Feature Reference - The Game Maker's Companion: Game Development - page 357

Line 4 sets up a variable delta that contains the difference in angle between the
direction the missile is currently going in and the direction it needs to turn into to face
the target, and we use point_direction to get this angle. If it's positive, it means the
missile must turn counter-clockwise (increase its angle of direction); if it's negative,
the missile must turn clockwise (decrease its angle of direction).
Line 5 checks if the difference is bigger than a 180-degree turn (the function abs
always returns a positive value, which helps us in our if statement). If it is, we invert
delta because it will be quicker to turn the other way!
Line 6 changes the direction with 4 degrees multiplied by the sign of delta : if delta
is negative, direction is lowered by 4; otherwise, it is increased by 4. We only do this,
however, if the difference is greater than 4, otherwise our missile will wiggle toward the
targeted direction. The final line adjusts the image's angle to the direction.
7. That's all we need to track it. Now for some destructive power, add a Collision event
with obj_asteroid and include a Create Effect action ( draw tab) with Type firework ,
Size small , and Where set to above objects , and check the Relative box.
8. Include a Play Sound action and set Sound to snd_explosion .
9. In the same event, include two Destroy Instance actions, one that Applies to Self and
one to the Other .
10. Finally, include an Other , Outside Room event and include a Wrap Screen action, in
both directions.

Result: Reference/Result/homing_missiles1.gmk

Try out a few shots, and notice how our missile never fails to meet its target—unless another
asteroid gets in the way! It even turns around if the asteroid wraps to the other side of the screen,
and if its target was destroyed by another missile, it just keeps flying straight until it hits
something. You'll notice that its effectiveness is dependent on the speed with which we allow it to
make turns—the slower we let it turn, the longer it takes before it is back on track toward its
target. Now it's up to you to invent all kinds of variations on the theme—do you want it to set a
different target if its target is destroyed? Or should it self-destruct immediately or after a given
amount of time? One cool variation is to make the missile heat-seeking, targeting anything that is
within a certain range.

Changing the Missile to Heat-Seeking the Nearest Object
1.

Open obj_missile and delete the Create event.

2. Select the Step event and open the Execute Code action. At line 2, insert the following

two lines:
target = instance_nearest(x,y,obj_asteroid);
if ( distance_to_object(target) > 100 ) target = noone;

Instead of setting a target only once in the Create event, this changes target to the
nearest instance every step. But if the target is more than 100 pixels away, we set it to
noone —which means that the missile will just fly straight without changing direction.

Result: Reference/Result/homing_missiles2.gmk