Stomp, don't sprint!

Publication date: 3 August 2012
Originally published, in a much shorter version, 2012 in Atomic: Maximum Power Computing
Last modified 29-Nov-2015.

I love giant robots. Who doesn't love giant robots? Strange little people with no friends, that's who.

But there is a problem with a lot of giant robots:

They're not actually giant. Not really. There's something wrong with their physics.

There are lots of reasons why the giant robots of comic books, cartoons and tabletop and computer games couldn't really work in the real world. The bigger your Gigantor or BattleMech or Metal Gear is, the more outrageous become the requirements for the materials, motors and power supply. And, in the end, you get something that isn't necessarily even very useful; it's difficult to argue that a BattleMech wouldn't, in reality, be easy meat for tracked, wheeled and flying vehicles built with similar technology.

But never mind that. Superman can fly, the Hulk's purple pants are magic, and BattleMechs are useful weapons. I'm fine with all that.

The physics problem, rather, is gravity.

Earth gravity is about 9.8 metres per second squared. If you drop an object near the surface of the earth, and it's got enough room to fall, it'll be moving at 9.8 metres per second after the first second of falling (ignoring air resistance), 19.6 m/s after second two, and so on.

Gravity is, of course, the same for everybody. Ants, people, buildings, fictional giant robots. (As long as said robots are not operating on a planet with lower or higher gravity, of course, which they almost always aren't.)

If you're a ten-millimetre-long ant, nine point eight metres per second squared is 980 times your body length per second squared. If you stand on your hind legs and tip forward a little bit, you'll be on the ground almost instantly.

(The same goes for humans shrunk to the size of ants or smaller, as in Fantastic Voyage. Unless your mind is accelerated by the same factor by which your body has been shrunk, walking will be impossible.)

When a two-metre-tall human tilts forward, they'll take 200 times as long to hit the ground as a ten-millimetre-tall ant. Which is just as well, since the square-cube law makes humans a lot more susceptible to falling damage than ants; when we do fall over, we need time to try to get our arms out in front of us.

For another example, look at the lava bombs falling from this volcano:

That's not in slow motion. The scale is just really big. Proportionally speaking, larger things fall slower in the same gravitational field. It takes them longer to fall a distance equal to their own length.

OK, now let's consider a 20-metre-tall giant robot. If your brain's transplanted into this magnificent body, standard gravitational acceleration will now look, to you, the same as a tenth of a gee would, to a two-metre human. That's less than lunar gravity. So, in order to walk with a human-like gait, you'd have to take proportionally slower steps, because walking is essentially a controlled fall. Trying to walk at a normal human pace will leave your huge titanium legs flapping around in the air, as you majestically fall over.

UPDATE: Here's a great demonstration of this, using a physics simulation to show what you have to do to get human-shaped things, from human-sized to more than a kilometre in height, to all walk in the same gravity without falling over:

Even the human-sized walk looks clumsy because of the limitations of the simulation software, but you get the idea.

Not many people have had the chance to walk in lunar gravity, and the only people we've even seen doing it have been wearing clumsy space suits that'll make you walk funny under any gravity. The most relatable analogy for non-astronauts is walking along the bottom of a swimming pool, perhaps while holding some small weights to keep you down and give you a tenth of your out-of-water weight. Even if you didn't have the resistance of the water to contend with, you couldn't walk at a normal pace down there; you'd keep bouncing up off the bottom.

You could get to a reasonable speed if you leant way forward so you could push back on the bottom of the pool much more than upwards; a similarly alarming stance could allow a human-shaped giant robot to make good speed. (Until it tripped over something.) But a giant robot trying to do that, like someone in an old-fashioned diving suit trying to run across the bottom of the ocean, would probably just dig trenches with its feet.

Some giant-robot scenarios take this into account, one way or another. But most of them don't. In most of them, the robots walk and run pretty much like humans - perhaps a bit slower, but not nearly slowly enough to match their size.

The result is like using player.setscale in a Bethesda game to become a giant:

You still move just as you did before, so it looks as if the world's shrunk, not as if you've grown.

OK, now let's look at "MechWarrior: Phoenix", the finest trailer ever created for a game that does not exist.

(That trailer was made for and/or by the people who are now finally making MechWarrior Online, and it's available on their site - with bonus animatics! But it has nothing much to do with the actual game in development. UPDATE: As of November 2012, Mechwarrior Online is in open beta, and stuff you earn in the open beta will carry over into the full game. It's an Allegedly Free Game that really is playable for free without getting steamrolled by paying customers. Except for your first few... dozen... fights, when even if you pay $30 to jump straight into the most expensive Atlas, you won't know what you're doing and will die in embarrassing ways. All Mechwarrior Online is so far is a basic multiplayer online giant-robot team deathmatch, but it's a really good one. And free. Highly recommended.)

In the "Mechwarrior: Phoenix" video, a building collapses, and a 'Mech on its roof falls into it, way too fast. (Unless, again, this planet has much higher than normal gravity.)

The walk animations for the Warhammer (the 'Mech getting its arse kicked) and the Atlas (the 'Mech doing the kicking) don't move the torso up and down much, but they do it a bit, and the centre of gravity of each machine then falls too fast to get the next foot onto the ground. The walk animation on the Warhammer as it backs up, though, is really good. The torso stays pretty much level, sort of making the 'Mech look as if it's tiptoeing, but it's physically plausible.

Now, look at the walking animation for the actual MechWarrior Online version of the Atlas:

It doesn't raise its feet very far, but gravity does have to pull it down for each new step, and that happens way too quickly for a machine that's meant to be at least 15 metres tall. (Again, unless this planet has high gravity, which raises the question of why anybody's fighting over ownership of a planet where only powerlifters are able to walk.)

Here's MechWarrior Online's 35-ton high-speed Jenner:

It has a pretty human-like gait, pushing itself into the air with each stride, and again it gets magically sucked back down onto the ground in time for the next step, instead of bounding through the air like a kangaroo. It comes back down from a jump-jet leap too fast, too. But when a dead 'Mech falls over, or a piece gets shot off one, it falls at a more realistic speed.

While we're ogling giant walkers that have strong disagreements with each other, behold the generally magnificent Code Guardian:

It's deliberately cartoony, but it still has the gravity problem. Whether the robot is gigantic, or close to human in scale...

(World War.)

...it'll move like a human.

The game I first really saw this in was Shogo: Mobile Armor Division back in 1998.

That game has on-foot and giant-robot sections, and they look and feel the same.

(The standard old-First-Person-Shooter "skating" effect doesn't help at all: As with numerous other games of that vintage, contact between anybody's feet and the ground has a purely coincidental relationship with their movement.)

I mean, seriously. Clearly, this is depicting a human-sized dude on rollerskates who's occasionally called upon to shoot a tank the size of a shoebox.

The gigantic... let's call them robots... in Neon Genesis Evangelion move like people too, despite being big enough to use skyscrapers as furniture.

(Well, sometimes they're 200 metres tall, sometimes they're 40. Wildly varying scale of ships and monsters and other things created by special effects is a common sci-fi problem.)

The Evas have magical hypertechnology, so maybe they're sucking themselves onto the ground with a tractor beam, or something. This could also account for the whole "skating robot" genre of games and anime, your Armored Cores and such...

...which I try to pretend does not exist.

Front Mission switches it up, with some stomping, plus a lot of skating.

But magical ultratech is clearly not the case for more "realistic" giant-robot scenarios, definitely including the "dieselpunk" mech scenarios like Gear Krieg or Steel Battalion.

The Steel Battalion games do it pretty well, actually, though serious questions need to be asked about how a bipedal mecha with the profile of a soccer goal manages to stay upright when it lifts one foot off the ground. The various MechWarriors, and Chromehounds...

...are also not too bad, Chromehounds partly because it lets you avoid the whole problem by just putting tracks or wheels on your mech. If you don't, though, the faster Chromehounds mechs still scurry around as if they're about four feet tall:

(And, sometimes, Chromehounds players managed to exploit a glitch and create a mech that looks like a block of flats on teeny little legs but zooms around like Speedy Gonzales. And then, there's whatever this is.)

(Oh, and there's even a flipside version of this: Unexplained moon-gravity in human-scale games. My favourite example of this is Carmageddon II.)

In Space Marine, the Titan Invictus more or less moves as a BattleMech should:

But, as a middle-sized Titan in Warhammer's deliberately over-the-top universe, it's meant to be about four times the height of a 'Mech.

Any walking machine that can plausibly push its feet down onto the ground, like the Warhammer in reverse in the MechWarrior: Phoenix video, can avoid this problem entirely. Star Wars AT-ATs do this; they're quadrupeds that only lift one foot at a time. The zippy chicken-walking AT-STs seen briefly in The Empire Strikes Back and humiliated by cavemen in Return of the Jedi are pretty good, too; they're strangely twitchy, but they don't seem to do anything very outrageous with their centre of gravity.

Now, for comparison, look at just about any old guy-in-a-rubber-suit giant monster movie. Those create, or at least attempt to create, the impression of great size by overcranking the camera when they filmed Haruo Nakajima in his Gojira suit, and then playing the film back at normal speed. The slow-down usually wasn't enough, but at least they were trying.

Animations and computer games aren't restricted by the actual size of a guy in a suit, the maximum speed you can run a film camera, or the limitations of model-making, so they could do even better. But they seldom do.

For games, at least, there is a very good reason for this: Playability.

Realistic locomotion for very large walking machines, and fun gameplay in an action game where you pilot said machines, are fundamentally incompatible. Most players aren't interested in controlling something that steers like an ocean liner, and they really don't want to control something that, if proper physics is obeyed, will simply fall the hell over if you suddenly change direction.

This creates a problem. When you're driving your own BattleMech in a first-person game, you can't see the machine's legs and probably don't much care what they're doing. But you can see other 'Mechs, and you'll be annoyed if you've lined up a perfect shot, leading your target by exactly the right amount, only to have the target suddenly magically flick from moving one way to moving another. It's sort of the opposite of the reason why rocket projectiles in first-person shooters don't move nearly as fast as rocket projectiles in the real world. Realistically-fast rockets would be overpowered weapons unless you reduce their damage until they just work like ordinary guns with different graphics, and instant course changes would make hitting enemies excessively difficult.

So giant robots in action games shouldn't be able to motor around like the Doom Marine...

...or they won't look or feel giant in any way. They'll just be re-dressed humans, like the mecha in Shogo or Evangelion.

But they also need to be responsive to player commands, and not as realistically ponderous as, say, the mecha in Ring of Red...

...which is a turn-based game with a bit of golf-game-like clicking. The 3D video is just for decoration.

The solution to this problem is what we get: Basically, walking-robot physics subjected to the same cheats and workarounds you see applied to car-handling physics in arcade driving games and most GTA-like games.

Forget realistic physics (which is fiendishly difficult to simulate at the best of times; adding more "realism" to physics engines can easily produce less realistic results). Simplify to make it fun. Try not to make it look too silly as a result.

I think it's an underestimation of the intelligence of consumers of sci-fi movies and TV to make giant robots move very much like human beings; present it correctly, and people can figure it out. See, for instance, the 2005 War of the Worlds, which has immensely tall bendy-legged tripod baddies with a walk realistic for their size, and in which there are almost no scenes not shot from a human viewpoint. The whole film, for me, looked far more "real" as a result of this.

And there's no reason why you can't have realistically ponderous mecha-walking combined with explosively fast movement of any parts of the mecha not dependant upon gravity. This makes possible scenes like the famous (in certain circles) Brutal Punch BattleTech picture. But no, Reality Is Unrealistic, so Space must Be Slow Motion, and 'Mechs Must Walk Like People.

The result certainly doesn't have to be bad, though. MechWarrior Online is looking great even though its 'Mechs walk too much like people, and it's hard to criticise the look of the upcoming Hawken either...