Part 2¶
So now you have a background and a bird that flys up and down, let’s add the pipes that the bird must fly through.
Adding the pipes¶
You’ve already seen that we can create Actor objects and move these around the screen. If you’ve forgotten, go and look at where you use barry_the_bird in your code to see how it works.
So we need to add two pipes, and if you look in the images directory you’ll see we have two files named top.png and bottom.png. How do we add these to the game?
Let’s create two new actors, at the end of your code add:
top_pipe = Actor('top', (300,0))
bottom_pipe = Actor('bottom', (300,500))
We mustn’t forget to draw these pipes, so add this to your draw function:
top_pipe.draw()
bottom_pipe.draw()
- Press play to see the effect.
Great, we have some pipes! But they are just sitting there on the page and there’s no gap between them.
Can you figure out what the (300, 0) and (300, 500) parts do? (Try changing them)
Mind the gap¶
How do we make a gap for our bird to fly through? We need to make sure we position the pipes in the right place on the screen, and to do this we need to know how big each image is.
Let’s see what we can find out from an Actor, add this code at the end of your program:
print(top_pipe.width, top_pipe.height)
- Press play and look in the bottom half of your code window.
You should see two numbers appear (I get 100 500), that’s the width and height of the top pipe in pixels.
Using that info, we can change the construction of the top and bottom pipes to this:
gap = 140
top_pipe = Actor('top', (300, 0))
bottom_pipe = Actor('bottom', (300, top_pipe.height + gap))
The two values separated by a comma at the end of each Actor line control the x (left to right), and y (top to bottom) position of the Actor. So (300,0) puts the pipe at 300 pixels away from the left edge, and 0 pixels down from the top of the window.
Hang on a minute, where did that maths come from? Well, gap is a new variable we’re creating, and we say that the height of the top pipe plus gap should be the y value of the bottom pipe. Try changing the gap value and see what happens.
- Press play to check we made a gap
What’s the biggest gap you can make and still see the bottom pipe?
You know when to press play¶
You’ve been pressing play a lot haven’t you? That’s how you see the effect of your code changes.
From now on we won’t always tell you when to press play. We encourage you to try running your code when you’ve completed each litte code box, or whenever you feel like it.
Sometimes you’ll see an error, in which case: read the last line of the error, think about what you just changed and get investigating. You can always ask a mentor if you need help.
Flying forwards¶
So now that we have the pipes, let’s get them moving.
Note: Smoke and Mirrors
You might be thinking “doesn’t the player fly forward and the pipes stay still?”
If you thought that, then the game fooled you. It’s a good trick. It looks like the player is moving forward. But in reality, the player stays still and everything else moves backwards.
So let’s make the pipes move. Just as we did with our bird we can create a speed variable for the pipes and use this to move them. We want the following line of code to run once when we run the program, so it needs to go at the end of your code:
scroll_speed = 1
And they need to move continously, so this code goes in the update function:
top_pipe.x += scroll_speed
bottom_pipe.x += scroll_speed
- Press play
Oh no! Why are the pipes moving the wrong way?
Can you fix it? Hint: when setting the speed, what’s the opposite of 1?
More pipes¶
We need more pipes, one set is not enough. But actually we have enough already, we can just loop them round when they go off the screen.
To do this, let’s meet the very handy if statement and two of its friends, greater-than > and less-than <…
What do you think this code does? Open a new Mu script and type it in:
a = 10
if a > 5:
print("Wow a is big")
- To run this you’ll need to save it first, just pick a filename such as
test.py. - Look at the bottom of the Mu editor to see the output from your code
What about this code?:
a = 10
if a < 5:
print("Wow!")
print("a is small")
print ("The End")
Try changing the a = 10 line to make all 3 print statements run
So as you can see (hopefully!) if tests something, in the first example if the variable a is greater than 5, and then does whatever you tell it to do.
There are two tricky things to get right with if statements:
- Exactly what are you testing? What goes after the
if? - Get your indentation right – how many spaces at the start of the line – so that the right code is run.
Looping the pipes¶
OK, let’s get to work in the update function, as that’s where we move the pipes. Add this code to the end of the function, and make sure you indent it (add spaces to the beginning of the line) so that it really is inside the function. Ask a mentor for help if this doesn’t make sense.
if top_pipe.x < 0:
top_pipe.x = WIDTH
OK, that’s not bad, but two problems…
- Only the top pipe moves
- The pipe dissappears too quickly, before it’s left the side of the screen
- Can you fix these issues?