One year ago, Epic Games announced that it was transitioning Unreal Engine 4 to a subscription model. After a successful year of experimenting, the company is going one step further: As of today, the engine will now be completely free for all to download.

Current subscribers will receive a prorated refund for their most recent monthly charge; anyone who ever had a paid subscription will also receive a $30 credit for the Unreal Engine Marketplace.

Staying in place is the company's royalty arrangement: After the first $3,000 earned (per quarter, or $12,000 per year) developers will owe Epic a five percent royalty on gross revenue for each project they develop with Unreal Engine 4.

"The more barriers we can eliminate ... the more potential we see in our business," Epic Games founder Tim Sweeney tells Gamasutra. "If a game succeeds, we succeed along with the developer. We profit along with them on it."

When asked if this is a smart business move, Sweeney said it "absolutely" is. Though the royalty model has only been in place for a year (so not that many products have shipped under it) the engine's audience has only grown since it became more accessible.

"Our user population really quickly increased over that year by a factor of 10," Sweeney said.

And triple-A developers who toyed with the engine on their off-hours have become advocates for using it in their day jobs, leading to increased uptake for Unreal Engine 4, Sweeney says.

(Of course, triple-A studios are still free to negotiate traditional licenses with the company and avoid or reduce royalty payments altogether.)

How live source access changed everything

Since April last year, Epic has offered live source access to the engine for subscribed developers, and it's been a rousing success, Sweeney says.

Was it scary? "We felt that way last year, when we launched and opened the source code to everybody in the world. We thought it was a good idea, because we knew developers who had the source would do far more than developers just working with the tools," Sweeney says.

So how did it play out? "It worked out incredibly well. Not only has there been no downside for us, but we've seen hundreds of people in the community improve the source, and give it back to us. We've been really happy, and that's really given us the confidence," he says, to move forward with opening the engine up.

In the past, new engine features would be developed in secret and unveiled. Only licensed developers who'd paid Epic hefty fees could see inside the tech. Here's an example of how it works now: Epic's new, experimental global illumination code is being written by a programmer at Epic called Daniel Wright. His work is "uploaded to GitHub every day as he writes it," Sweeney says.

"We're developing the engine as openly as possible now. Now everybody who creates an account for free can see absolutely everything we're doing, in the same way that the Linux kernel is developed in the open."

Sweeney chose his words carefully there, as there's a significant difference from Linux: It's governed by Epic's license, not an open one. There could be no "truly free" version of Unreal Engine 4. However, when asked, Sweeney did clarify: "Developers using UE4 are free to fork the engine and share their changes with the UE4 community, so there could be alternative distributions. However, each would still be governed by Epic's commercial license."

A "common vocabulary" for photo-real 3D

The new accessibility of Unreal Engine has lead to its uptake in the architecture industry, film, and of course virtual reality -- game and non-game applications both. "We're really seeing these different worlds come together now," Sweeney says. There are "huge opportunities for sharing between these non-gaming fields."

The Unreal Engine community of 2015 "doesn't look like the isolated game development community as in the past," says Sweeney. VR and augmented reality are "going to make 3D graphics ubiquitous in a way even smartphones have not," he says. Unreal Engine will create "a common vocabulary for everybody in all fields who is doing realistic 3D."

Unreal Dev Grants update: A year of free money from Epic

I asked Sweeney for an update on Unreal Dev Grants -- the company's newly launched program that aims to disburse a total award of $5 million to Unreal developers.

In the first few days of the program, says Sweeney, Epic received over 100 submissions. "We're really going through the different proposals in detail. We're going to announce the first several developers at GDC."

"This will probably be going for over a year," Sweeney says. "We've really been stunned with the number of the submissions and the quality of them." He's been struck, in particular, by a number of "indie games we had no idea about," and Unreal's "uses outside the game industry," as detailed above.