Creating hazards

So our player can move and shoot. That's great, but what fun is that when there are no obstacles? Hazards offer a great challenge and can even be the core of your gameplay. How boring would Mario Kart be without banana peels? Or Pitfall without…well, the pits? Video game hazards come in many forms and, if you play games, you've likely encountered most, if not all of them. Spikes, lasers, electrical traps, and falling objects are a few examples, but they have at least one thing in common: if your player character touches them, he/she will be hurt or killed. Now, in most games the latter isn't permanent, what with health bars and lives (we'll get into those later), but it remains a major part of the gameplay. Let's look at creating some simple hazards that can give your players a hard time.

Getting ready

In-game hazards can come in many forms and various visual styles. Like the projectiles we discussed previously they can be static images, but they look much better as animated sprites. We're going to build a spike trap; you can use your own art, but I've included an animated sprite in the downloadable files.

1. Create a sprite, name it spr_hazard_spike, and load the necessary images. Before we move, on you're going to need to set up the sprite's collision mask. Sprites are assigned a collision mask by default but you can modify it to your own needs. We only want the trap to be activated and for the player to take damage when the spikes are actually touched, as opposed to the area around the trap.
2. Under Collision Checking make sure Precise collision checking is selected.
3. Click Modify Mask and, under Shape, select Precise. To the left of that is a slider that allows you to specify Alpha Tolerance. This is useful because it allows you to shrink and grow the mask according to the sprite's outline (the empty space around it being an alpha map).

Once you save your selections we're ready to get started.

How to do it...

1. Begin by creating a new object called obj_hazard_spike and assigning the sprite you made previously.
2. Add a Create event and drag and drop two instances of Set Variable. One will set image_speed to 0, the other will set trap_set to 1.
3. Add a Collision event with obj_player as the target.
4. Drag and drop Test Variable to the Actions box, checking that trap_set is equal to 1.
5. Place both a Start and End Block in the Actions box. Within this block you'll need two Set Variables; one that sets trap_set to 0, and another that sets image_speed to 1.
6. Still within the block, set Alarm 0 to 60. Your Actions box should now look like this:
   

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7. Create the event for Alarm 0.
8. In the Actions box, drag and drop Set Variable, setting image_speed to 1.
9. Drag and drop Set Alarm, setting Alarm 1 to 30.
10. Add the event for Alarm 1, where you'll drag Set Variable and have it set trap_set to 1.
11. Add a Step Event and drag Test Variable to the Actions box and have it test whether or not image_index is equal to 4.
12. This should be followed by a Start and End Block. Within this block should be a Test Variable that checks whether or not alarm[0] is greater than 0, as well as another Start and End Block.
13. Within this block you'll need a Set Variable that sets image_speed to 0. When you're done, the Actions box should look like this:
    
14. Finally, click Add Event, then Other, and select Animation End.
15. All you need in the Actions box here is a Set Variable that sets image_speed to 0.

Got all that? It's a lot to do all at once, but we'll go through what it does, step by step. The good news is that you now have a spike trap that springs when touched by the player and stays sprung for a bit before retracting. There's a good reason behind this that we'll explore a little further once we get into player health.

How it works...

The trap you've just made uses collision detection and alarms to set and reset variables that control the trap's state. On creation, GameMaker sets the image_speed to 0, keeping the animation on the 1st frame, and establishes the variable trap_set, which will only be used by this particular instance of the spike trap. Think of trap_set as a sort of on/off switch, using binary (0 or 1) to dictate its current state. When the player collides with the trap, GameMaker checks whether or not the trap is on. If trap_set is equal to 1 (on), GameMaker proceeds to set image_speed to 1 (so the animation will begin to play), set Alarm 0 to 60 steps, and reset trap_set to 0 so that the spike trap will not be activated over and over again. This is to give the player a chance to move away from the trap to avoid being injured a second time. The Step Event checks every single step to see whether or not the sprite's animation has advanced to frame number 4. If it has, it then checks whether or not Alarm 0 is still counting down. If it is, image_speed is set to 0, so that the animation remains on the 4th frame (in the up position) until Alarm 0 is finished counting down. When it has, the image_speed is set back to 1, allowing the animation to continue, and Alarm 1 is set to 30 steps. The animation can continue (retracting the spikes) but once it reaches the last frame, the Animation End event then sets image_speed to 0 again, preventing the animation from looping. The last thing to happen in this long list of events is the end of Alarm 1, which simply gives trap_set a value of 1, thereby resetting the trap to be used again.

It's a lot to take in at once, but creating this trap is a great demonstration of the programming logic we're going to get into later on. For now, enjoy your new spike trap!