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Horizon Forbidden West: how the Decima engine evolves for PS5

A generational leap from Guerrilla Games?

Guerrilla Games' Decima Engine is evolving - and in more than one direction. Over the next couple of months we will see its existing iteration deployed on PC with the arrival of conversions of both Death Stranding and Horizon Zero Dawn. However, it's in the reveal of Horizon Forbidden West for PlayStation 5 that we see the very latest work on Decima and our first look at how Guerrilla intends to deploy its technology on next generation console technology. With that in mind, we decided to take a look at the Forbidden West reveal trailer, to get some idea of the studio's next-gen vision.

The big challenge facing Guerrilla was exactly how to expand on its existing work, simply because the iteration of Decima as seen in Death Stranding is already in a league of its own. However, as advanced as the tech is, it is fundamentally shackled to the constraints of a 2013 console design, and some shortcuts and compromises are in evidence - so for example, interaction with the world's rich foliage is somewhat minimal. While the Forbidden West trailer does not show off much in the way of direct character interaction with foliage, we do get to see scenes where tall grass flows in the breeze with a rolling pattern dictated by the wind. There's also secondary motion from foliage on the back of the gigantic tortoise mech, reacting to the behemoth's primary movements. There is still the sense that this is work-in-progress though: in the scene where the trackers subdue the bot, there's no physical reaction from foliage when it stamps the ground or falls down - something we'd expect to see addressed in the final code.

Actual foliage density is also an area where Guerrilla is making strides with Horizon Forbidden West. In the original game, the focus on detail is mostly in the foreground surrounding Aloy, with shadow maps, detail and overall LOD cut dramatically further into the distance (I really can't wait to see how this changes on PC). For Death Stranding, the priorities changed since that game had less foliage, which is expensive to render. Foliage wasn't quite so dense, but perhaps because of that, more of it could rendered further into the mid-distance, and even smaller elements like rocks closer to the camera received an upgrade in detail.

Cover image for YouTube videoHorizon Forbidden West PS5 Trailer Analysis: The Decima Engine Evolves!
A video breakdown of the Horizon Forbidden West trailer, captured from PlayStation 5.

Horizon Forbidden West essentially delivers a 'best of both worlds' scenario - and then some. Texture detail and mesh density is on the upper end of the scale compared to Death Stranding, but there's the great technological leap brought about by AMD's generational leap in rendering power, meaning more foliage pushed out further into the distance. Even smaller scale detail is retained further away from the camera, even casting high precision shadows. Compare and contrast with the original Horizon: here, shadow maps tended to be of a rather lower resolution unless they were immediately next to the player camera. This generational change of asset detail and shadow detail is best seen in the closing shot of the trailer, where you can see swaying foliage moving far out into the distance.

The density and dynamism of the world in this trailer is not just limited to to the ground, as there has been a nice upgrade in sky rendering too. We've discussed Guerrilla's approach to volumetric cloud rendering in the past, and let's just say that this is a remarkable system. Internally, it's known as Nubis and it consumes a relatively high amount of GPU time - 2ms on the base PS4. Decima only renders a small portion of the clouds in one frame, but as movement is gradual, all clouds can be rendered across multiple frames. This means that detail could be preserved when the clouds are at a great distance parallaxed from the camera, but they could not show off sudden dramatic changes or fast movement without obvious artefacting. In real life though, it is generally hard to see clouds moving at speed anyway since you are not flying at the speed of a jet fighter - in short, it's a good compromise for the PS4's hardware limits.

However, Forbidden West sees a fast-moving, swirling cloud formation. The centre of the cloudscape is moving at a very high parallax, even from the camera view on the ground, which genuinely would not be possible in the old system as we understand it. Not only that, but the resolution of the clouds themselves looks decidedly higher. Based on what we're seeing here, I would imagine the extra power in the PS5 GPU has allowed Guerrilla to deliver an animated volumetric sky that would not suffer greatly from the visible artefacting that would happen based on the constraints of the current console generation. Interestingly, the effects we're seeing here look very much like the 'future work' section of Guerrilla's Siggraph 2017 Nubis presentation.

Beyond these larger elements, some of the smaller details found in the trailer caught my eye. I really liked the water rendering systems, for example. Wave crests on the ocean actually break - something rarely seen in games - and in the trailer's final shot, the effect is seen occuring way out into the distance. What also caught my eye was the way large waves break and deform vertically as they hit the rocky shoals. Water rendering is generally excellent, right down to frothy bubbles in waterfalls - and accuracy extends to interaction too. In the scene with the injured fox, water level rises and intersects accurately with the animal, which even appears to float a little. This is genuinely impressive, as water intersecting with geometry causes a lot of visible clipping in many games.

Cover image for YouTube videoDeath Stranding Tech Review: The Digital Foundry Verdict
Death Stranding was a stunning technological achievement for the Decima Engine on PS4 and PS4 Pro... and now it's heading to PC.

But it is this level of realism that does make me wonder about the extent to which the Horizon Forbidden West trailer represents in-game fidelity, or whether we're looking at cutscene-specific systems. For example, is the fox in the water reacting to a game-wide real-time fluid simulation, or is it actually pre-calculated 'baked' geometry animations made specifically for cutscenes? It could go either way but I do expect to see improved water rendering generally, particularly as we saw improvements here between Horizon Zero Dawn and Death Stranding. In the scene with the giant mech 'tortoise' rising from the water, the fluid displacement moves in a similar manner to the black ooze from Death Stranding.

In terms of character models, I think the upgrades are a little more modest and subtle, which is in itself not a bad thing - the character models in the first game are genuinely great and almost unparalleled in Death Stranding. That said, even superb models like Aloy in the first game saw some compromise - like visible polygon edges on her ears, for example, which look much rounder and more realistic in Forbidden West. Another upgrade is the small fuzzy hair seen on character silhouettes of Aloy that are backlit by the sun - this is something common to humans that is very rarely rendered in video games, as rendering budgets and art authoring really cannot always afford to plaster transparency textures across the surface of the skin on character models - so I am very curious if Guerrilla is using a traditional 'card'-based system for the fuzzy hair here, or something else entirely. This kind of detail is found across the trailer to help sell the smaller graces enabled by the generational leap in GPU hardware.

Of course, it's worth keeping in mind that what we've seen is a short trailer - a teaser, really - and there are still many questions we have, specifically regarding lighting and certain levels of asset quality. The trailer showed no 'over the shoulder' camera shots from gameplay, with more of an emphasis on sweeping cinematic shots or shallow close-ups lavish in their use of depth of field. There's a shot that focuses on the beach, with two incredibly detailed crabs in view. Given how the camera is zoomed in and how the depth of field looks, I would assume they are hero assets for a cutscene, and not necessarily the quality of the model a player should come to expect should they happen across a crab in the game world.

Cover image for YouTube video[4K HDR] Horizon: The Frozen Wilds PS4 Pro - Tech Breakdown and Engine Analysis
There's a ton of behind-the-scenes Decima content to enjoy in this video analysis of the Horizon Zero Dawn Frozen Wilds DLC.

The same goes for that small pebbles on the beach that they are on - I would be very pleasantly surprised if the terrain in the game rendered at this level of detail if you just stopped and stared closely at the ground in-game. By their nature, cutscenes are carefully choreographed and the render time can be used with far more precision than running in-game, and higher detail 'hero' models are often deployed. This may also apply for certain aspects of the lighting, where some scenes suggest that additional lights are placed to simulate the effects of indirect lighting. By extension, the models in real gameplay may not look quite the same because they are lit differently and more manually in cutscenes. This also means that we're unclear as to how indirect lighting itself presents during gameplay - because as far as we can see, there isn't any in the trailer.

That said, Horizon Forbidden West doesn't seem to be using ray tracing for reflections in the footage seen so far. Decima-flavour screen-space reflection is used on reflections for distant water, at least. In one of the last shots in the trailer, pterodactyls obscure the terrain in the distance and as a consequence, the reflection beneath lacks that information. While PS5 offers up hardware accelerated ray tracing as a tool for Guerrilla to utilise, it may not be the best fit for a game like this, which has a lot of organic, natural scenes and rough surfaces.

What we've seen of Horizon Forbidden West so far is obviously highly impressive - and if the game does indeed make its projected 2021 release date, there's probably far more that Guerrilla can show in the not too distant future. While the launch doesn't sound too far away, that's not to say that there won't be scope for more visual improvements: the first game released in February 2017 and we went hands-on with an earlier build at the PS4 Pro reveal in September 2016, and a lot was changed and improved in those final months. Suffice to say, based on how little we've seen so far in combination with the amount of development time left to the studio, there may well be yet more 'next-gen upgrades' that have yet to be revealed.

Before that, we'll be seeing Decima again, albeit from another perspective as Death Stranding and Horizon Zero Dawn are set to arrive on PC in short order. Here, I'd expect to see PC-style improvements in precision to existing effects, pushed out LODs and the like - but I'd be surprised to see any of the next-gen work seen in Forbidden West retrofitted into the older games. However, I'm fascinated to see how some of Decima's foundations transition across - what API will be used? What's CPU utilisation going to look like? How does Decima's checkerboarding look compared to native rendering? And of course, the big one: what do these games look like and how well do they run at 60fps or higher?