Mutant Design and Behavior

I’d like to talk a little about the Mutant creature – the first creature (and first enemy) the player encounters in Singularity. Since this was our first chance to expose the player to the weird creatures of Katorga-12, we wanted to make it memorable. I’ll talk a little about the look of the character but I’d mainly like to talk about how its behavior was developed, and what makes it unique.

Zombie or Mutant?

This creature was basically supposed to be a human who was mutated by E-99. Our main focus when designing this character was to make him creepy and scary. The first attempt at a look for it was a more traditional zombie-like character:

We used this model for a while but eventually realized we needed something different. The creature we created wasn’t scary – if anything, it was a bit goofy looking when you dropped it in a level and let it run around.

The final version looks a lot less human. It looks more sickly and mutated, with long gangly arms and legs.

Behaviors

The behavior for the Mutant started out simple. It had two possible states: a Circle state where it would circle strafe around the player, and a Melee state where it would move towards the player and attack when it gets close enough. If there are multiple mutants, we only allow two of them to be in Melee at the same time. Once a mutant closes in and attacks you, he plays a “retreat” animation that takes him out of melee range. What all this means is that when the player is fighting a bunch of mutants, you will see them taking turns moving in to attack, backing away, and circling around.

This made the creature functional, but it did not make it scary. They were very slow and it was easy to avoid their attacks. So the next thing we added is a sprint for them. The AI for the mutant would continually check if it should be running towards you. This was random but it was also modified by certain things. For example, if the mutant was farther away from you, or if you had recently damaged it, it had a higher chance to sprint towards you. We then gave the Mutant a jump and leap attack that it could use while sprinting. The result is that you would have a pack of slow moving Mutants, but every so often, one would surprise you and run full speed towards you.

Dismemberment

The next thing we decided to do with the Mutants is add a limb-severing system. They are supposed to be badly mutated and fragile, so we wanted to convey that this creature was barely holding together. We already had the ability to sever limbs when an enemy died – for example, if the killing shot hit an arm, the arm would fly off as the enemy died. What I did was take that system and extend it one step further. I added the ability to assign hit points to individual limbs. So the Mutant has a general hit point pool for the creature to determine when he dies, and then each limb has its own amount of hit points. When the creature takes damage, it checks to see if you hit any of those limbs and applies the damage directly to that limb’s hit points. When the limb reaches 0 hit points, it is detached from the body and flies off, accompanied by blood effects.

This added some interesting challenges though, because now we could get creatures that were still alive but missing limbs. For example, the Mutant’s melee attacks needed to have knowledge of what limbs were remaining, so that you wouldn’t get a mutant trying to do an attack with his right arm when it had already been blown off. Since the player could shoot off both the right and the left arms, we had to give the Mutant a “bite” attack to use as a last resort.

The most interesting challenge was what to do when you blew off a leg. You obviously couldn’t have the Mutant continue to walk around. So we ended up doing two things. When a leg is reduced to 0 hit points, we play an animation of the Mutant falling to the ground. This gets him from standing to being on the ground. Then we replace his normal movement with a crawling animation and force him to just continually crawl towards the player and try to attack.

This made the creatures a lot more dynamic. If a mutant falls to the ground, is it really dead? Or is it going to get up and start crawling towards you?

After this whole process was done, we had a dynamic, unpredictable creature that should provide an interesting challenge for the player. Oh, and watch out for when it tries to tackle you…

Mario Giombi
Software Engineer
Raven Software