Forza Horizon 3 Uses Voxel-Based GI, Playground Simulated Car Materials to the Sub-Pixel Level


Forza Horizon 3 is one of the best-looking racing games available on the market, that's for sure. In a lengthy interview with the Guardian, Playground Games' Creative Director Ralph Fulton explained some of the technical details behind the engine.

To begin with, the game uses a voxel-based GI solution for real-time lighting.

Forza Horizon 3 uses a voxel-based global illumination system to calculate light bounces in real-time. This system uses the HDR sky, other light sources and offline-generated occlusion data to work out which surfaces the light bounces onto.

For an easy way to see the impact that bounced light has on a scene, look at locations with complex shadows like the entrance to a cave or crevices in a rock formation – in Forza Horizon 2, those shadows would have had a single colour value which meant the shadow would be uniform everywhere there was no direct light. In Forza Horizon 3, we calculate how light – and hence colour – bounces into partially occluded areas so shadows have a much higher variation in darkness and colour. The depths of a tunnel, with no direct and minimal indirect light, will be almost black; whereas the edges of shadow will bleed naturally from dark to light based on indirect bounced light.

A huge part of the graphics in Forza Horizon 3 is the sky, which is rendered with incredible care. Here's why Playground Games exerted a painstaking effort to make it look as real as possible.

People have asked us, why are you so obsessed with the sky? Well, it’s a massive part of our frame. In a driving game you have sky, car, terrain, road. But it’s more than that: the sky is the lightsource, so everything in the world benefits from a realistic system.

Through the detail of the light and the reflections, the cars, the leaves on the trees, the tarmac, all of them benefit from the much higher detail lighting data that’s coming from the sky. So the whole scene looks better - not just the blue bit at the top.

Of course, the cars have received particular attention. The developer simulated materials like carbon fibre, alloy metal types and leather in the interiors up to the sub-pixel level. But even the sea was crafted in a meticulous way, which is certainly peculiar for a racing game. For this specific task, Playground collaborated with Rare.

The sea in Forza Horizon 3 is also the sea in Sea of Thieves. Doing proper sea tech is incredibly complex. We knew the guys up the road at Rare were working on the same problem. They’re doing more deepsea stuff, but we got in touch with them, shared a bit of code, and we optimised it back and forth, so we’ve both benefited.

Our challenge was making the sea interact with the beach, so lapping waves, foam, wetting and drying the sand correctly – that was a whole other area of work. The 12 Apostles area is a sort of hero image for us, and we wanted the car to be down there racing. If you’re going to be on a beach, the sea has to look good. That work paid off because driving the cars into the sea looks awesome.

Forza Horizon 3 is available now on Windows 10 PC and Xbox One with full cross-platform play. Learn why it's a great racing game from our detailed review.