#simulation for simulation''s sake

[askagamedev]

My Twitter is melting down over Tears of the Kingdom having a chain wrap around a wheel to lift a door open. Can you explain why senior programmers are calling this “black magic”?

Physics simulation starts off simple when you have simple situations, but each additional factor creates additional layers of complexity that the simulation might not handle all that well. Let me try to give you an example of such.

Let's go with a basic physics problem - I am standing still and drop a ball I am holding in front of me. What should happen? The ball starts with a velocity of 0, gravity pulls it downward, and it falls over time. Creating a general equation to simulate this is not that difficult - it's pretty basic physics.

Now let's add another layer of complexity. I'm still holding the same ball, but now I am standing on a mine car that is moving at a constant velocity along a straight rail. I drop the ball over the edge of the mine car. What should happen? The movement of the ball should take the movement of the mine car into account in addition to gravity, causing it to move differently from the above example. Creating a general equation to simulate this result is much more difficult.

And then we add another layer of complexity. I'm holding the same ball and standing on the same mine cart, but instead of the mine cart moving at a constant velocity, the cart got a big initial push and is now slowing down. In addition, we're no longer on a straight rail, but we're actually going in circles along a circular rail. I drop the ball over the side of the mine cart. What should happen? How many different factors must the physics simulation take into account in order to calculate the result correctly?

We can make this even more complex. Instead of dropping the ball, let's throw the ball. Instead of it being a ball, let's make it bola - a rope with balls on either end - that we're twirling over our heads before we release it. And we'll be doing the release while on an accelerating mine cart on a circular rail.

What the engineers on Tears of the Kingdom did was build a physics simulation system that can elegantly handle multiple layers of complexity. That's an impressive engineering feat, which is why so many skilled engineers are impressed by it. It's very similar to the fawning over the rope physics from the Last of Us Part 2 a few years back, and for the same reason - it's an exhibition of physics simulation elegantly handling multiple layers of complexity.

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[askagamedev]

Hi Dev. Pardon if I'm imposing. Have you ever seen "Firearms Expert Reacts"? It's a very interesting and educational series of YouTube videos where John Ferguson, the director of the Royal Armories Museum, reacts to video game firearms. He raises curious points that I would love an extended perspective on from an experienced dev like you. The Counter Strike and R6 Seige videos are great ones.

I've seen a number of <Some Expert Reacts To Things In Some Medium> videos about a number of topics, and the general gist is usually the same - real life goals tend to be different than goals in a different medium (narrative, gameplay, etc.), which means that factual accuracy often takes a back seat to the needs of the narrative/gameplay/etc. or even the needs of the schedule. Accuracy mistakes tend to fall into one of two major buckets - either we knew and we did it on purpose because we needed it for gameplay reasons, or we just didn’t know about it. It’s something of a mirror of yesterday’s post about the film Free Guy - our primary goal is to create compelling gameplay, and that can often require us to sacrifice real life accuracy.

In real life, the difference between firearms tends to be the way they are constructed and their firing behavior. The way they kick when fired, the weight, the placement of mass, the size, and so on all affect how a gun handles. In a game, however, we all use the same body and there is no sense of weight. Guns have to function differently or they’re basically just different skins of each other. If we offer a choice of guns, the guns must feel and play distinctly from each other in order to make the choice of weapon meaningful. This necessity often sacrifices real life accuracy like firing rate, grouping, reload speed, ammunition caliber, and so on.

When it comes to the artistic depictions, we’re usually operating off of reference photos rather than being able to examine the actual weapons. Artists are not often experienced with actual firearms and do not have weapon engineering experience, so they often have to guess about what the various small pieces of real-life weaponry do. Reference is often incomplete since we can see what it looks like, but not have any real clear idea how much it weighs, where that weight is distributed, how it behaves in motion, and so on. That just means our artists have to fill in the gaps on the fly and that often results in weird inaccuracies.

Generally speaking, the need of the gameplay we’re making supercedes the need for accuracy. These kinds of issues can be extrapolated for accuracy depicting other topics as well - history, science, culture, sports, and so on. Sometimes we purposely make things inaccurate for the sake of the gameplay experience we want to create. Other times it’s because we just don’t know and we have to guess.

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[askagamedev]

Is there any particular reason simulated physics in games usually feels so floaty/bouncy? Floppy ragdolls, swords sliding down hills, table settings prone to pop off the table, it just seems really difficult for physics in games to be made to feel right. What's up with that?

Mostly it’s because physics simulation is just an approximation, and that physical objects moving in reality have a whole lot more properties than the approximations can handle.

If you’ve ever taken a physics class in school, you must have noticed that the material tends to do things to simplify, such as assuming that the objects are in a vacuum, or that the velocity is applied instantaneously, and so on. These all serve to simplify the equations for the sake of teaching a principle. Real world physics are extremely complicated. There’s air resistance, friction, qualities of the materials that are involved, objects touching objects touching objects, and so forth and so on. There is also no cap on computing power for the real world to run at a high frame rate.

When a sword slides down a hill in a game, it’s because there’s some kind of gravity being applied to the object on an angled surface. Most game physics simulations just take basic properties into account like mass, various forces, and the (basic) properties of the material it’s on. We make approximations such that most visible things don’t behave super weirdly. Our physics simulation formulas are often incomplete - they work most of the time, but under the right conditions, for example, falling objects can steadily increase in their acceleration. This is why, in Skyrim (sometimes), objects falling and bouncing down a hill can get launched off into space - it’s because their physics simulation didn’t account for all of the factors at play in that situation.

Most of the issues stem from simplifications and assumptions we make to ensure that the physics simulation isn’t so complicated that it affects the frame rate. If you remember my old [Corpse Volcano story], there was always a noticeable performance hitch when I shot the bodies because there were suddenly dozens of physics objects (corpses) all exerting force on each other at the same time. I suspect that, as processing power improves, physics simulation will also continue to improve. We devs get more annoyed about the weird physics artifacts that we see than you do, because we are the ones who spend our time trying to make it look right to begin with.

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[askagamedev]

Here is one I noticed no game does even the realistic ones. Why don't games, especially on more realistic FPS, have the player loose the bullets remaining in a magazine when they swap them?

The problem with this is that it’s not very playable. It’s not easy to intuitively explain to the player why the remaining ammunition store is going down for reasons that aren’t shooting those bullets, and it can become a headache trying to keep track of all of the rest of the data. The natural question that arises from losing the remaining bullets in a magazine becomes why the other magazines in the game are all full. You could try to store the number of magazines, with individual bullet counts per magazine, but then you have to track all of those as well. Imagine this scenario:

You’re playing as a soldier, and you find a new weapon. How many bullets does the gun come with? Will there be magazines with it, or just what’s in the chamber?

You find a box of ammo, which contains six magazines. Are the magazines all full? Do you need to have the player spend a few seconds refilling all of the magazines to full?

When you are reloading in the middle of a fire fight, do you need to select a specific magazine? What if you grab one and it happens to be almost empty?

The main issue is that there’s usually simulation for simulation’s sake, and there’s simulation for streamlining so that the player can get to the fun part. Tracking ammunition carefully like this is a lot of engineering hassle and doesn’t really add a lot to the game, which is why it usually isn’t done. I could see a feature like this specifically added for a hard core realistic sim, but we don’t have many of those. AAA shooters are more about the action movie experience than they are about simulating reality.