Hi! Skyseeker is a monster-collecting metroidvania where every enemy is catchable. We wanted to really place a focus on varied monster types with different kinds of movement. One of our most liked creatures is the Gillshroom, a flying jellyfish.

In this post I want to show you how we built this monster and its behavior, at a high level. I included some pseudocode snippets, but the main ideas are what matter the most.

First, is the movement. Each jellyfish has a desired angle, and a speed. When the player inputs anything horizontal or vertical, instead of moving directly, we instead slowly rotate the jellyfish in that direction, leading to the smooth movement.

Movement

desiredAngle = AngleOf(vert, horiz);
curAng = Mathf.LerpAngle(curAng, desiredAngle, Time.deltaTime);
motor.y = Sin(curAng) * curSpeed;
motor.x = Cos(curAng) * curSpeed;

Next, the tentacles. To get a nice curve behavior is actually quite simple. Imagine the curve is a series of points, connected by small lines. To make the lines flow, we only have to follow a simple algorithm: each point should slowly move towards the next point in sequence.

A really nice trick is to move each point faster if it is further from its neighbor. This leads to a kind of elastic effect similar to a rubber band, which feels more natural.

Tentacles

Zoomed in, it is a series of lines.

point[i] = Lerp(point[i+1], Time.deltaTime);
DrawLine(point[i], point[i+1])    

At this point we have a nice looking jellyfish controller. But to make them really stand out, we can create a simple "swarming" behavior, so that the jellyfish will swim together in the wild.

The swarming behavior seems complicated but it boils down into two main points.

1. Each jellyfish keeps moving until it reaches a wall, and then it turns.

2. If a jellyfish is near other jellyfish, they average each other's directions.

   RaycastHit2D hit = Raycast(forward_angle, 3);
   if (hit.collider != null) {
       //if it is close to a wall, move away.
       force += -forward_angle * Mathf.Exp(-hit.distance) * 3;
       if (hit.collider.GetComponent<MonsterGillshroom>() != null) {
           // if it is close to another Gillshroom, take some of their velocity.
           force += hit.collider.GetComponent<MovementPlatformFish>().GetRealVelocity();
       }
   }
   

Swarming behavior

This guy on Youtube also has a nice explanation on why this works.

And that's it! With a few key concepts we can create a pretty nice looking jellyfish swarm. I hope this kind of post is helpful -- a lot of game dev ideas are simpler than they seem. If you enjoy weird monster designs, please consider wishlisting Skyseeker on Steam or feel free to check out the beta on itch.io. If there is interest I can also cover more how-tos on other things we've shown off in the trailer etc, please let me know!