Assassin’s Creed Shadows marks a significant technological evolution for Ubisoft’s flagship series, with a revitalised Anvil Engine that taps into some of the latest rendering technologies, alongside other innovative systems. We’ve already seen how transformative the new game’s real-time ray-traced global illumination (RTGI) system is, replacing an older, pre-computed or ‘baked’ GI solution that first debuted over ten years ago in DF favourite, Assassin’s Creed Unity. However, the new engine goes further – much further.
Before Shadows was delayed last year, we were offered the opportunity to spend time with the game’s rendering engineers to discuss how Anvil has evolved and finally, earlier this week, we had the chance to talk directly to the team. As you’ll see in the embedded video on this page and the tech interview below, we get to grips with the key enhancements in the new game. Ray tracing takes centrestage, with RTGI delivering a per-pixel lighting system that dynamically adjusts to the game’s expansive, seasonally shifting open world, delivering realistic effects like bounce light from moving sources and light filtering through materials such as Japanese shoji dividers. Additionally, ray-traced reflections enhance water and glossy surfaces with a subtle sheen.
Although it’s not mentioned in the interview, the Atmos system is covered in the video – and it’s great stuff, simulating weather and wind based on environmental factors like humidity and temperature. This dynamism affects the world procedurally: trees sway, hair moves, and particles like leaves or snow react to wind. Interactivity is further emphasised with a physics-driven destruction system, enabling players to break bamboo, cut through thin walls, or slice cloth, echoing the environmental interactivity of games from the mid-2000s, such as the epoch-making Crysis.
Visually, the micropolygon geometry system – similar to Unreal Engine’s Nanite – eliminates level-of-detail (LOD) popping for rigid objects like buildings and rocks. While not yet applied to vegetation or characters, future expansions are planned to enhance fidelity further. That’s just a broad overview of what we learned about the new game – be sure to check out the video below to see how all of this technology plays out. Meanwhile, many thanks to Engine Technical architects Nicolas Lopez and Pierre Fortin, along with Project Lead Programmer Rendering, Sébastien Daigneault for spending (a great deal of) time with us in discussing the game and putting this coverage together. As always, the remarks below are lightly edited for clarity and brevity.
We’ve seen how ray traced global illumination is a game-changer in AC Shadows – how does the RTGI function at a technical level?
Our hybrid RTGI system combines two steps: per-pixel ray tracing (screen space rays and world space rays) and DDGI-like probe cascades. We first trace per-pixel rays in screen space, trying to solve RT intersections without having to use costly hardware rays. If no hit is found in screen space, we continue and ray trace in world space. These are DXR rays, and they traverse the acceleration structure (BVH) until they hit or miss. All the hits are then relit in a deferred manner to produce the first bounce. Then the result is summed to our ray-traced probes that act as an irradiance cache to bring subsequent bounces, and the result is finally denoised.
How are dynamic lights beyond the sun and the moon handled?
With direct lighting, we use tile z-binning to handle local lights, a technique presented at Siggraph by Activision. Instead of partitioning the frustum into 3D clusters, this decomposes it into 2D tiles and 1D z-bins, which provided a 10 percent speed-up in lighting cost for AC Shadows. In RT though, we insert dynamic lights into an omnidirectional clustered lighting structure, which works like traditional clustered lighting but the clustered volume is mapped onto a uniform grid around the camera instead of onto frustum voxels. With this, we observed a 10x speed-up versus more naive implementations.
How are transparencies shaded? What about multiple bounces? And how is specularity handled?
GI is applied to transparent objects using RT probes only when running with RTGI. The first bounce comes from per-pixel RT and is what we call the secondary ray. Subsequent bounces come from the RT probes that act as an irradiance cache. For specular reflections, most platforms will use a combination of screen space reflections, gbuffer local cube maps (baked, but relightable at runtime) and a dynamic cube map at the player position as a fallback. On PC and PS5 Pro though, specular RT is available to replace those three systems and improve the overall fidelity. Specular RT is very similar to diffuse RT in terms of design, although we had to iron out some BVH quality issues that were otherwise barely noticeable.
What does the BVH structure (the geometry against which rays are traced) look like, what’s included and how is it updated?
We include only static opaque geometry in the BVH. It includes alpha tested geometry, such as vegetation, but also dynamic moving objects that are not skinned (non deformed). For tree leaves, we use a trick to fake alpha test with scaled opaque triangles. It sounds counterintuitive but it works very well in practice and is a very good approximation for specular RT as well. With this we save the massive cost of DXR “any hits” while still producing renders close to the reference.
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What GI solution is used for the open world, versus the hideout?
We use a baked GI open world on console modes that don’t include RTGI. For PCs that don’t support hardware RT, we developed a custom compute-shader-based software RT solution that allows for a fully dynamic hideout without compromising on quality. The scoped/sandboxed nature of the hideout means we can afford the higher GPU cost of this system here. We developed this after the initial delay of AC Shadows, as we took a step back and analysed where we could offer a more polished experience for players. The same tech is also used on Steam Deck.
Why does the game ship with two GI solutions rather than being RT-only, given the likes of Indiana Jones and Avatar: Frontiers of Pandora?
The simple reason is that we believe forcing RT on players today, notably on PC but also in performance modes on consoles, compels us to make qualitative sacrifices that we were simply not willing to take. We’re proud of the baked GI system that we’ve continuously improved from AC Unity – with time of day in AC Syndicate, sparse GI in AC Origins, multi-state GI in AC Valhalla and seasons in AC Shadows.