Collision Physics

Gaslands is a tabletop wargame in which heavily-armed vehicles career dangerous into either other, and high-speed collisions are virtually guaranteed.

Recently, new player Dan Frohlich provided some interesting feedback about his first game, and asked for a little background on how the Gaslands collision physics ended up in their final state.

“In the first game I ran for my crew, there was a head on collision between two cars each in 3rd gear. After the collision resolved, one of the players gave me a funny look and said: “We just drive through each other with a couple more hazard tokens?”

Mike, could you comment on the design decisions or iterations that drove the collision system?

Why don’t collisions have an effect on a vehicle’s vector (speed and heading)?”

That is an excellent topic for a blog post, and I’m happy to take the bait. I designed and discarded a whole host of systems for managing collisions, and I’d love to talk a bit more in depth about why we ended up with this one.

Within the simulation of the game, we are dealing with two fast heavy objects colliding. The real-world physical outcome of their impact is a result of their vectors and will be affected by their relative weights.

In game terms, there are many possible approaches to handling collisions, and I’ll talk through some the things I’d tried on the way to the right solution for Gaslands. As you’ll see, I’d experimented with different systems that affected the location, facing, or speed of one or both vehicles, and I’ll try to show how they proved problematic.

Scatter Dice

In the earliest versions of the game, skid checks were bad things to be avoided, rather than a tactical feature of the game. Three of the skid dice faces acted like scatter dice, randomly generating a direction for the skidding vehicle to rotate to or travel in. This provided a natural initial design for the collision physics:

“(v1) After the smash attack is resolved, the target vehicle makes an immediate SKID CHECK.

After the skid check is resolved, if either car is in a position such that it cannot move away in its next movement, place the attacking car on the far side of the target vehicle, such that both cars can drive off in their next turn.”

As you can probably guess from the second paragraph, the first paragraph was throwing up some weird positioning issues. Cars would collide, get randomly pointed or moved somewhere they likely wouldn’t fit properly, and then neither car would be able to move off next turn. (You couldn’t drive through other cars at this point). It was dynamic, sure, but it felt glitchy and not cinematic.

Throughout the development of the collision physical engine, two issues kept coming up: (1) how can you position the vehicles so that the inertia and vectors of the two vehicles “feels” right, and (2) how can you do so in a way that is crisp and not open to player interpretation?

As a prideful games designer, I wanted my game to avoid unclear rules situations that lead to confusion and disagreement wherever possible. Whilst some players will be able to resolve things quickly and in an amicable fashion, I basically feel that it’s lazy and careless on the rules author’s part if the outcome a rule is not clear to “most” players. Either the rule is not written it clearly enough or the rule is not right and needs to be changed. Otherwise you leave an opening for the game not to be fun for some players, and that’s literally the opposite of what we’re trying to achieve here!

Needless to say, with “scatter dice” skid check these collision rules inevitable ended up with a vehicle overlapping another vehicles or terrain and requiring a good deal of finicking to place. It wasn’t crisp enough.

Trading Places

A weird thing is that we sort of intuitively know how two objects should behave when they collide. Drive a toy car into another car whilst making a screetching noise, you’ll have a pretty strong sense of how the cars will act as a result. Both will change direction to some degree, based on their relative vectors and weights. Easy. Obvious, right? It turns out writing a coherent and simple rules system to capture this intuition is really freaking hard. I pushed toy cars into each other all day long and could not find a simple system to describe the results.

Here was the next version of the collision rules:

“(v2) After the SMASH ATTACK is resolved, the active vehicle immediately lowers its CURRENT GEAR value by a 1.

After the SMASH ATTACK is resolved, if the collision was not a head-on collision: change the orientation of the passive car to match the orientation of the active car, and then change the orientation of the active car to match the orientation of the passive car at the start of the SMASH ATTACK.

After the SMASH ATTACK is resolved, if the collision was a HEAD-ON COLLISION: swap the physical locations of the two vehicles, maintaining their current facing.

Ideally, both cars should be able to drive off in their next turn.

Author’s Note: These rules sometime result in situations that are a bit weird, and I’m very much open to ideas here!”

This version of the rules, I was hoping a rule as intuitively simple as “exchange the directions of the two vehicles” would provide an elegant solution that was simple enough for younger players, and didn’t involve either randomness or geometry.  Actually, it made the cars feel sort of “rubber”… like they were bouncing off each other. Also, as the angle between the relative directions of the two cars increased, the effect got weirder and weirder, leading to that second paragraph, which we there to avoid cars ping-ponging off in opposition directions, which looked ludicrous!

In this version, the vehicle doing the smashing loses a GEAR, which was intended to simulate the lost momentum from the collision, but in gameplay it overly punished “close combat” as you had to be in a gear higher than the current gear phase, else smash attacks would cause you to lose all you further activations in a turn.

Also, “Ideally, both cars should be able to drive off in their next turn” might actually be the worst piece of rules text I ever wrote for Gaslands.

I was trying to short circuit the vector calculation, to find something that was simply enough for an eight-year-old to figure out, but which gave the effect of heavy, fast-moving objects smashing into each other.

Protracted Calculations

I iterated on a system to change the facing of the vehicles, trying to find some way to simulate the vector summing without actually requiring the players to get a protractor and a calculator out!

For example, here’s a sketch from my notebook for a scatter system that attempted to provide such a vector calculation.

The idea was to place some sort of clock-face template (imagine the scatter template from Blood Bowl) centred on the target vehicle and to point the “1” value at the oncoming attacker. A 2D6 roll would then spin the target vehicle to a new facing, with the probabilities being that it would end up roughly the other direction. The system could have been expanded. Perhaps rather than setting the clock at “1”, you set it at the current gear of the attacking vehicle, to provide variance in the distance likely spun. (Although it would have actually needed to be “seven minus the current gear of the attacking vehicle”, which is more complex).

In playtesting, co-conspirator Glenn came up with an unusual idea for simulating collision physics involving bases with small holes and using pencil tips as pivot points.

This nicely simulated the experience of pushing two toys cars together, and provided reasonable “right” feeling movement, but I felt it added too much complexity to the game, and I wasn’t keen on either the fact that it required cars to be based or the fact that it require physical pushing of models. Also, it would have required a custom component for the game that couldn’t easily be produced at home by players.

Occum’s Razor

At the start of 2016, I added the rule that allowed vehicles to ignore obstacles if they begin their activation in contact with them. This rule was added to provide a less frustrating and more exciting experience during death races. Gaslands without this rule involved a lot more reversing in first gear to escape being snagged on obstacles, and I really didn’t want Gaslands to become some kind of post-apocalyptic parallel-parking simulator.

This new rule provided a new option for the collision physics engine that hadn’t be possible before. We could do nothing! Not certain how this would feel on the tabletop, I replaced the rule with the simplest rule I could imagine. I even placed an explanation into the beta rules text to attempt to justify this intuitive odd solution to the problem.

(v3) After the SMASH ATTACK is resolved, BOTH the active and passive vehicles immediately gain TWO SHIFT TOKENS each.

Author’s note: after a smash attack is resolved, you might think it odd that the target vehicle doesn’t get moved or spun. Every game has to make abstractions, and with the two new shift tokens, you may find the smashed vehicle ends up moving unpredictably in its next move and hopeful describes the carnage.”

This was fast and simple. Everybody just gets two hazard tokens and has to deal with that. No finnicking about with overlapping vehicles, no vector calculations, no scatter dice.

But how do the two vehicles get past each other?

With much of Gaslands, players are invited to imagine what cinematic movement might have led to a particular maneuver or situation arises. Here it’s no different, perhaps one car dodges to the side at the late minute, suffering a scrape but swerving back and recovering their line shortly after. Perhaps one of the vehicles goes straight over, or under the vehicle.

If one of the vehicles is wrecked, then more descriptive action takes place as it flips.

If one of the vehicles happens to wipe out at the end of its next activation, more likely because of the two hazard tokens, you can trace the action back to the collision and imagine a cinematic visuals that causes this outcome.

Of course, the problem with this solution goes right back to Dan’s original point. The players’ expectations is that a collision is going to cause the vehicles to change position. The leap of faith this rule asks players to make is to wait for the next activation of these vehicles to find out the result, like some sort of gasoline-fueled “what happened next”.

Whilst the “snapshot” at the end of a collision now might not describe the carnage, if neither vehicle has moved, it’s equally likely that either one or both vehicle now has a dangerous number of hazard tokens and will do something carnageful next activation, or one or both vehicles wipe out.

Here’s the final text of the rule as it appears in the Gaslands rulebook:

“If one or both of the participants in the collision chose smash attack as their reaction then all the vehicles involved in the collision gain +2 hazard tokens each.

If both participants in the collision choose evade as their reaction then all vehicles involved in the collision gain +1 hazard token each.”

Whilst the “snapshot” at the end of a collision now might seem a little static, Gaslands is a game where the cinematic physical unfolds throughout the game. The influence of the two hazard tokens and the lost hull point will undoubtedly be felt in the following activations, and hopefully the cinematic carnage will unfold, even if the collision rules themselves appear to freeze-frame the action at the point of impact rather than the explosive aftermath. All games have to make abstractions in the name of gameplay.

Fast & Furious

I think the final version of the rules provides a faster, less complex game than all the other options I considered, and it still provided the cinematic visual fun that I was looking to give players. In play, it keeps the game fast and avoids adding another fiddly area of the rules, (“Final Position” is fiddly enough on it’s own *sigh*).

Our playtesting experience showed me that the game is no less fun for not having a collision system that alters the direction or position of the vehicles at the point at which the collision occurs, even though that seems like the obvious thing it should do. However, I hope that I’ve shown here why even obvious things need to be tested and challenged, and why you always need to be open to discarding things if they aren’t right for the game.

At the beginning of the game’s development, I stated that part  of my vision for the game was to have “the minimum rules required to create maximum fun.”

One of the things that I learnt during Gaslands is that there is an endless supply of cool ideas for cool-sounding features.  Part of the skill as a games designer is figuring out whether the game is actually more fun for having these feature or not, and then being brutal about removing the things that are only adding more rules, rather than more fun.

If you want to crash to toy cars together at high-speed to see what happens, get yourself some toy cars right away. If you also want to receive two hazard tokens for doing so, get your oily butt off to our Gaslands Store and pick up a copy of the new post-apocalyptic death-racing game.