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State of the Art: The slimes of Slime Rancher

Design of a slime

I've spent a looooooot of time with my slimes in Slime Rancher [official site] and watching them bounce and coo as they wiggle free of my corrals and wobble off like determined balloon-toddlers has been a delight. But how does one convert a ball into a creature with such a strong sense of spirit? How do you keep their little slime modifications from becoming a confusing mulch of wings and ears? What happened to the meteor slime? How does Gremlins figure in the design of Slime Rancher's monsters? And will anyone listen to me when I tell them puddle slimes are actually cuter than tabby slimes???

Let's do a slime art and design interview with game designer/Monomi Park studio co-founder Nick Popovich and find out...

As always, you can click on an image for a super-size version to really examine the detail! I'll also include a few of my own screenshots to illustrate how a few elements look in the full-release version of the game.

Pip: Let's start with the look of the slimes and how you settled on that. Was it a case of always wanting little blobby creatures or were they more complex but then you simplified them/placeholders you decided to keep?

Nick Popovich: It was always going to be slimes because they’re simple, limbless creatures. In the early days of development I was the only artist and I’m a terrible animator, so limbs were out. And since we were going for a very physics-intensive experience, a creature that is actually just a spherical collider was the most efficient.

Adorable smiles having a hug

Vac design for the sucking up of slimes

But additionally, slimes were a great fit since I wanted the player to be able to shoot them anywhere, get them stuck in places, etc and never look broken. A slime dangling upside down stuck in the branches of a tree and smiling looks 'correct' in Slime Rancher because the slimes are these goofy blobs. You don’t expect the same behaviors of a person or animal so there is less in the game that breaks the immersion.

Pip: The basic slime is a pink ball whereas all of the rest have a modification which helps players see what they are immediately - cat ears, a radiation ring, lava cracks... Can you tell me about the process of picking out the designs which worked and how you refined them? If it's easier we could take the phosphor slime as an example.

A selection of early slimes

Nick: Each slime is designed to have a different silhouette so the player can parse which slime is which at a glance. We often call that ‘passing the squint test’ as ideally the information you’re trying to display can still be discerned even if the player is squinting. And though the slimes all have unique colors, we specifically don’t rely on them as that information is less useful to those that are colorblind (1 in 12 men in the world).

The phosphor in particular followed the same rules as the other slimes: whatever element it brought to the table visually that sets it apart form a pink slime can only live in that spot on a slime body for that slime type. In other words, when we added wings to the phosphor, no other slime could have a feature that was attached to its body where the wings are, as a largo version would cause conflicts. It’s a huge design hurdle as it means that essentially you could make a single slime that has every single unique feature of every slime all at once. It would be a mess, but it would exist without overlapping features.

Pip: I'm also curious about the quantum slimes which project a ghost form as a glitchy slime it can actually take the position of. Did you have to go through a few forms so that their abilities were easily legible to players?

Nick: No actually, that one was on a post-it note on my desk forever that was something like 'we gotta do this some day!' and one day we just felt brave enough to try it. It’s a unique behavior and, unfortunately, it’s really difficult to manage in the game so a lot of players avoid them.

[I actually love them and just keep them vaguely in place away from other slimes but near a lot of fruit trees - Pip]

Quantum slime concepts

Pip: On a related note I'd be interested to know if that's a difficult behaviour to model in the game - a projection which can become the thing itself?

Nick: One of the most challenging aspects of the quantum slime’s design is the same challenge we face with any slime: whatever we do, it can’t be so complex that it’s difficult to render or compute because the player could have HUNDREDS of them in the scene. The quantum was the worst of all because those hundreds become many hundreds since it can basically replicate itself.

Watch on YouTube

So the ‘ghosts’ that the quantum produces have a bunch of the slime logic stripped out of them and only are doing what is truly necessary to function, allowing them to still be 'cheap enough' to run in game without everything breaking.

Pip: I imagine the distinctive features of each slime also help players with forms of colour blindness recognise them quickly. Are there any which were a challenge there? I imagine crystal and rock slimes seem similar if you don't see them rolling around or creating those shard explosions.

Nick: Yes, the rock and crystal are so close, probably one of the worst examples of making each largo distinct. But since the two slimes are sort of cousins, they have similar features. But we were sure to have the rock’s spike crown be drab, and rounded off, with the crystal being shiny and very pointy: all adjectives that are color-blind agnostic.

Crystal slime concept

I feel like this is something we do well. And I feel like many games don’t pay enough attention to this sort of thing. All games are communicating systems of information to the player at all times, coupled with the noise and chaos that those systems and the players produce. You NEED to ensure that every bit of that information is as easy to parse as possible just as you should be clear in your speech when talking with others or grammatically correct and well-structured in your writing. It’s the same thing.

Hunter slime concept art

Pip: Plorts must be an even bigger challenge on that front so how did you approach it?

Nick: Plorts are easier since you don’t need to worry about largo-style combinations. But you do need to distill the slime’s ‘essence’ into an even smaller, simpler design. That is sometimes tough.

Pip: There's a degree of slimes becoming more complex as you progress in the game, or more exotic-looking but I'm interested in how you decided on the order of unlocking them - how closely does your in-game progression relate to the complexity/colourfulness of the slime designs?

Nick: We wanted each slime to stand out on its own, but I think as we went along we needed to come up with wilder ideas for each since we were maybe trying to compete with our own previous designs. The quantum slimes in that regard are almost an act of hubris!

[In the following image you can actually see a sketch of the cancelled meteor slime we talk about later - Pip]

Tier 4 slimes concept

But definitely the slimes in the late game do more because their plorts are more valuable so the tension they should create on the ranch needs to be increased over the earlier ones. It’s why the slimes from the Glass Desert each do two ‘bad’ behaviors instead of one.

Glass Desert sketches

Pip: The slimes are clearly designed to be adorable but I'd love to hear about the ways you refined their level of cuteness over time.

Nick: A key factor in their amicable quality is that, for the most part, the slimes are designed to be respond with positivity or awe to any given situation. Save for a few instances where they run in fear, like from the tarr, a slime will smile and coo if left alone, if hanging with its buddies, or if stuck in a tree. That unflinching positivity is not found in a lot of games and its something we knew would be refreshing so we clung to it all through development.

Similarly, their audio is higher pitched, which is always cute, and they say a mix of gibberish and word-like emotes like ‘whoa!’ that allow you to humanize them just enough without expecting any more form them. It’s certainly a fine line.

Pip: I know a lot of people love the tabby slimes but the puddle slimes are definitely where it's at. Especially if you give them a rubber ducky. (This is not a question, just a statement of fact!)

Nick: Can’t argue with that.

Puddle slimes playing with their rubber ducky

Pip: There's also the darker side of the game - can you tell me about how you came up with the design of the tarr?

Nick: Even in its earlier days, I wanted Slime Rancher to take some inspiration from the movie, Gremlins. I liked that there were strict rules you needed to follow or the cute little gremlin you had became something terrible. So instead of having conventional enemies or a villain in Slime Rancher, we stuck with a monster that only really appears if you let it happen or make a mistake.

The tarr design

The tarr’s design were dark and prismatic from the beginning. I always pictured it looking like when you mix a bunch of paint together and you see those swirls of color, or like an oil slick with a colorful sheen to it.

Pip: I also wanted to ask about the areas of the game where there are absences of effects. I'm thinking of things like when you need to feed chickens you've raised to meat-eating slimes - how did you work out how to portray those moments and keep the game's perky tone?

Nick: The trick is to not make the player feel bad about their actions. If meat eating slimes at adorable bunnies, we’d be in trouble. But we chose chickens instead and made their expressions intentionally pretty stupid-looking. The chickens are absolutely victims in Slime Rancher, but if the player can laugh their way through their demise, the tone of the game isn’t affected. Also, no blood. That helps. I’ve never drawn a drop of blood in any of the games I’ve made and I don’t plan to.


Pip: I know you had to cut the meteor slime from the game but can you tell me a little about that and why it didn't work out?

Nick: We never had final art for the meteor slime [you can see a sketch earlier in this article, though - Pip] but it was the aforementioned dual ‘bad behaviors’ of the Glass Desert slimes that made us pivot on the meteor slime. It was going to be magnetic, like an inverse boom slime and also come crashing into the world as a meteor. It was very interesting but that second bit wouldn’t make sense on the ranch, and the mosaic slimes were already raining fire, so we pivoted.

It might seem like a shame that it’s not in the game, but things are kept on the cutting room floor for a reason.

Pip: As a final question, the slime shapes are echoed frequently in the architecture of the world when you get to the late game - that feels like a strong statement in terms of hinting at lore or nibbling at the idea of a civilisation. Is that playful ambiguity or is it part of some environmental storytelling you want to develop post-release?

Ruins sketch

House sketch

Modular Ranch design

Modular Ranch designs

Nick: It’s a bit of both but leaning more on the former. I don’t think we’ll ever explicitly state who made the ruins or how the slimes came to be as it’s more fun that way. It was also somewhat born from necessity: you can’t make every bit of the world natural or it would get boring fast. We needed to inject some variety into the world with ‘man-made’ structures as it allows us a great deal more freedom in the art and level design departments.

But that doesn’t mean it was done without purpose lore-wise, it’s just that most of it might live inside my head forever!

Pip: Thank you for your time!

Beatrix Lebeau


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