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Metro Exodus and Real-Time Ray-Tracing - A Glimpse Into the Future
Post by KostaAndreadis @ 12:35pm 19/02/19 | Comments
The release of Metro Exodus sees the PC version implementing real-time ray-tracing in a way that showcases where interactive technology is heading.

It’s the visual effect that took everyone by surprise when NVIDIA announced its RTX line of graphics cards last year. Namely because the computational requirement to render an interactive scene with real-time ray tracing for lighting and other effects is so huge - the tech has been though of for many years as, The Future. That thing that will perhaps one day make the games we play look and feel more realistic. One day.

And that’s not photorealism where one mistakes a digital human for the real thing, but the look and feel of a modern-day big budget computer-generated cinematic. Where everything looks better because light, shadows, and reflections all behave realistically due to all that stuff being calculated off light-rays bouncing around. And doing their thing.

Previously we delved into the still very new technology, as part of Battlefield V’s real-time ray-tracing implementation. Where the technology was utilised to accurately reflect images and light. Ray-traced reflections. In execution, an impressive technical demonstration of the future of videogame explosions – where big fiery balls of, err, flame light up and bounce off objects whether visible or not. Seeing something behind you reflected in a mirror-like surface.

As the only real-world example that took advantage of NVIDIA’s RTX line and the still very new DirectX DXR technology, the newness of it meant that it took a few patches before performance was in a state that felt good enough to turn, as NVIDIA puts it, RTX On. Still, a technology showcase that although impressive - was far from essential. After all, Battlefield V is traditionally an online shooter so a cut into performance for an impressive visual effect might not be worth it.

With the release of Metro Exodus though, the story changes. As the second major release to support the new RTX-powered technology, instead of real-time ray-traced reflections, the development team at 4A Games has utilised ray-tracing for global illumination. With the results ranging from subtle to barely noticeable to stunning and even the equivalent of, say, going from night into day.

Experiencing Metro Exodus, with ray-tracing enabled, is a glimpse into the future of videogames.

Global Illumination – What Is It?

Based on name alone, it’s not hard to understand what a reflection is – and how tracing light and rays from a source can be used to make all that stuff look realistic in ways that faking it can only hint at. Global illumination though, is something else. A term used specifically in the world of computer graphics and a reference to dealing with indirect light. That’s light that bounces off surfaces to add additional luminance to a room, add colour to a shadow, and more.

To better understand the concept, take a direct light source like a torch or lamp as an example. In the world of videogames, a torch or lamp can easily become a direct source of light, where nearby objects will cast shadows or brighten up depending on their proximity to the source. Where indirect light comes into play is simple. Say that the light from a lamp hits a white wall, not only will the light from the lamp brighten up the room but additional light from the now brightly lit wall will too. From there the light from the wall can hit the floor and the light from the floor can then bounce back to the wall. Add a table into the mix and you can begin to see how lighting in this way accurately presents shadow and colour.

The effect, even without ray-tracing is taxing on hardware. Global illumination adds depth to shadows, gives object more realistic colour, and even result in the light from outside a window can bouncing off a surface to light up a room that doesn’t have its own light source. The only down-side when it comes to games – is that until now, global illumination was handled via pre-calculations. Creating light-maps of how certain scenes should look and using that to create some truly impressive effects. Effects that due to the real-time interactive nature of videogames are never quite completely realistic.

Metro Exodus – Leading Us All to the Ray-Traced Promise Land

In the case of Metro Exodus, the result of its real-time ray-traced global illumination is the most natural and impressive lighting ever seen in a game. A feature that becomes more impressive the more you play. At a glance the effect might be subtle in a lot of scenes, barely noticeable. Taking the following two images, the first with RTX ray-tracing on, and the second with RTX Off – you might need a minute to notice any differences.

But even in this less noticeable example, there’s a lot happening. Like in one of those picture-puzzles that presents you with a challenge to ‘Spot the 10 Differences’.

With ray-tracing on, the snow is brighter - based on how snow reacts in the real-world and becoming brighter due to rays bouncing around in a realistic manner. This in turn makes parts of the train closer to the snow brighter, and those obscured casting darker and more realistic shadows. In the second and admittedly darker image, parts of the train obscured by direct sources of light – including snow – are clearly visible where in the ray-traced image they are more realistically presented.

It’s subtle sure, and you wouldn’t be at fault for viewing this one example and thinking that it was merely ‘different’. And not exactly ground-breaking or, as this feature intends to argue – the future of videogames.

So, let’s switch gears and showcase a more drastic example.

In this image which takes the bright light coming in through the windows, the global-illumination powered by ray-tracing offers a stunning example of realistic depth to shadows. Turning it off and the result is a flat image. Taking the train console and roof as an example there’s a world of difference. But, also in the depiction of the characters too. Who animate and move around a ray-traced environment with shadows and light presented more realistically.

Going even further here’s an indoor location – not tied to any specific mission and lit by ray-traced global illumination. The wreck of a rusty sea vessel in a sea of bright sands and desert heat. In the following clip, the ray-traced image is more natural, and unlike the outdoor examples to follow – darker. With light coming in through half-buried windows creating true depth as it bounces around and getting dimmer the further you go – the key take-away is that this ray-traced global illumination is being handled in real-time.

The difference is night and day.

Even though Metro Exodus features many dark and creepy locations that give off massive survival horror vibes, the time of day in the above example means that this location, fake or approximate lighting gives off a flat and unrealistic image. Metro Exodus and many games, like the recent remake of Resident Evil 2 play with light and shadow in ways that truly impress - without ray-tracing. But the addition of RTX global illumination takes it all to the next level. Where the effect has a seamless feel, never breaking the illusion. And something that could really-never be faked to this level in a game with large open areas and a dynamic day/night and weather system.

One area where ray-traced global illumination seems to provide the most realistic benefit is also with that thing you might immediately notice. Whether or not the image is brighter. With RTX On, objects like snow and sand appear far brighter and less textured than with RTX Off. In the following example we can see a bright moonlit exterior, with more accurately whiter sand and a mostly dim entrance to a pitch-black interior.

Turning off RTX ray-tracing and the entrance is completely covered in shadow with no realistic moonlight or light from the sand. In fact, the sand or dirt is coloured in a way that looks and feels unrealistic.

Surprisingly one of the side-effects of ray-traced global illumination in Metro Exodus has a very different look to a lot of locations. In well-lit and more defined cinematic places with multiple direct light sources, the visual difference can be subtle. But wide-open areas can look like scenes from a different game. Or the same game on a different platform, a port – rendered differently giving off a visibly different presentation. The following clip showcases the shift from subtle, more realistic shadows and light – to an entire environment that looks different.

The question to ask isn’t whether or not RTX On looks more realistic – but whether it feels that way. And you might even prefer the look of RTX Off. Which comes down to perhaps the greatest complement that we could give the effect. RTX Off looks more like a game versus the cinematic quality that comes with RTX On.

The Future is Now

Although these examples were hand-picked to properly showcase the difference that comes with the effect, ray-tracing becomes more impressive the more you play. Especially in the case of Metro Exodus, a moody, immersive, and stunning post-apocalyptic journey through various landscapes across post-war Russia.

In the following, and final example we get to see perhaps the subtlest of differences possible. Where without ray-tracing objects close to a window are baked in shadow. In the ray-traced scene the light from surrounding windows accurately light the room in a circular fashion.

Heading downstairs and the effect is reversed, where without ray-tracing the number of windows leads to a brightly lit interior. In the ray-traced version it’s not simply darker but lit in such a way that each connecting room feels natural.

In a way Metro is the perfect game to implement such a ground-breaking feature. The global illumination ensures that it all feels, well, natural. And again, that’s not in a photorealistic sense - but in the way everything in one scene feels like part of a single picture. And how every scene, location, and moment in turn feels like one part of an even bigger picture. Every tiny detail that might change the look or feel thanks ray-tracing adds up. No matter how small or subtle. And with ray-tracing on, Metro Exodus paint a very big picture.
A picture that gives a glimpse into the future.

All footage captured at 4K with an NVIDIA RTX 2080 Ti with both graphic detail and RTX set to Ultra. Performance-wise with NVIDIA's DLSS AI rendering frame-rates sit in the 60fps range, albeit with a softer overall image.

For our full Metro Exodus review - head here.
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