Valve announced its long-rumoured Source 2 engine at GDC 2015 in San Francisco this week.

The company said that, unlike other engines that have licensing fees and royalty costs attached, Source 2 will be entirely free. The only condition is that any game made with Source 2 must also be released on Valve's proprietary storefront and PC gaming portal, Steam.

Speaking with Rock, Paper, Shotgun, Valve’s Erik Johnson explained that games don't need to be exclusive to Steam, only that the platform must be included among the distribution channels used.

Elsewhere at GDC, Epic announced that Unreal Engine 4 would be free. The only stipulation on the engine's use is a 5 percent royalty on gross revenue after the first US$3,000 per product, per quarter.

Meanwhile, Unity boss John Riccitiello described royalty systems as "f**king around", and said his company's new Unity 5 engine would be free for amateur dabblers but come with a license for professional shops: either US$1,500 up front or a US$75 monthly subscription.