Let's see how to create a trail behind a moving instance. In this example, we'll use a dotted trail behind a bee!

You might want to be familiar with the following concepts and functions for this example:

instance_create_depth()
image_angle
point_distance()
instance_number()

It's good to break down the behavior we want to see into smaller pieces. Let's think about what we want to make, and that should give us some clues.

Create a dash-mark at the bee's current position.
Make sure the dashes have enough distance between them before creating another.
Angle the dashes to match the bee's angle.
Limit the number of dashes so the oldest dashes will disappear first.

The words in bold give me clues about what kind of functions I'll need to get the job done. Now let's check out some code!

CREATE event in obj_bee:

last_dash_instance = instance_create_depth(x,y,depth+1, obj_dash);

We're creating an instance of a single dash and placing it under the bee right as it's created. The obj_dash is just a sprite of a single dash-mark. We'll store a reference to it in a variable called last_dash_instance so that in the STEP event we check when we're too far away from it and make a new one.

STEP event in obj_bee:

var _dash_distance = 96;
if point_distance(x, y, last_dash_instance.x, last_dash_instance.y) > _dash_distance 
{
	last_dash_instance = instance_create_depth(x,y,depth, obj_dash);
	last_dash_instance.image_angle = image_angle;
		
	if instance_number(obj_dash) > 8
	{
		var _dash = instance_find(obj_dash, 0);
		instance_destroy(_dash);
	}
}

We'll store a local variable called _dash_distance to determine how close the dashes can be placed together. I've put 96 here, but you can put a bigger number for a bigger gaps between dashes, or a smaller number for smaller gaps.

Now, we're just checking if the distance between the bee and the newest created dash-mark is greater than _dash_distance. If it is, we're going to our variable last_dash_instance to a newly created instance of obj_dash, and set the new dark-mark's angle to that of the bee.

We then check if there's more than 8 instances of obj_dash on-screen. If there are, we use instance_find to find the oldest one, then we'll destroy it. This will make sure only the 8 most recently created instances of obj_dash will ever be trailing behind the bee.

It's a simple effect that looks nice behind bugs, leaves, spaceships, and other things that move across the sky.

Make a Dashed Trail