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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!


at acceleron) for me it was un-nessasary to explain about the < and > as that is primary school level maths but i could see how it would b helpful for some people

i fully agree with this i would also like to know why once the ship goes offscreen it cant come back on

i want to add a couple suggestions aswell first couldnt [y of self] be used to simplify the code?
and second inside the otherwise if statement it needs to be 1 instead of -1 because say the ships width is 35 your making x 34 pixels from the edge which is actually one pixel over the edge
this is a minor issue but just though i might point it out =)

0 11 years, 10 months ago
I agree with pgarc7a that this guide isn't specific enough and leaves a lot of room for potential error on the user's side.

I would also like to learn more about why the Ship stops working when it's moved off screen. Perhaps the guide could offer a brief explanation of this, or link to another article that might help explain it?

0 11 years, 11 months ago
i gotta make a humble recommendation,in step 66 its not clear whats to be chosen, for example in the first "if", if one has little idea of maths, he'll not be looking for the green "0(0", and also may not figure to choose the "x of self" from the menu in actor, also in "otherwise if", first one has to choose "0)0", the first block is fine cos is "x of self" again, but in the second block another 2 blocks "0-0" has to be chosen, being the first "screen width" and the other "width of self", im good visually and i remember some maths, but other person wont uderstand it for sure, and i think id take little for you stencyl guys to change it, sorry my english
1 11 years, 11 months ago
0 11 years, 11 months ago
Make sure that you have added the above code to the "ship" itself, not any other actor or "level one". Also I know you said the code is the same, but it's easy to accidentally use "x-speed" instead of "x" or have the > and < the wrong way.
0 12 years, 1 day ago
@ohlachbiz: My guess would be that you are setting the x position of the ship to something offscreen. If you can take a screenshot of you event and post it somewhere, I'm sure someone can figure it out.
0 12 years, 3 days ago
can anybody help me. I have but the exact same code in but every time i go to the right of the screen i keep disappearing
0 12 years, 1 week ago

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