Teach Your Kids To Code: A Parent-friendly Guide to Python Programming

200 Chapter 8

picx += speedx

picy += speedy

if picx <= 0 or picx + pic.get_width() >= 800:

speedx = -speedx

if picy <= 0 or picy + pic.get_height() >= 600:

speedy = -speedy

screen.fill(BLACK)

screen.blit(pic, (picx, picy))

pygame.display.update()

timer.tick(60)

pygame.quit() # Exit

The rebounds look realistic. If the smiley is coming toward the

bottom edge at a 45-degree angle down and to the right, it bounces

off at a 45-degree angle up and to the right. You can experiment

with different values of speedx and speedy (say, 3 and 5 , or 7 and 4 ) to

see the angles change for every bounce.

Just for fun, you can comment out the line screen.fill(BLACK) in

SmileyBounce2.py to see the path traveled by our smiley face as it

bounces off each edge of the screen. When you comment out a line,

you turn it into a comment by putting a hash mark at the begin-

ning, as follows:

# screen.fill(BLACK)

This tells the program to ignore the instruction on that line.

Now the screen is not erased after each smiley face is drawn, and

you’ll see a pattern created by the trail your animation is leaving

behind, like in Figure 8-9. Because each new smiley is drawn over

the previous one, the result looks like cool, retro 3-D screensaver

artwork as it draws.

Our collision-detection logic has allowed us to create the illusion

of a solid smiley face bouncing off all four edges of a solid drawing

screen. This is an improvement over our original version, which

let the smiley slide off into oblivion. When we create games that

allow the user to interact with pieces on the screen, and that allow

those pieces to look as if they’re interacting with one another—like

in Tetris, for example—we’re using the same kind of collision detec-

tion and boundary checking that we built here.