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Ray Tracing: Explained

We explain the technology of ray tracing, its working, pros and cons, and compare it to other rendering methods.

Since the launch of Nvidia’s RTX 2000 series of cards, “ray-tracing” has been a buzzword in the PC gaming market. Now what exactly is ray tracing, how does it work, and why is it such a major breakthrough for the gaming industry? This article will explain it all. 


Key Takeaways

  • Ray tracing is a graphics rendering technique that uses complex algorithms to trace the path of light in games and animations to make the lighting as realistic as possible. Light reflects, refracts, and forms shadows realistically, just like in real life. 
  • The one major drawback is that it takes a lot of computational power. Also, games use real-time ray tracing, which requires capable hardware (RT cores for NVIDIA, and RT accelerators for AMD). 
  • Compared to rasterization, ray tracing is a lot more intensive. At the same time, it generates much more realistic lighting and soft shadows that rasterization cannot.
  • Path tracing is a much more computationally intensive form of ray tracing that provides better illumination and the ability to form accurate soft shadows
  • Lumen is another intensive form of ray tracing (powered by Unreal Engine 5) and unlocks the ability for real-time global illumination and the formation of indirect reflections.

What Is Ray Tracing? 

The basic meaning of ray tracing is in its name. It is a graphics rendering method that uses an algorithm to trace light rays to mimic the behavior of light in real life. It applies the real-life physical properties of light, like reflections and refractions, to display realistic lighting in animation. 

Formation of shadows by light (Image by Johannes Plenio, 500px.com)

Tracing the path of billions of rays coming from a light source would be nearly impossible for a computer to handle. So instead, ray tracing traces the path of light rays emitted from the camera’s POV. 

The Process

Light is made of tiny particles called photons. A beam of light is emitted from the camera or the viewer’s point (in the case of video games) and is traced back to its source. The beam’s hundreds or thousands of photons encounter multiple objects or surfaces. The surface may be reflective (causing the light to bounce away in a different direction), refractive (absorbing some of the light), or may block the light (forming shadows). Eventually, the rays reach the light source.

A basic visualization of ray tracing (Image by Nvidia)

This process is carried out for each pixel, as a single ray of light is passed through each pixel.

The simplest way to think of ray tracing practically is by looking at any object in your room illuminated by light. From that object, move your eyes toward the light source. That is exactly what happens in ray tracing.

Ray Tracing In Games (Real-Time Ray Tracing) 

In games, ray tracing needs to be done in real-time. This is very challenging as it requires a lot of computational power. Real-time ray tracing was only made possible in 2018 as Nvidia launched their Turing-powered RTX 2000 cards. These cards featured new Ray Tracing (RT) cores that are capable of performing real-time ray tracing. AMD hopped on board in 2020 with their RX 6000 series GPUs, which featured dedicated RT accelerators.

Ray tracing in Shadow of the Tomb Raider
Ray tracing in Shadow of the Tomb Raider (Image by Square Enix)

Read: How To Choose The Right Graphics Card For Your PC

The type of ray-tracing (used in big-budget animated movies) is offline ray-tracing, which works on the same principle. The difference is that the ray-traced frames are generated offline on supercomputers with thousands of cores. Pixar’s Toy Story (1995) was the first movie to use the technology. 

Ray Tracing Vs Rasterization

Rasterization is a rendering technique that converts a 3D mesh of 2D shapes (polygons) into 2D pixels on your screen. The vertices (corners) of these polygons contain information about the texture and color of the pixels. After further processing of the pixels by a shader, their final color is determined based on the lighting in the scene. 

Ray tracing vs rasterization
Ray tracing vs Rasterization (Image by ResearchGate.net)

Although rasterization itself takes significant computational power, it is much less intensive than ray tracing. Rasterization cannot portray accurate shadows, reflections, refractions, and lighting. Instead, it “fakes” them in an attempt to make the game look as visually pleasing as possible. 

Since real-time ray tracing was made possible in 2018, most video games have used hybrid rasterization with ray tracing. This technique combines a rasterized game engine with ray tracing. Rasterization generates most objects on display, while ray tracing generates accurate lighting, shadows, and reflections. 

Ray Tracing Rasterization
Ray tracing is a rendering technique used to simulate realistic lighting in video games and animation. Rasterization is also a rendering technique. It converts a 3D mesh of polygons into 2D pixels on your screen.
Ray tracing is much more computationally intensive. Rasterization is not as intensive and thus is also the widely used rendering technique in video games.
Ray tracing uses algorithms to accurately mimic the physical properties of light, such as reflections and refractions. Rasterization “fakes” the behavior of light and does not always generate accurate shadows and reflections.
Real-time ray tracing (used in video games) requires dedicated hardware. (RT cores or RT accelerators) Rasterization does not require dedicated hardware.

Ray Tracing Vs Path Tracing 

Path tracing is essentially a more intensive mode of ray tracing. In path tracing, multiple rays are sent through a single pixel instead of one ray for each pixel. However, the algorithm only traces certain rays that follow the most likely path of light. 

Path tracing
Path tracing process (Image by TechSpot)

When a ray reflects or refracts after hitting an object, it becomes a separate, independent ray. The ray then proceeds to bounce off in another direction. The process may repeat multiple times before the ray eventually reaches the light source. In this way, path tracing provides more illumination than ray tracing, which can be seen below.

Path tracing vs ray tracing vs rasterization
Path tracing vs ray tracing (vs rasterization) (Image by Nvidia)

Path tracing calculates soft shadows ray tracing is unable to do. Overall, it is more realistic than ray tracing. 

Ray Tracing Path Tracing
In ray tracing, only a single ray is passed through each pixel. In path tracing, multiple rays are passed through a single pixel.
Ray tracing traces the path of every light ray that passes through a pixel. Path tracing only traces the path of certain few light rays. (those that follow the most accurate path)
Ray tracing does not calculate soft shadows accurately and is not as illuminating. Path tracing is a lot more illuminating and calculates soft shadows accurately.
It is not as computationally intensive. More computationally intensive than ray tracing.

Ray Tracing Vs Lumen

Like path tracing, Lumen is a more intensified form of ray tracing. It is used as the default lighting system in Unreal Engine 5. It uses multiple forms of software-based ray tracing (hardware ray tracing is also an option) and does not need a dedicated ray tracing hardware. The process of software ray tracing is much more complex than normal ray tracing.

Lumen vs Ray Tracing
Global Illumination: Lumen vs Ray Tracing (Image from lunas.pro)

The main goal of Lumen is to provide real-time global illumination and indirect reflections. Global illumination is the process in which light bounces off one surface (thus becomes indirect light) and goes on to hit another surface. This causes effects like color bleeding and soft indirect shadows, providing incredibly realistic and true-to-life lighting in video games. Such effects are not possible in normal real-time ray tracing. 

Lumen Ray Tracing
Focused on simulating real-time global illumination and indirect lighting. It cannot simulate dynamic GI or indirect lighting
By default, it is software-based and thus does not rely on dedicated hardware. Real-time ray tracing requires dedicated hardware. (RT cores or RT accelerators)
Needs a lot more computational power. It is less computationally expensive than Lumen.

Final Words

Ray tracing is a technology for the future that can stimulate true-to-life lighting in video games and animations. It is, however, computationally expensive, and achieving a stable 60 FPS, especially on higher resolutions, is still only possible on fairly expensive GPUs. However, with hardware advancing more and more every year, ray tracing is becoming easier to run and more common too. 

Nvidia GeForce RTX 4090
Nvidia’s RTX 4090 offers the latest and greatest in real-time ray tracing support (Image by Tech4Gamers)

In any case, Nvidia pulled off a revolutionary move by enabling the capabilities of real-time ray tracing with the introduction of their Turing-based RTX cards. It’s exciting to think about the levels of realism we will continue to see in games, thanks to ray tracing. 

Also Check: What are Tensor Cores?

Frequently Asked Questions

Is ray tracing worth it?

Ray tracing enables games to display realistic lighting, reflections, and refractions. It is certainly worth it, given you can afford the hardware to handle it.

How to turn on ray tracing?

First, ensure the game you’re playing has ray-tracing support. Once you’re sure it does, you need to proceed to the “advanced video settings” or “graphics settings” tab in-game and look for “Ray Tracing” or “Ray Tracing Mode” and choose your desired setting.

Is ray-tracing for RTX only?

Video games perform real-time ray tracing and require a GPU with RT cores (in the case of Nvidia) or RT accelerators (in the case of AMD). For other software-based ray tracing applications (like Lumen), such cores aren’t necessary.

What is the difference between ray tracing and path tracing?

Ray tracing sends a single ray through each pixel and traces the path of all the rays. Path tracing sends multiple rays through a pixel but only traces the path of certain rays. Furthermore, path tracing provides better illumination than ray tracing.

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