The decades-old Doom 3 engine proves perfectly suited to new indie game Skin Deep

Unreal Engine and Unity may be the most popular engines for game developers today, but there are still some who try to go against the grain, such as Wax Heads using Godot. But Skin Deep is even more radical, as it's a new game made using Id Tech 4 – more popularly referred to as the Doom 3 engine – which is now more than 20 years old.

Having traditionally made its engines open-source, this was the last open-source engine from id Software before it was bought by Bethesda, which still has a passionate community, although Skin Deep is a rare example of the old tech being used for a commercial release, the other being developer Blendo Games' 2016 cyberpunk adventure game Quadilateral Cowboy.

"I've been using various iterations of the Id Tech engines since the late '90s, so I'm just very comfortable with knowing the ins and outs of it," Blendo founder and creative director Brendon Chung tells me. "The Id Tech engines, generally speaking, are made just for first-person games and not really anything else. which I think is great. I think there's something really nice about using a very small sharp tool that's great for doing one thing really well."

The pros of using old tech

Whereas Epic and Unity continue iterating and updating their engines, which can come with its own issues of optimisation, the fact that the Doom 3 engine is so old means it's solid and stable. The downside is that it lacks the accessible features and documentation of a modern engine, perhaps not a huge issue when you're largely a solo developer with expertise on it already.

But Skin Deep is also Blendo's most ambitious project to date, a sci-fi immersive sim that's both systemically elaborate while also having a strong narrative focus, and the developer's first to use voice acting. In other words, it requires more hands who can also understand how to use the engine.

Fortunately, joining the team was Suzanne Will, responsible for the level and systems design, art, and programming, who at the time had been making maps in Half-Life, which uses Valve's Source engine, itself based on a heavily modified version of the Quake engine. "The Source Engine and the Doom 3 engine are kind of both different offshoots of similar ancestral technology, so adjusting from one to the other wasn't too bad," she explains.

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The beauty of open source

As they're making an immersive sim, it was also beneficial that one of the most significant mods made with id Tech 4's open-source license was The Dark Mod, a fan-made take on the Thief games, one of the earliest examples of the genre. "A lot of our documentation and even some of our tools and code is thanks to The Dark Mod, which has a really good community," Will adds.

Skin Deep also makes use of id Tech 4's unique features, such as its implementation of GUI where you can interact with a computer terminal and seamlessly move its cursor while reading an email. Aesthetically however, Skin Deep couldn't be more different from the dark and gritty look of Doom 3, opting for something more colourful and stylised. This is after all a game where you are rescuing talking cuboid cats, while it also makes use of jump-cuts previously employed in Blendo's short heist adventure Thirty Flights of Loving.

For the interiors, Will says 70s sci-fi such as Alien and Star Wars had a big influence, especially with "very bold and tactile" decor. "We were drawing on the contemporary interior design of the time with that 70s wood paneling look, lots of beige, leather and autumn tones as a base," she explains. "Then we explored a lot more colour. It was very important for people playing the game to differentiate spaces very clearly at a glance." Inspiration for the exteriors meanwhile came from chunky designs from other games like Home World and The Signal from Tolva, as well as an earlier Blendo title, the VR-based Flotilla 2.

Having a game that features interiors and exteriors was however a challenge. "The engine is extremely good at doing hallways, it's kind of laser focused on doing that really well, but it's not really built for doing big sprawling outside areas where you can see a bunch of things," Chung explains. "But we really wanted to allow the player to also at any point break open a window and float outside, go to a different airlock and then go back inside the ship at a different point."

It takes some creative solutions to get the game changing between interiors and exteriors seamlessly by controlling what's visible. "We set up some tech for doing visibility fading at distance for windows, which was an idea we actually pinched from Source," Will adds. "So the window is transparent when you're up close, but then as you move further away, it fades to an opaque cube map, but it just kind of looks like a very shiny window from far away."

In other words, you can still teach an old engine new tricks. Will is already using the engine on another personal project but perhaps Skin Deep's commercial release will be a lightning rod for other developers. Chung concludes, "Our team has put a pretty wild amount of stuff in it to update it to the year 2025, so we're secretly hoping that more people take this tech and make other cool stuff with it."

Skin Deep releases for PC on 30 April. A demo is available on Steam. Inspired to create? Then read our guide to the best game development software.