Real-time ray tracing in games has been making waves since Nvidia launched its RTX 20 series cards back in 2018. What was little more than a gimmick, initially, gradually evolved into a feature that gamers consistently look for in their favorite games. More recently, the path tracing vs. ray tracing debate has been making waves, with the former showcasing more advanced and realistic visuals than the latter. With a few more years and GPU generations, path tracing may finally become mainstream.
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Rasterization, Ray Tracing, and Path Tracing
Let’s break down path tracing vs. ray tracing to see why they are needed in gaming graphics.
Traditionally, developers have been using a technique called rasterization to render gaming visuals. Rasterization is the translation of mathematical vector shapes into pixels so that they can be viewed on computer monitors and other pixel-based displays (unlike older CRT displays).
Admittedly, rasterization has been pretty effective in approximating the real-world behavior of light by using “pre-rendered” lighting with static light sources. But, as the demand for photorealistic gaming visuals grew – alongside the technology to power them – real-time ray tracing finally became available to gamers.
Unlike rasterization, ray tracing attempts to simulate the real-world behavior of light in every frame of a game or application. As light can reflect, refract, or get absorbed after interacting with multiple objects in reality, the same behavior of light rays is calculated for every single pixel of a frame. This amounts to calculating the behavior of millions of simulated rays for just one frame. This is why ray tracing is so computationally heavy.
Ray tracing itself isn’t anything new. Developed in 1969, ray tracing has been used by movie studios to varying degrees since the 1970s. But Hollywood has the luxury of “pre-rendering” every frame of a movie by using “render farms” that can sometimes take weeks to render a single ray-traced scene.
Real-time ray tracing in games is different, as it needs to perform this complex calculation in a dynamically changing environment for every single frame. This is also a reason most ray-traced games – even when using hardware acceleration of modern GPUs – only use one or two elements of ray tracing in combination with rasterization, such as ray-traced reflections or shadows.
Now, we come to path tracing. At its core, path tracing is just a modified version of ray tracing. Instead of tracing the path of every single light ray – whether primary or bounced (secondary) rays – path tracing only calculates the most likely path the light would take when shot through a scene.
Conceived in the 1980s, path tracing needs more complex mathematics involving multiple rays for every single pixel, but in the end, amounts to lower overhead for the rendering hardware, i.e., your GPU. Path tracing is an approximation of ray tracing using Monte Carlo simulations – probability models that help predict the most probable outcomes in situations where precise prediction is impossibly difficult.
There are other variations of ray tracing, such as photon mapping (developed in 2001), a two-step global illumination technique. The first step uses light rays throughout a scene to create a photon map containing color and illumination data, and the second step renders the scene in real time, using the preprocessed photon map. Photon mapping can technically reduce the rendering overhead even further, but it is quite complex and prone to some artifacts when there is less powerful hardware.
Path Tracing Looks Better Than Ray Tracing
If path tracing just approximates the effect of ray tracing, why is it any better? Shouldn’t the result be inferior to ray tracing?
No. The path tracing vs. ray tracing debate is a little more nuanced. Path tracing – sometimes called full ray tracing – has actually been shown to produce more realistic lighting in games. Recent games like Cyberpunk 2077 and Alan Wake 2 have showcased what full-fledged ray tracing (with help from path tracing) can look like.
Path tracing has even been implemented in older titles like Quake 2, Portal, The Elder Scrolls V: Skyrim, and Half-Life, transforming them from dated graphics to technological demos.
Path tracing can produce more realistic lighting due to the larger data sample it has access to. Unlike ray tracing, path tracing shoots multiple light rays for every pixel into the scene, sampling a handful of them to arrive at the final output. This enables the algorithm – assisted by denoising, thanks to Nvidia’s DLSS – to produce more defined shadows, reflections, and global illumination.
Path Tracing Is Technically Easier on GPUs
Despite the more complex calculations, denoising, and AI trickery involved in path tracing, it ends up being easier on GPUs – at least in theory.
Games developed from the ground up using path tracing would benefit the most from this lesser overhead. This hasn’t been the case too often. Path tracing has been introduced in various games as an update, years after the original release.
Even Alan Wake 2, which Nvidia claims is a “fully ray-traced game,” doesn’t use path tracing universally. It’s a different, as it looks breathtaking even without that, being one of the best games on the PS5 as well.
The way things are now, such titles are ridiculously hard to run at playable frame rates, even with the most high-end GPUs. Nvidia’s RTX 40 series needs to use an array of AI features just to render these games at lower resolutions and upscale them to produce acceptable results.
But, in a few more years, as AI in games revolutionizes the gaming industry, path tracing implementations will become much easier on mainstream GPUs.
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Path Tracing Isn’t a New Technique
Since the 2000s, path tracing has been used by movie studios, such as Disney and Sony, to render animated films with full path tracing. The theory has been sound since the 1980s, and now real-time path tracing has arrived in popular, albeit prohibitively heavy, games for people to see it happen in front of their eyes.
Path tracing isn’t a radically new technique that will take a long time to trickle down to more games and more affordable hardware. As companies like Nvidia, AMD, and Intel compete to improve their GPUs and invest in advanced AI algorithms, path tracing will only become more prevalent in games as well as other media. And this will happen sooner rather than later.
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Ray Tracing, Path Tracing, and Rasterization Will Work Together
As advanced and impressive as path tracing is, it still can’t do everything perfectly. Its biggest advantage of having access to more data is also its biggest drawback.
Ray tracing is still superior in cases where the available data is small (such as dark dungeons) or in the case of reflective and refractive surfaces (such as water bodies or glass). This is why ray racing will not suddenly disappear from the scene with the advent of path tracing in games.
Developers, as always, will likely use a combination of rendering techniques to produce the most realistic scene in any situation. Hence, both ray tracing and path tracing, along with rasterization, will together power games in the near future.
Every few years, a new technology makes news, as it promises to make games more photorealistic than ever. First, it was shaders, then tessellation, followed by ray tracing, and now, path tracing. But, these technologies also take a huge toll on gaming performance. If you’re feeling your current PC isn’t able to take advantage of these new-age techniques in games, you may need to buy a new GPU. I only hope that as we wait for fully path-traced games of the future, GPUs of the future will also become accessible to the average gamer.
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Image credit: Close-up view of a modern GPU card with circuit by DepositPhotos
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