for example, as 'fownleft'), the computer would not recognize that we meant to type 'downleft' instead of 'fownleft'. This bug would cause our program to behave strangely.

But if we use constant variables, and accidentally type the variable name FOWNLEFT instead of the name DOWNLEFT, Python would notice that there is no such variable named FOWNLEFT and crash the program with an error. This would still be a pretty bad bug, but at least we would know immediately about it and could fix it. Otherwise it may be hard to notice that there is a bug at all.

19.   MOVESPEED = 4

We will use a constant variable to determine how fast the blocks should move. A value of 4 here means that each block will move 4 pixels on each iteration through the game loop.

Setting Up Constant Variables for Color

21. # set up the colors
22. BLACK = (0, 0, 0)
23. RED = (255, 0, 0)
24. GREEN = (0, 255, 0)
25. BLUE = (0, 0, 2 55)

We set up constant variables for the colors we will use. Remember, Pygame uses a tuple of three int values for the amounts of red, green, and blue called an RGB value. The integers are from 0 to 255. Unlike our "Hello World" program, this program doesn't use the white color, so we left it out.

Again, the use of constant variables is for readability. The computer doesn't care if we use a variable named GREEN for the color green. But if we later look at this program, it is easier to know that GREEN stands for the color green rather than a bunch of int values in a tuple.

Setting Up The Block Data Structures

27. # set up the block data structure
28. b1 = {'rect' :pygame.Rect(300 , 80, 50, 100), 'color' :RED, 'dir':UPRIGHT}

We will set up a dictionary to be the data structure that represents each block. (Dictionaries were introduced at the end of the Hangman chapter.) The dictionary will have the keys of 'rect' (with a Rect object for a value), 'color' (with a tuple of three ints for a value), and 'dir' (with one of our direction constant variables for a value).

We will store one of these data structures in a variable named b1. This block will have its top left corner located at an X-coordinate of 300 and Y-coordinate of 80. It will have a width of 50 pixels and a height of 100 pixels. Its color will be red (so we'll use our RED constant variable, which has the tuple (255, 0, 0) stored in it). And its direction will be set to UPRIGHT.

29. b2 = {'rect':pygame.Rect(200, 200, 20, 20), 'color':GREEN, 'dir':UPLEFT}
30. b3 = {'rect':pygame.Rect(10 0, 150, 60, 60), 'color':BLUE, 'dir':DOWNLEFT}

Here we create two more similar data structures for blocks that will be different sizes, positions, colors, and directions.

31. blocks = [b1, b2, b3]

On line 31 we put all of these data structures in a list, and store the list in a variable named rectangles.

rectangles is a list. rectangles[0] would be the dictionary data structure in r1. rectangles[0]['color'] would be the 'color' key in r1 (which we stored the value in RED in), so the expression rectangles[0]['color'] would evaluate to (255, 0, 0). In this way we can refer to any of the values in any of the block data structures by starting with rectangles.

Running the Game Loop

33 . # run the game loop 34. while True:

Inside the game loop, we want to move all of the blocks around the screen in the direction that they are going, then bounce the block if they have hit a wall, then draw all of the blocks to the windowSurface surface, and finally call

pygame.display.update() to draw the surface to the screen. Also, we will call pygame.event.get() to check if the QUIT event has been generated by the user closing the window.

The for loop to check all of the events in the list returned by pygame.event.get() is the same as in our "Hello World!" program, so we will skip its explanation and go on to line 44.

41.     # draw the black background onto the surface
42.     windowSurface.fill(BLACK)

Before we draw any of the blocks on the windowSurface surface, we want to fill the entire surface with black so that anything we previously drew on the surface is covered. Once we have blacked out the entire surface, we can redraw the blocks with the code below.

Moving Each Block

44.     for b in blocks:

We want to update the position of each block, so we must loop through the rectangles list and perform the same code on each block's data structure. Inside the loop, we will refer to the current block as simply r so it will be easy to type.

45.         # move the block data structure
46.          if b['dir'] == DOWNLEFT:
47.             b['rect'].left -= MOVESPEED
48.             b [ 'rect'] .top += MOVESPEED
49.          if b['dir'] == DOWNRIGHT:
50.             b['rect'].left += MOVESPEED
51.             b [ 'rect'] .top += MOVESPEED
52.          if b['dir'] == UPLEFT:
53.             b['rect'].left -= MOVESPEED
54.             b [ 'rect'] .top -= MOVESPEED
55.          if b['dir'] == UPRIGHT:
56.             b ['rect'] .left += MOVESPEED
57.             b['rect'].top -= MOVESPEED

The new value that we want to set the left and top attributes to depends on the direction the block is moving. Remember that the X-coordinates start at 0 on the very left edge of the window, and increase as you go right. The Y-coordinates start at 0 on the very top of the window, and increase as you go down. So if the direction of the block (which, remember, is stored in the 'dir' key) is either DOWNLEFT or DOWNRIGHT, we want to increase the top attribute. If the direction is UPLEFT or UPRIGHT, we want to decrease the top attribute.

If the direction of the block is DOWNRIGHT or UPRIGHT, we want to increase the left attribute. If the direction is DOWNLEFT or UPLEFT, we want to decrease the left attribute.

We could have also modified right instead of the left attribute, or the bottom attribute instead of the top attribute, because Pygame will update the Rect object either way. Either way, we want to change the value of these attributes by the integer stored in MOVESPEED, which stores how many pixels over we will move the block.

Checking if the Block has Bounced

59.         # check if the block has move out of the window
60.          if b ['rect'] .top < 0:
61.             # block has moved past the top
62.              if b['dir'] == UPLEFT:
63.                 b['dir'] = DOWNLEFT
64.              if b['dir'] == UPRIGHT:
65.                 b['dir'] = DOWNRIGHT

After we have moved the block, we want to check if the block has gone past the edge of the window. If it has, we want to "bounce" the block, which in the code means set a new value for the block's 'dir' key. When the direction is set, the block will move in the new direction on the next iteration of the game loop.

We need to check if the block has moved passed each of the four edges of the window. In the above if statement, we decide the block has moved past the top edge of the window if the block's Rect object's top attribute is less than 0. If it is, then we need to change the direction based on what direction the block was moving.

Changing the Direction of the Bouncing Block

Look at the bouncing diagram earlier in this chapter. In order to move past the top edge of the window, the block had to either be moving in the UPLEFT or UPRIGHT directions. If the block was moving in the UPLEFT direction, the new direction (according to our bounce diagram) will be DOWNLEFT. If the block was moving in the UPRIGHT direction, the new direction will be DOWNRIGHT.

66.          if b['rect'].bottom > WINDOWHEIGHT:
67.             # block has moved past the bottom
68.              if b['dir'] == DOWNLEFT:
69.                 b['dir'] = UPLEFT
70.              if b['dir'] == DOWNRIGHT:
71.                 b['dir'] = UPRIGHT

Here we see if the block has moved past the bottom edge of the window by checking if the bottom attribute (not the top attribute) is greater than the value in WINDOWHEIGHT. Remember that the Y-coordinates start at 0 at the top of the window and increase to WINDOWHEIGHT because we passed WINDOWHEIGHT as the height in our call to pygame.display.set_mode().

The rest of the code changes the direction based on what our bounce diagram says.

72.          if b ['rect'] .left < 0:
73.             # block has moved past the left side
74.              if b['dir'] == DOWNLEFT:
75.                 b['dir'] = DOWNRIGHT
76.              if b['dir'] == UPLEFT:
77.                 b['dir'] = UPRIGHT

This is similar to the above code, but checks if the left side of the block has moved to the left of the left edge of the window. Remember, the X-coordinates start at 0 on the left edge of the window and increase to WINDOWWIDTH on the right edge of the window.

78.          if b [ 'rect'] .right > WINDOWWIDTH:
79.             # block has moved past the right side
80.              if b['dir'] == DOWNRIGHT:
81.                 b['dir'] = DOWNLEFT
82.              if b['dir'] == UPRIGHT:
83.                 b['dir'] = UPLEFT

This code is similar to the previous pieces of code, but it checks if the block has moved past the rightmost edge of the window.

Drawing the Blocks on the Window in Their New Positions

85.         # draw the block onto the surface
86.         pygame.draw.rect(windowSurface, b['color'], b ['rect' ])

Now that we have moved the block (and set a new direction if the block has bounced off the window's edges), we want to draw it on the windowSurface surface. We can draw this using the pygame.draw.rect() function. We pass windowSurface, because that is the Surface object we want to draw on. We pass the b['color'] value, because this is the color we want to use. Then we pass b['rect'], because that Rect object has the information about the position and size of the rectangle we want to draw.

This is the last line of the for loop. We want to run the moving, bouncing, and drawing code on each of the blocks stored in the blocks list, which is why we loop through each of them. Also, if we wanted to add new blocks or remove blocks from our program, we only have to modify the blocks list and the rest of the code still works.

Drawing the Window on the Screen

88.     # draw the window onto the screen
89.     pygame.display.update()
90.     time.sleep(0.02)

After we have run this code on each of the blocks in the blocks list, we want to finally call pygame.display.update() so that the windowSurface surface is draw on the screen. After this line, we loop back to the start of the game loop and begin the process all over again. This way, the blocks are constantly moving a little, bouncing off the walls, and being drawn on the screen in their new positions. Meanwhile, we also check if the QUIT event has been generated by the Pygame library (which happens if the player closes the window or shuts down their computer). In that case we terminate the program.

The call to the time.sleep() function is there because the computer can move, bounce, and draw the blocks so fast that if the program ran at full speed, all the blocks would just look like a blur. (Try commenting out the time.sleep(0.02) line and running the program to see this.) This call to time.sleep() will stop the program for 20 milliseconds. There are 1000 milliseconds in a second, so 0.001 seconds equals 1 millisecond and 0.02 equals 20 milliseconds.

Some Small Modifications

Drawing as Fast as Possible

Just for fun, let's make some small modifications to our program so we can see what it does. Try adding a # in front of line 90 (the time.sleep(0.2) line) of our animation program. This will cause Python to ignore this line because it is now a comment. Now try running the program.

Without the time.sleep() function call to intentionally slow down the program, your computer will run through the game loop as fast as possible. This will make the rectangles bounce around the screen so fast, they'll only look like a blur. Now you can see why it is important for us to slow down the program with this line.

Drawing Trails of Blocks

Remove the # from the front of line 90 so that the line is no longer a comment and becomes part of the program again. This time, comment out line 42 (the windowSurface.fill(BLACK) line) by adding a # to the front of the line. Now run the program.

Without the call to windowSurface.fill(BLACK), we do not black out the entire window before drawing the rectangles in their new position. This will cause trails of rectangles to appear on the screen instead of individual rectangles. The trails appear because all the old rectangles that are drawn in previous iterations through the game loop don't disappear.

Remember that the blocks are not really moving. We are just redrawing the entire window over and over again. On each iteration through the game loop, we redraw the entire window with new blocks that are located a few pixels over each time. When the program runs very fast, we make it is just one block each time. In order to see that we are just redrawing the blocks over and over again, change line 90 to time.sleep(1.0). This will make the program (and the drawing) fifty times slower than normal. You will see each drawing being replaced by the next drawing every second.

Summary: Pygame Programming

This chapter has presented a whole new way of creating computer programs. Our programs before would stop and wait for the player to enter text. However, in our animation program, we are constantly updating the data structures of things without waiting for input from the player. Remember in our Hangman and Tic Tac Toe games we had data structures that would represent the state of the board, and these data structures would be passed to a drawBoard() function to be displayed on the screen. Our animation program is very similar. The blocks variable held a list of data structures representing things to be drawn to the screen, and these are drawn to the screen inside the game loop.

But without calls to input(), how do we get input from the player? In our next chapter, we will cover how our program can know when the player presses any key on the keyboard. We will also learn of a concept called collision detection, which is used in many graphical computer games.