Paper.io 2: A Natural Difficulty Curve

Paper.io 2 is the follow up to Voodoo’s breakout hit Paper.io.

Paper.io is what originally landed Voodoo on the mobile gaming map. Now, after a dozen more games and becoming a top mobile publisher, Voodoo comes out with the sequel.

I’m going to take a look at Paper.io 2 from a game design perspective. I will look at the overall flow of the game, the design of the difficulty curve, and a psychological trick that keeps players coming back.

Basic Description

The objective of the game is to control 100% of the arena. You play as a small, ever moving square with a small starting territory that corresponds to your square’s color.

There are other players that start the same way and have the same goal.

In your own territory, you are safe. When you leave, you create a trail that draws onto the arena. If you make it back to your territory then you gain control of the territory you just traversed.

If someone else hits your trail, you die, your territory is lost, and you must start over.

A major difference between Paper.io 2 and the original is how you move. In the original, you could only move in 4 directions: up, down, left, and right. Now you can move in any direction. The arena is now circular instead of square to complement that change.

Game Flow

The game starts with a screen displaying your current character and your high score, which is displayed as a percentage of the highest area you have conquered.

You may select any characters you have unlocked and then start the game.

You conquer territory and kill other players until you die. You are then presented with an opportunity to watch an ad to get an extra life or decline the offer to return to the main screen.

Game Mechanics

First of all, this game is not played with other humans. You are playing against bots. This means it’s a free-for-all between you and a bunch of AI.

You are dropped into an ongoing game, which means some bots could already have taken a chunk of the arena.

New bots are also added throughout the game. So no matter how many bots you kill, they will just keep coming. The only aspect of their spawning that you can rely on is that they will never spawn in your own territory.

The number of bots compared to how much territory you have conquered directly correlates to the difficulty of the game.

At the start of the game, you spawn with 0.66% of the playfield. If there are 10 other bots, then there is 99.34% space for them to occupy. If you own 50% of the arena and there are still 10 bots, then they are in the other 50%, a much more concentrated area. This makes it harder to venture out of your territory without bumping into someone else.

This is one of the ways the game gets more difficult. Less space but the same amount of bots. Towards the end of the game, any free space would get used to spawn another bot, creating a hectic situation.

This balance of space to bot count could be managed itself by AI, potentially decreasing the amount of bots as space becomes cramped.

The other way the game could manage difficulty is the AI of the individual bots. The bots are tasked with taking space and killing you or the other bots.

In order to increase the difficulty, the bots just need to be more aggressive toward the player, attacking you whenever you leave the safety of your own territory.

If you kill an aggressive bot, the game just spawns another in it’s place. The life of one individual bot doesn’t matter much.

I could be over analyzing this and I don’t actually have any data to back that claim up. But it’s the only other major way the game could scale difficulty.

Getting to 20% or even 40% has not been too difficult. However, I think it would take a long, long time to achieve 100% due to the difficulty curve. At a minimum, you would have to be very conservative when taking territory which would end up in a very long play session.

My high score of 48% took about 10 minutes. And guess what, there is no pause button!

Strategy

So knowing what we know about the AI, we can come up with some strategies to get the most area possible.

1. Take the walls

The edge of the arena is the safest place since there are no enemies there. You put yourself up against a wall instead of being surrounded by the bots. You also can’t die by hitting the walls (unlike Paper.io 1).

2. Wait until you know you can kill someone

The AI are bound to slip up. It’s pretty easy to chill in your territory and watch a bot until they leave their space. Then, once they are a distance farther from their own territory than you are to their trail, you can strike with high certainty to kill them.

3. Take enemies space directly after killing them

When you kill a bot, their territory disappears. This is the best time to take a risk and capture the space. This is because you know other bots are not likely to be there. Just be careful of a new bot spawning in.

4. Don’t venture out too far

This might be obvious, but my deaths always come from going out too far. Someone then comes into my field of view and I realize there is no way for me to make it back.

This limited field of view is what makes this game so difficult. If you could see the entire playfield at all times, then winning becomes much easier.

Metagame

The only metagame is unlocking new skins.

This is achieved by completing certain stat based challenges within the core game like getting 75% of the arena space or killing 25 players in a single round.

The skin changes the character and also the territory that you own, changing it’s color or pattern.

Metagame isn’t as required in a pure competitive game like this one. There is nothing to upgrade or currency to gain. The motivation comes down to skillfully earning that 100% victory.

Psychological Tricks

Speaking of that 100% victory. It’s displayed as a percentage on purpose.

One of the things we’ve been trained to do in games is fill those percentage bars to 100% to ‘complete’ whatever task it was asking us to do.

The Zeigarnik effect, discovered in 1927, describes the phenomenon of finding it easier to recall a task that you have started, but not yet completed.

Completing a task gives a sense of closure, a release of tension, and apparently, a departure from our memory.

Now most games have progress bars that tap into this effect and our drive to be completionists. Quest logs in RPGs or leveling systems in multiplayer games are just a couple examples.

So why do I point it out for this game in particular? Because there’s only one quest. Just get that one bar to 100%. Should be simple, right? The thing is, I think you’ll be playing for a long time to complete this one quest.

Monetization

There are 3 ads and 1 IAP in the game:

1. A banner ad at the bottom of the screen at all times.
2. An interstitial ad after you die.
3. A rewarded ad that revives you 1 time during the game.
4. One IAP to remove ads.

This is a very common formula for Voodoo games. It’s sticking ads in all the places that don’t intrude on gameplay. The revive mechanic is very common in casual games. Players will use it when they capture enough territory that they think it merits a use to not lose their progress (me included).

I’m still surprised Voodoo doesn’t include skins that can only be purchased using IAPs. It’s something Ketchapp and many other casual developers do. It doesn’t change the gameplay but is available to those who really want them. The omission of these must mean that Voodoo doesn’t think they are worth the development cost of putting in.

Another IAP that Voodoo could include is paying for extra lives. I’m actually very happy this is not included since that’s just pay to win. Voodoo would then be incentivized to make the game even harder, forcing you to pay money in order to get 100% completion.

With what they currently have in the game, I feel the ads are fair.

Game Juice

Here I like to highlight aspects of the game that don’t affect gameplay, but really help make the game ‘feel’ good.

1. The game shows the percentage you acquired after every successful territory grab.

2. Particles shoot up when you are in someone else’s territory. (see above)

3. The game has a crown on whoever has the highest percentage. AKA the king.

This may not have a gameplay affect but it can have a psychological one.

If you see the king, you are more likely to want to kill that player. If you are the king, then it feels awesome to know you are the top dog.

4. The death animation is a 3 step process that draws out your agony.

First your line disappears. Then you explode in a satisfying animation. Then your territory fades away.

This is all much better than everything disappearing at once. It’s more satisfying to kill other players and more dreadful when you die.

5. You are shown your territory after you die.

This is a really cool improvement from the previous title. Since you have a limited view of the game space, it’s nice to get a whole screen view of the territory you conquered.

Similar Games

There are many, many io type games where you are playing against a bunch of bots or humans to become the biggest or strongest.

In terms of this specific io game, the most similar is String.io. It’s another space taking game and has the same free form movement of this sequel. There are hex shaped spaces that you take. I assume this was probably a little easier to program than the free form shape in Paper.io 2.

Another similar game is Hexar.io, which is basically the same as string.io, but with different metagame mechanics.

Of course, I should probably show you what the original Paper.io looked like as well.

Bugs

Not something I would normally write about but there is definitely a game breaking bug in here. There has been multiple instances where I have owned 40%+ of the arena and then suddenly I have <1%. Like someone took some of my territory and instead of taking away that small chunk, the game took away the larger area. Somehow the logic to determine which territory to take away is off.

Either way, I suddenly lost all my territory in a very not obvious manner. This deters me from continuing to play the game. Why should I keep trying to get 100% when I keep dying like this?

I caught it happening in this video. I was at 11% and then my space disappears. I did a video instead of a gif because you can hear my live reaction haha.

Conclusion

Paper.io 2 takes the classic io competitive gameplay and makes enough tweaks to the original to have a quality sequel.

For me, by far the most interesting aspect is the AI and difficulty curve.

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As you conquer space, you naturally make the game more difficult for yourself. It’s really quite elegant.

It’s still unclear how much tweaking of bot AI and bot spawning there is. Who knows, if I was able to get to higher percentages, then maybe I would see a noticeable change.

Check out the Friv 100 Games website for more game design analysis!

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