Graphics and Animation

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.