Crash Course 2: Invaders - Part 9

by Hectate (Updated on 2015-02-01)

Part 9: Keeping the Ship On Screen

You probably played around with the game so far and noticed something; the ship can go off the screen (and doesn't come back!). Let's fix that, we need to restrict the Ship’s position so it can’t go past the screen’s edge.

Step 66: Create a new When Updating Event for the Ship. We need to add an If and an Otherwise If block inside.

We will use the first to check if the Ship’s position on screen is beyond the left-most edge of the screen (an X position of 0). The second will check if it’s past the right-most edge (a value equal to the screen’s width) of the screen.

Note that it will need to take into account the Ship’s width. Stencyl measures an Actor’s x position from the left-most edge of the graphic; if we didn't account for the Ship's width, the right side of the ship could go further than the right side of the screen.

Step 67: For our conditions, we need to compare two numbers; the Ship's position and the left or right edge of the screen. Let's grab the Greater Than and the Less Than blocks from palette. You can find them in the Flow category, Conditions sub-category, in the Equality section.

Put the Less Than (shaped like < ) block in the top If block's empty space. Put the Greater Than (shaped like > ) in the second, Otherwise If block's empty space.

Step 68: As mentioned previously, one of the numbers we want to compare is the Ship's position. We need a block that tells us that, and we can find it under the Actor category, Position sub-category, Position section; visible as x of Self.

We want to drag two of these blocks out; one for each of our comparison blocks (the < and > blocks). We need one in the left side for both. We need one more change for these two blocks, we want them to say "x (on screen)" instead of just "x". We can click on each one and change them easily with the drop-down menus that appear.

Step 69: Now that we have our first value to compare, we need a second value to compare it against. For our first comparison, we are just going to compare to 0; since an X value of zero is the left side of the screen. All we have to do is type a 0 into the right half of the Less Than block.

The second, Greater Than, comparison will be slightly more complex, however. As previously noted, we have to account for the width of the actor on the right side. Let's get the following blocks and drop them side-by-side in the workspace.

  • Minus (found in Numbers & Text > Math > Arithmetic)
  • Screen Width (found in Scene > View > Screen Bounds)
  • Width of Self (found in Actor > Properties > Size)

We then need to drag the Screen Width into the left side of the Minus block, and the Width of Self into the right side. The result will be a stack of blocks that looks like the below image.

We need to drag this set of blocks into the other half of our second condition. Be sure to grab the Minus block to move them in, or we might only get one of the blocks inside of it instead.

Here's a image of what we should have so far.

Step 70: The goal of setting up these two conditions is to identify when the Ship has gone too far and has reached the edge of the screen. Since we now have those conditions set up, we can have something happen when they are true.

Given that our goal is to keep the Ship on the screen, we want to reset the position of our Ship Actor so that it is back on-screen anytime these conditions are met. To do so, we need two of the Set X to [ ] for Self blocks; one for each condition. Let's drag a couple out and put them where they belong.

Step 71: Just like with the comparison for our condition, the first value is rather easy. We just need to put a 1 into the block so it reads Set X to 1 for Self. Simply put, if our Ship is ever at 0 or less X position (further left than the left edge of the screen), we are going to force it back to 1 instead. This will prevent it from ever going off the left side of the screen.

Again, like our second comparison for our condition, the second value requires a few more blocks to do what we want it to. In this case, let's get the following blocks. Remember that you can ALT-drag on our existing blocks to duplicate them; you should do this to grab the entire stack out of the comparison.

  • Minus
  • Screen Width
  • Width of Self
  • Minus (yes, we need a second one)

Again, we need the same math to subtract the width of the Ship from the width of the Screen. This time though, we put that stack of blocks inside the left side of the second Minus block. On the right side, we enter a value of 1 so our Ship gets replaced 1 pixel inside of the right edge of the screen.

Step 72: Now we just need to take the above stack of blocks and insert it into the second Set X to [ ] for Self block in our conditionals. The complete set should look just like the image below.

Once you’ve got this new Event set up, test your game. If it’s working, and your Ship stays on screen, it’s time to move on to our next major Event, enabling our Ship to fire the Bullets we created earlier.



Click here to go to Part 10.

Disclaimer: Use comments to provide feedback and point out issues with the article (typo, wrong info, etc.). If you're seeking help for your game, please ask a question on the forums. Thanks!


Andre Fernando

My ship can not reach the limits on the right side. Everything is correct but the ship only goes to a little of the half of the right side as if it already was the edge there.

0 1 week, 2 days ago
I also had the problem with the right corner. Found out that I had mistakenly used "X-center" instead of "X(on screen)" in the left comparison block for "otherwise if".
0 1 week, 3 days ago
As a test project I wanted to do a simple platformer, so I started to build on the platformer level you can download from here.
I've made a bunch of self made sprites for the regular idle animations for left and right, and the walk cycles for walking in those directions too.
But as I put the movements and such in place for it and added the sprites in, the character just slides around in the idle animation, or spins around really fast!
And then I took the already made example player, replaces it's sprites with my new ones (just to see if that's give any hints to how to solve the original one I was making), and when done, it started to slide and spin as well despite working perfectly with the original sprites!
The hitboxes between the animations are different sizes, could that have something to do with it?

- Je

0 3 months, 3 weeks ago
******************* To fix the the problem with the right corner, just set the value to minus one (-1) . I found no need to calculate (screen width minus ship width) minus one ****************************
0 7 months, 1 week ago
ship trapped in the right corner.
0 9 months, 3 weeks ago
My own self-created shop, the result is: ship does not move to the screen was marginal.
0 9 months, 3 weeks ago
great tutorial for newbie. thanks
0 10 months, 2 weeks ago
Very understandable for newbie. Made some mistakes,but corrected easily.I get stuck on translation differences "otherwise if" and "if".

0 10 months, 4 weeks ago
when i click test the game I get an error telling me I nee to import an OGG sounds
whats wrong?

1 1 year, 1 month ago
I do not seem to have the ability to choose "SELF" I have been able to limp along without it, as the first behaviors dealt with the (Singleton) ship, but, when dealing with bullets or aliens, i'll need SELF
0 1 year, 1 month ago

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