This is The Mechanic, where Alex Wiltshire invites developers to discuss the inner workings of their games. This time, Doom [official site].

Doom, the new one, has one heck of a sense of forward momentum. It’s a game of aggression and constant movement. You’re the Doom Marine: you move like the wind and your shots are unbroken by the need to reload.

At the heart of how Doom creates this response in players is a single feature which, paradoxically, is all about pausing your interaction with the game, pressing you so close to the enemy that they often fill the screen. It’s a feature, after all, that was intended to capture something special about the original Doom that had little to do with movement, but it turned out to trigger all kinds of secondary effects. The feature was:

THE MECHANIC: Glory kills

The origin of glory kills lies in an animatic, a visual prototype which was made by id’s animation team during the great overhaul of the Doom project some three years ago. The aim was to finally finish the game by bringing Doom back to its roots, and for animation director Shinichiro Hara, that was all about one thing.

“To me, when I hear Doom, what comes to my mind is the melting demon stuff. Shoot the enemy and he just melts,” he tells me. The stuff, in fact, that Brutal Doom is all about. So they built a prototype which was built on the elaborate hit detection features they’d made for Rage, which they’d recently launched, with the addition of a new gore system. Shoot an enemy in the shoulder and it would spin and fall and then try to stand up again, hit it harder in a leg and it might blow it clean off.

But it wasn’t working. “I just felt like the actual visual wasn’t there,” Hara says. “The way you remember Doom, you remember the melting demon right in front of your face, but the enemies were still AI, small, and the level of detail was getting lost. They just didn’t have that impact.” And so they tried something else, bringing the action right up into the player’s face, driven through a set animation, and showed the concept off in the animatic, which featured what game director Marty Stratton calls “synced moments of melee between the player and the enemy”.

Taking control from the player was a controversial concept. “We do everything we can to not take player control away, throughout the game, whether for a cinematic or anything,” says Stratton.

“But I wasn’t too concerned about that, because it’s kind of like the throw moves in fighting games, you know?” Hara says. If you decide to hit the button combination for a throw in a fighting game, you know you’re happy to commit yourself to watch a short animation of your character executing the action. For Hara, these moments of synched melee would work the same way, and besides, the effect was just too good. “You could really show off, like an enemy shattering in front of your camera, and to me that was the right direction to go in.”

The length of these sequences was crucial. A lot of the animatic’s animations were about three seconds long, which didn’t feel good to play once they were incorporated in a playable prototype. The team went through many iterations, reducing the timings and tuning them so they balanced how long players lost control against expressive animation. “It was a challenge,” says Hara. “How can we store the same amount of information and deliver it to the player?” If you’re looking for real artistry in animation, Doom’s glory kills are where it’s at. They’re pulling elaborate moves in just 30 to 60 frames, or between a second and a second and a half at the outside.

They’re also aided by a detail that helps them flow into play. The death animation can continue after the player has been disconnected from the sequence. So you’ve torn a mancubus’ heart out and stuffed it down its throat in about 45 frames, and at this point you’re free to move again, but the mancubus will still be going through its death sequence, ending in blowing up. “Or maybe the player punches twice and then the enemy’s head blows up and it runs around without a head for a while, whether players look at that or not. Adding that was a pretty huge leap, actually, because we could still have a lot of cool presentation, but without comprising the gameplay,” Hara says.

The next challenge with glory kills was one of disorientation and even, in Hara’s case, nausea (despite working at the most celebrated FPS studio in the world, he experiences motion sickness in FPSes). It was caused by the camera exiting the animation looking in an unexpected direction. Rage’s hit detection would again provide the answer. Part of the system enabled the game to query where the player was looking at; in Rage it meant the game could play a specific animation dependent on where an enemy was hit, and for Doom it’s coerced into triggering a specific glory kill animation depending on which part of the enemy the player is looking at, and also into ending the animation with the same view. “By doing that, it made it a lot more seamless, but we also managed to add more variations without using randomised stuff. I don’t really like randomisation, because it’s not really player choice,” Hara says. “In this system it’s the player’s choice to to rip an arm off and smack them with their own arm.” He lets out this irrepressible giggle at the thought of it.

This was where glory kills almost got out of hand, because suddenly different animations were required for each part of each enemy’s body. I ask how many animations were made, and Hara bursts out laughing again. “I don’t know, man!” “I think it’s safe to say, so many,” says Stratton. Committing to the set of features that were rapidly growing around glory kills was a big decision for the team to have to make, because it would define a great deal of how the game would have to be developed. “Per enemy at least you have…” Hara counts them up. “12 glory kill animations. Stunned is one. And then special ones like chainsaw or berserk. Actually, I take it back. I think it was 16 per enemy, because you have directionals.” Later he corrects himself again when he remembers there are animations that play if enemies are close to walls.

And numbers aside, the animations made big demands of character rigging, because they had to be able to be torn in two, blown into chunks, have limbs detached, splint down the middle. The sheer amount of complexity also presented issues for the game in general. “It’s one of those things, it takes away a lot of your flexibility as you want to make changes down the road,” says Stratton. “There were times when we wanted to make a change to a character model or something like that. You basically get to this point of no return a lot of times.”

“We did a kind of scary thing from a dev’s point of view: we had to be right, right from the start,” says Hara. The complexities even came close to preventing the chainsaw from being part of the game, simply because of the trouble of integrating a set of animations of enemies being cut in half.

Another feature of the glory kill system is the stagger mechanic, which came from a different thread of ideas that had origins in a problem that id had with Rage. They’d seen how players tended to react to skinny mutants, which were quick melee enemies, by shooting while running backwards. And yet many of Doom’s enemies are melee-based. How to keep players moving forward? The answer was adding a stagger state to enemies when they reach a certain percentage of their health, as inspired by Resident Evil 4. “It’s my very favourite game,” says Hara. Shoot one of Resi’s ganadoes in the foot and it’ll grab it, initiating a state in which you can perform a special melee attack which leaves it lying on the ground, enabling you to slash at it with your knife. “It’s very satisfying because you save ammo, but it’s not Doom,” Hara says. For Doom they wanted it to be a guaranteed kill, so it was integrated with the glory kill system, promoting that forward motion as you fire on enemies, stagger them and finish them off in a single hit.

Reinforcing this yet further is another complementary system which drops health and ammo after a glory kill, giving these aggressive pushes a payoff, closing a virtuous circle of violence. But they’re balanced so that players are kept on the edge, teetering on life and never fully stocked after a period of combat. “We want them to use that downtime once out of combat to look for secrets, larger health and ammo packs, better ways to upgrade themselves,” says Stratton, referencing exploration, another piece of the Doom archetype that they wanted the new game to capture.

There are other, subtler and strategic effects to the glory kill system, too. You’ll find yourself farming enemies for health and ammo drops, taking care to avoid causing too much damage to weak zombies and imps. The animation introduces moments of invulnerability that can shield you against attacks and give you thinking time before you make your next move. The way glory kills can be triggered from a short distance away from the enemy can also allow you to cover ground quicker than running, especially if you equip Seek and Destroy, a rune which increases the trigger distance. “It became a central part of the game early on and spawned a lot of ideas for runes and a ton of stuff that really is the bread and butter of the game,” says Stratton. If you really like glory killing, there’s even a rune that makes staggered enemies next to invulnerable, so you don’t accidentally finish them off before initiating their glory kill.

In fact, the glory kill became so central to the game that it even plays into the characterisation of the Marine, introducing brevity and savagery into his movements. Stratton laughs. “Well, he kind of glory kills everything.” That moment of loss of control turned out to have extraordinary reach.