2. Create a Scrolling Backdrop

In this screencast, you will learn how scrolling backdrops work, then, you’ll program your own background sprites to scroll. Watch this screencast first to learn what to do, then you’ll get the chance to try programming it on your own.

In this starter project, three different cave sprites act as the background. The first cave sprite is red, the second is blue, and the third is green.

In this example, you can see that one cave sprite closely follows the next, forming what looks like a continuous, scrolling background. In the starter project, you’ll notice that there is already code created for each cave sprite.

This code starts the first cave at x = 480, the second at 2 times 480, and the third at 3 times 480.

This code also contains a variable: distance traveled. Each time these loops run, each sprite goes to a new position based on the value of distance traveled.

You can imagine how this might look outside of the Scratch viewer. Imagine you have three sprites that are all 480 pixels wide. You need to set the first to start with an x value of 480, the second to start with an x value of 2 times 480 (or 960), and the third to start at an x value of 3 times 480 (or 1,140). To move these sprites, you’ll need to use a variable called “distance traveled,” which will go up continually as the sprite moves.

To get your backdrops to scroll, you’ll need to program the distance traveled variable to increase throughout the game. To start, click the player sprite and drag out a “change distance traveled block” from the data menu. The cave sprites start running their code when the flag is clicked, so to test how this block works with the cave sprites, add a flag event here as well.

Click the flag a few times to test this code.

You should see that each time the program runs, the value of “distance traveled” goes up by 1 and the caves move 1 step to the left.

If you try a higher value, like 50, you can scroll through all of the caves just by running the code a few times.

Great, now you need to program the computer to increase the value of distance traveled repeatedly.

Add a forever loop to your block stack and try that.

Whoa, that scrolled really fast! This program can only be run one time.

If you try it again, nothing happens. This is a common type of bug that computer scientists encounter. The distance traveled variable holds a value. When you start the game, the variable is still holding the value of distance traveled from the last game. To fix this, add a “set distance traveled to 0” block that will run right after the flag is clicked. Try that.

Nice work! Now distance traveled starts at 0, and the computer is constantly increasing its value. What do you think about the speed at which the background is changing? Too slow?

Too fast? Tinker with the value in the “change distance traveled by” block until the sprites scroll across the screen at a pace you like.

In the next screencast, you’ll control the rising and falling of the player sprite with an if-else statement.