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Glossary:Ray tracing (RT)

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Comparison of Metro Exodus and its Enhanced Edition.[1]
Comparison of Metro Exodus and its Enhanced Edition.[1]

For a list of games, see games with ray tracing support.

Ray tracing (RT) is the generic name, umbrella terminology, and description used to describe various rendering methods or techniques and types in which light is realistically simulated to interact with the environment, as opposed to traditional light rendering methods where light data is hand-crafted based on the expected or current scene/environment. In relation to video games it refers to the simulation of light travelling from designated light sources in the environment (sun/sky, flashlight, lamps, fire, etc) to offer more realistic lighting enabling enhanced visuals and greater player immersion. However if not enough rays or bounces are used to fully establish the path of light, the consequences may be negative with visuals suffering from noise and flickering.

In 2018 the Nvidia GeForce RTX 20 series of graphics cards were the first to integrate hardware accelerated real-time ray tracing in consumer level hardware, enabling its use in Battlefield V and a few other titles.[2] AMD followed this up in 2021 with their release of the Radeon RX 6000 series.[3]

As a consequence of calculating complex and advance light trajectories in real time at a playable framerate, the feature is quite demanding compared to traditional light rendering methods, necessitating the use of supportive features such as advanced ray tracing denoisers and high-fidelity upscaling and frame interpolation technologies to improve visuals without increasing render resolution or ray or bounce count, reducing the overall system requirements of ray tracing. Despite these measures the requirements and performance impact remained high, causing slow adoption of the feature among consumers and developers; especially as early hardware generations with ray tracing support was unable to combine the feature with other often sought-after features at the time, such using a 4K resolution or achieving a high frame rate.

General information

Ray tracing (graphics)
Path tracing
Ray Tracing vs Path Tracing

Rendering methods

As of 2023, the predominant rendering methods used to implement ray tracing features are ray tracing (also known as "normal" or "regular" ray tracing) as well as path tracing (also known as "full" or "real" ray tracing).

  • Ray tracing describes when the path of light is traced from a light source to any adjacent environment or object and back to said source.
  • Path tracing continues the traversal of the light around the adjacent area until the light reaches within the camera's view or a set number of bounces is reached.

Ray tracing is often separated and exposed as four different types depending on what they cover, with separate toggles in the game settings:

  • Ray-Traced Reflections uses ray tracing to allow glass, water and other reflective surfaces to more accurately reflect the world around them from the perspective the player is viewing them in, even allowing objects or environments that might have otherwise been culled from being rendered to be visible to the player.
  • Ray-Traced Shadows makes use of ray tracing to enable more realistic shadows of objects and characters based on surrounding light and character or object movement.
  • Ray-Traced Ambient Occlusion uses ray tracing to allow more realistic occlusion of ambient light against nearby objects and characters
  • Ray-Traced Global Illumination uses ray tracing for general environment light sources (such as a lamp or the sun) to generate more natural and realistic looking lighting.

When Ray Tracing was first being added to games, often only one or two of the aformentioned types were implemented as the computational cost and development time required made it a risky feature for developers to use, but now that ray tracing has become more popular and performant on mid-range graphics cards, more and more games are either adopting Ray Tracing/RTGI as the default lighting method or implement all aforementioned types as separate options.

Games which add ray tracing features close to the end of development or as an update can look subpar, jarring or out of place as a result of the ray tracing not being integrated well with the rest of the graphics pipeline, some notable cases are Metro Exodus where the base game has some ray tracing that isn't too noticeable because it was put on top of the rasterized lighting system, whereas it's enhanced edition made the lighting system only ray traced and made it a more significant part of the graphics pipeline leading to a much different visual experience, look at the picture comparison on this page to see the difference.

While new games with ray tracing took a while to release, some modders and Nvidia decided to add ray tracing to older games to show the contrast between it and traditional lighting, which has lead to things like Doom PRBoom Ray traced and Half-Life 1 Ray traced by user Sultim-t, Minecraft Vulkanite by user cortex, and Quake 2 RTX, Portal with RTX and Nvidia's own RTX Remix software. Generally these modifications also change some of the games' assets to better make use of the ray traced lighting.

Path tracing is far less common due to the increased computational cost in comparison to regular ray tracing and covers all lighting scenarios described above rendered at much higher quality leading to increased visual fidelity and immersion. As of December 2023 there have only been a few games released to date which uses path tracing, including Portal With RTX, Cyberpunk 2077, DESORDRE : A Puzzle Game Adventure, and Alan Wake 2.

Software ray tracing

Software ray tracing refers to implementing ray tracing features using traditional shaders as opposed to relying on the discete ray tracing APIs (such as DirectX Raytracing (DXR) or Vulkan Raytacing (VKRT)), which allows the use of such effects even on graphics card lacking hardware accelerated support for ray tracing technologies, with optional functionality to improve quality and/or performance on ray tracing accelerated hardware.[4] Visual effects implemented this way usually have a lower quality level than that of full "hardware" ray tracing, though the visual effects may still deliver better visuals than a typical rasterized lighting system.

Software based ray tracing debuted in 2020 with the release of Crysis Remastered,[5] where Crytek used DirectX 11 (which otherwise lacks discrete ray tracing support) for the main renderer of the game while an additional Vulkan based backend renderer was used for all ray tracing features. Other games which use software ray tracing include Teardown[6], and Avatar: Frontiers of Pandora[7]

In 2021, Unreal Engine 5 would make software based ray tracing more accessible to developers through its default global illumination and reflections system called Lumen, which supports software based ray tracing out of the box, while having an option to be hardware accelerated potentially improving performance and visual quality on supported hardware.

References