What Is Ray Tracing? | Digital Trends

Ray tracing is a lighting technique that brings an extra level of realism to games. It emulates the way light reflects and refracts in the real world, providing a more believable environment than what’s typically seen using the static lighting in more traditional games. But what is ray tracing, exactly? And more importantly, how does it work?

A good graphics card can use ray tracing to enhance immersion, but not all GPUs can handle this technique. Read on to decide if ray tracing is essential to your gaming experience and if it justifies spending hundreds on an upgraded GPU.

Virtual photons

To understand just how ray tracing’s revolutionary lighting system works, we need to step back and understand how games previously rendered light and what needs to be emulated for a photorealistic experience.

Games without ray tracing rely on static “baked in” lighting. Developers place light sources within an environment that emit light evenly across any given view. Moreover, virtual models like NPCs and objects don’t contain any information about any other model, requiring the GPU to calculate light behavior during the rendering process. Surface textures can reflect light to mimic shininess, but only light emitted from a static source. Take the comparison of reflections in GTA V below as an example.

Overall, the GPU’s evolution has helped this process become more realistic in appearance over the years, but games still aren’t photorealistic in terms of real-world reflections, refractions, and general illumination. To accomplish this, the GPU needs the ability to trace virtual rays of light.

In the real world, visible light is a small part of the electromagnetic radiation family perceived by the human eye. It contains photons that behave both as a particle and as a wave. Photons have no real size or shape — they can only be created or destroyed.

That said, light could be identified as a stream of photons. The more photons you have, the brighter the perceived light. Reflection occurs when photons bounce off a surface. Refraction occurs when photons — which travel in a straight line — pass through a transparent substance and the line is redirected, or “bent.” Destroyed photons can be perceived as “absorbed.”

Ray tracing in games attempts to emulate the way light works in the real world. It traces the path of simulated light by tracking millions of virtual photons. The brighter the light, the more virtual photons the GPU must calculate, and the more surfaces it will reflect, refract, and scatter off and from.

Minecraft RTX ray tracing beta
Ray tracing alone makes a huge difference in Minecraft. Matthew Smith / Digital Trends

The process isn’t anything new. CGI has used ray tracing for decades, though the process required farms of computers in the early days to generate a full movie given a single frame could take hours or even days to render. Now home PCs can emulate ray-traced graphics in real time, leveraging hardware acceleration and clever lighting tricks to limit the number of rays to a manageable number.

But here’s the real eye-opener. Like any movie or TV show, scenes in CGI animation are typically “shot” using different angles. For each frame, you can move a camera to capture the action, zoom in, zoom out, or pan an entire area. And like animation, you must manipulate everything on a frame-by-frame basis to emulate movement. Piece all the footage together and you have a flowing story.

In games, you control a single camera that’s always in motion and always changing the viewpoint, especially in fast-paced games. In both CGI and ray-traced games, the GPU not only must calculate how light reflects and refracts in any given scene, but it also must calculate how it’s captured by the lens — your viewpoint. For games, that’s an enormous amount of computational work for a single PC or console.

Unfortunately, we still don’t have consumer-level PCs that can truly render ray-traced graphics at high framerates. Instead, we now have hardware that can cheat effectively.

Let’s get real

Ray tracing’s fundamental similarity to real life makes it an extremely realistic 3D rendering technique, even making blocky games like Minecraft look near photo-realistic in the right conditions. There’s just one problem: It’s extremely hard to simulate. Recreating the way light works in the real world is complicated and resource-intensive, requiring masses of computing power.

That’s why existing ray-tracing options in games, like Nvidia’s RTX-driven ray tracing, aren’t true to life. They’re not true ray tracing, whereby every point of light is simulated. Instead, the GPU “cheats” by using several smart approximations to deliver something close to the same visual effect, but without being quite as taxing on the hardware. This will likely change in future GPU generations, but for now, this is a step in the right direction.

Most ray tracing games now use a combination of traditional lighting techniques, typically called rasterization, and ray tracing on specific surfaces such as reflective puddles and metalwork. Battlefield V is a great example of that. You see the reflection of troops in water, the reflection of terrain on airplanes, and the reflection of explosions across a car’s paint. It’s possible to show reflections in modern 3D engines, but not at the level of detail shown in games like Battlefield V when ray tracing is enabled.

Ray tracing can also be leveraged for shadows to make them more dynamic and realistic looking. You’ll see that used to great effect in Shadow of the Tomb Raider.

Ray-traced lighting can create much more realistic shadows in dark and bright scenes, with softer edges and greater definition. Achieving that look without ray tracing is extraordinarily hard. Developers can only fake it through careful, controlled use of preset, static light sources. Placing all these “stage lights” takes a lot of time and effort — and even then, the result isn’t quite right.

Some games go the whole hog and use ray tracing for global illumination, effectively ray-tracing an entire scene. But that’s the most computationally expensive and needs the most powerful of modern graphics cards to run effectively. Metro Exodus uses this technique but the implementation isn’t perfect.

Because of that, half-measures like only ray-tracing shadows or reflective surfaces are popular. Other games leverage Nvidia technologies like denoising and Deep Learning Super Sampling to improve performance and to cover up some of the visual hiccups that occur from rendering fewer rays than would be necessary to create a truly ray-traced scene. Those are still reserved for pre-rendered screenshots and movies where high-powered servers can spend days rendering single frames.

The hardware behind the rays

To handle even these relatively modest implementations of ray tracing, Nvidia’s RTX 20-series graphics cards introduced hardware specifically built for ray tracing.

Nvidia’s Turing architecture — featured on 20-series GPUs — introduced RT cores alongside Nvidia’s CUDA and Tensor cores. RT cores are solely there to handle real-time ray tracing. In Turing cards, the RT cores performed decently, but it wasn’t until the recent Ampere launch where we saw them shine.

Nvidia released a breakdown of generating a single frame of Metro Exodus, where it showed how the rendering pipeline is laid out and how it is affected by ray tracing. While an RTX 2080 and GTX 1080 Ti might be roughly comparable in performance for non-ray-traced games, when ray tracing is applied to a scene, it can take much longer for the 1080 Ti, without the dedicated RT cores, to generate the same image.

The dedicated RT cores in RTX 20-series GPUs were a big selling point, but they didn’t quite deliver the performance Nvidia suggested. Even last-gen’s 2080 Ti struggled in supported ray tracing titles upon launch. The new RTX 3080 and 3090 feature newer RT cores, however, and the performance improvement is clear. Not only are these cards faster than their last-gen counterparts, but the new RT cores are faster, too. In many ways, the RTX 30-series cards feel like the GPUs Nvidia was promising with RTX in the first place.

Nvidia’s ray tracing method isn’t the only option available, however. There are also Reshade “path tracing” post-processing effects that deliver comparable visuals without anything like the same performance hit.

AMD has options for ray tracing now, too, which we’ll get to next.

You’ll still want a powerful graphics card for ray tracing no matter the implementation, but as the technique catches on with game developers, we may see a broader array of supporting hardware at much more affordable prices.

What about AMD?

AMD Radeon RX 6000

AMD has struggled over the past few years to deliver hardware-accelerated ray tracing, but that changed with the launch of the RX 6800, 6800 XT, and 6900 XT. These new cards feature DirectX 12 ray tracing support, and deliver excellent performance, even if AMD isn’t quite on Nvidia’s level in the ray-tracing department (read our RX 6800 XT vs. RTX 3080 and RX 6900 XT vs. RTX 3090 comparisons for more).

That hardly comes as a surprise considering the Big Navi architecture powering AMD’s RX 6000 cards is very much a first-generation of ray tracing acceleration. It’s the same architecture that powers the visuals in the PS5 and Xbox Series X, catering to a lower-tier of performance overall than Nvidia’s flagship cards. However, because ray tracing is a standout feature on next-gen consoles, we expect better support and optimizations moving forward. In the near future we will see the debut of AMD FidelityFX Super Resolution (FSR) for gaming PCs and also the latest versions of Microsoft Xbox.

How can you see ray tracing at home?

You’ll need a recent — and expensive — graphics card to see ray tracing at home. Hardware-accelerated ray tracing is only available on Nvidia RTX 20-series and 30-series GPUs, and on AMD’s RX 6000-series GPUs (GTX 10-series and 16-series cards support ray tracing but lack the RT cores to make it comfortably playable). It seems likely that RX 6000 and RTX 30-series cards will stay out of stock throughout 2021, and RTX 20-series cards have reached end of life, meaning stock is quickly dwindling from retailers. In late 2020, there aren’t many options, but that should hopefully change in the next few months. Once you’ve secured a graphics card, the rest is simple.

If you expect to play at resolutions above 1080p and with frame rates of 60 FPS or more, your best bet is to splurge for a top-of-the-line graphics card. At 4K, the RTX 3080 and RX 6800 XT are the standout cards, but you can get by with an RTX 3070 or RX 6800 if you’re willing to move to 1440p in certain titles.

There’s a limited selection of ray tracing-enabled games, but the number is growing. Top examples of ray tracing include early RTX demos, like Battlefield V, Shadow of the Tomb Raider, and Metro Exodus. More recent games like Control and MechWarrior 5: Mercenaries also look compelling. Stay in the Light is an indie horror game built using ray-traced shadows and reflections. The remastered Quake II with RTX ray tracing is another fantastic example. 

There are fewer ray tracing games on the market, but the industry grows. As the PS5 and Xbox Series X begin advertising ray tracing, competitors will soon follow. The multi-platform game Watch Dogs 2 is different from the new Watch Dogs: Legion as the new game has initiated ray tracing to work on consoles and computers.

Try using UL Benchmark’s Port Royal ray tracing to determine if your PC will work with ray tracing.

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