Forza Horizon 5 PC modded to add ray tracing in-game
Dramatic improvements for photo mode too.
Forza Horizon 5 ships on PC, Xbox Series X and Series S with hardware-accelerated ray tracing support to embellish vehicle reflections - but only in Forza Vista and garage modes. This is something of a shame, especially when photo mode in particular stands to benefit immensely. However, on the DF Supporter Program, one of our backers - Frosticles - produced an RT mod for the PC version (with an assist from Rotab for the Windows Store version), allowing for reflections to run in all modes of the game. So, how beneficial is it and what is the performance cost? Could it be enabled in future on Xbox Series X?
To begin with, let's clarify how standard reflections work in Forza Horizon 5, because it's fair to say that even without RT, they look great. If you can imagine six invisible cameras attached to the player vehicle, capturing imagery and mapping it onto a sphere, you get some idea of how the reflections are generated. Effectively, you're looking at cube-map reflections generated continuously in real-time. These reflections are not perspective-correct, but it's close enough and works well enough.
Hardware accelerated ray tracing further embellishes effect by adding RT reflections of the vehicle onto the car itself, something the cube-maps can't do. It's important to stress that it is only the car - not the environment - being traced and certainly not other cars. Anything outside of the player vehicle that makes its way into FH5's reflections all comes from that real-time cube-map image. The specific way that the reflections work also means that 'reflections within reflections' are possible - so wing mirrors reflected in the door glass 'work', though in this case, it's the cube-map from the mirror that is being reflected.
In many scenarios, the changes can be subtle - especially in-game - but close-up to the car, the difference can be extraordinary, so using the mod in photo-mode can be highly beneficial in getting a much better shot. In fact, the thumbnail image on the video (and the article thumbnail on the front-page) are based on a Playground press image, which does have ray tracing enabled: you can see the vehicle reflected in the mounted lights at the front of the car. It's also worth mentioning that the pre-rendered cinematics on the introduction also use the effect. And why not? The cars are the stars of the show and hardware-accelerated ray tracing makes these beautiful models look even better.
In my original PC tech review, I wasn't hugely impressed by PC scalability beyond the Series X quality mode, but engaging RT offers a cool upgrade - but it also has its limitations, of course. For starters, in-game, primarily you are seeing your car just from the back. This means that the impact of self-reflections varies according to the design of the vehicle. Secondly, hardware RT does not work within the vehicles - so the rear-view mirror does not reflect the interior of the car, for example. The final limitation is that when say that only the player car's detail is reflected, we mean it. The player model itself is never reflected. This is a bit of a shame for a potential in-game or photo mode application, but would have no impact for FH5's intended use in Forza Vista or the garage.
However, in addition to forcing RT on in all modes, the mod also access other parameters. The ray-per-pixel or reflection setting can be tweaked to increase or decrease reflection fidelity. One ray per pixel gives a good result, but moving beyond that effectively anti-aliases the reflection and adds a good deal of quality - at a cost to performance, of course. Another option is to increase the number of times rays bounce, above the default one bounce, adding further realism and quality. It's fascinating to test RT in all modes and to tweak the available parameters in Playground's implementation - it gives you some idea of scalability and tweakability in what is clearly a work-in-progress technology.
But let's say you own FH5 on PC and want to give it a go - what's the performance cost? First of all, you obviously need an Nvidia Turing, Ampere or AMD RDNA 2 GPU to make this feature work at all. And the cost of the feature depends on your GPU, what the rendering resolution is, and how close to the car you are. For my testing, I chose the RTX 3080 and RX 6800 XT and used the game's PC benchmark at 4K resolution. On the Nvidia side, RT reflections adds 1.7ms of render time in its default configuration. On the AMD card, it adds about 2.9ms. Bearing in mind that to run at 60fps, we're aiming for a 16.7ms render budget at the maximum, this is a significant cost and clearly, maintaining 4K60 is going to be more viable on the RTX 3080.
Could RT be enabled on Xbox Series X? Bearing in mind that the RX 6800 XT is considerably more powerful, that 2.9ms render cost would rise significantly, so perhaps it's not viable. However, PC gaming is all about options and delivering forward-looking features - and with that in mind, I would hope to see Playground Games incorporate it as an official feature in future. The mod seems to work quite well - the only issue I found concerned square patch artefacts on the rear car window of one specific vehicle. In the here and now though, it's obviously early days for RT in ForzaTech - but I'll be curious to see how the technology evolves for the upcoming Forza Motorsport.