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

198 Chapter 8

We can then modify our image position updates in the

game loop:

picx += speedx

picy += speedy

We change picx (the horizontal or x-position) by speedx (the

horizontal speed) and picy (the vertical or y-position) by speedy (the

vertical speed).

h tting Four Wallsi

The last part to figure out is the boundary collision detection for

each of the four edges of the screen (top and bottom in addition to

right and left). First, let’s modify the left and right boundaries to

match the new screen size (800 pixels wide) and to use the new

horizontal speed speedx:

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

speedx = -speedx

Notice that our left-edge-boundary case remains the same

at picx <= 0, because 0 is still the left boundary value when picx is at

the left of the screen. This time, though, our right-edge-boundary

case has changed to picx + pic.get_width() >= 800, because our screen

is now 800 pixels wide, and our image still starts at picx and then

draws its full width to the right. So when picx + pic.get_width()

equals 800 , our smiley face looks like it is touching the right side of

the drawing window.

We slightly changed the action that our left and right boundar-

ies trigger, from speed = -speed to speedx = -speedx. We now have two

components of our speed, and speedx will control the left and right

directions and speeds (negative

values of speedx will move the

smiley face left; positive values

will move it right). So when the

smiley hits the right edge of the

screen, we turn speedx negative to

make the image go back toward

the left, and when it hits the

left edge of the screen, we turn

speedx back to a positive value to

rebound the image to the right.