What is ray tracing? How it works, in games and in 3D rendering

Ray tracing shows up in game settings and in 3D render engines, and the idea behind both is the same. Path tracing, the variant behind film-quality renders, has its own comparison in path tracing vs ray tracing. Blender settings below are checked against the Blender 5.2 manual and source code.

What is ray tracing?

Ray tracing is a rendering technique that works out the color of each pixel by following rays through a virtual scene. A ray leaves the camera, passes through one pixel of the image and travels until it meets a surface. At that point the renderer knows what the pixel sees, and it can ask more questions by tracing more rays. Is a light visible from here, or is something in the way? What does this surface reflect? What lies behind this glass? To ray trace a scene is to answer those questions, ray by ray, for every pixel.

Because every answer comes from geometry the rays actually meet, shadows fall where objects block the light, mirrors show what stands in front of them, including things outside the frame, and glass bends the view behind it. That is the difference with rasterization, the technique real-time graphics are built on, which has to simulate those effects.

How ray tracing works, step by step

1. Primary rays. For each pixel, a ray starts at the camera and goes through that pixel into the scene.
2. Intersection. The renderer looks for the closest surface the ray hits. This is the step that costs the most, since it is repeated for every ray (more on that below).
3. Shadow rays. From the hit point, a ray goes toward each light. If an opaque object lies in between, the point is in shadow for that light.
4. Reflection rays. On a reflective surface, a ray leaves in the mirror direction, and whatever it hits appears in the reflection.
5. Refraction rays. On a transparent material, a ray continues through the surface, bent by refraction, to find what lies behind it.
6. Recursion. Reflection and refraction rays hit other surfaces, which send their own shadow, reflection and refraction rays, up to a set depth. All of it is combined into the color of the pixel.

These rays run backward compared with real light, which travels from the lamps to the eye. The Cycles manual explains the choice: tracing from the camera wastes no ray on light that never reaches it, but it makes some important light paths hard to find. The same manual names caustics, light focused by glass or water, among the effects this approach handles poorly; the guide to caustics in Blender shows how Cycles copes with them.

Where it comes from: Appel and Whitted

Arthur Appel published the first use of a computer to trace rays for shaded pictures in 1968; his non-recursive method is now called ray casting. Turner Whitted, then at Bell Labs, added the recursive part in "An Improved Illumination Model for Shaded Display", published in Communications of the ACM in June 1980. For each pixel, a tree of rays runs from the viewer to the first surface, on to other surfaces and to the light sources, and the shader uses it to render true reflection, shadows and refraction.

Whitted-style ray tracing has a recognizable look. A single shadow ray per light either reaches the light or does not, so a light can never be partly hidden and shadows have hard edges. One reflection ray and one refraction ray per hit make every mirror and every pane of glass perfectly sharp. Soft shadows and blurry reflections need many rays averaged together, the idea behind distributed ray tracing and then path tracing.

Finding what a ray hits: acceleration structures

Testing every ray against every triangle would be far too slow once a scene holds millions of them. Ray tracers first sort the geometry into an acceleration structure, such as a bounding volume hierarchy (BVH): a tree of nested boxes that lets a ray skip whole groups of triangles it cannot touch. DirectX Raytracing builds a two-level structure in a format the GPU can traverse quickly, and Cycles builds its own before rendering; its settings sit in Render Properties > Performance > Acceleration Structure, a panel that is not shown with an OptiX device. The guide to BVH in Blender explains how it is built and when it is rebuilt.

Ray tracing vs rasterization

Rasterization takes the opposite route. It projects each triangle onto the screen and finds which surface is visible in each pixel. The EEVEE manual sums it up: instead of computing each ray of light, rasterization determines what surface is visible from the camera, then estimates how light interacts with it. That is very fast, but each pixel only knows its own surface, so anything that depends on the rest of the scene, such as shadows, reflections or bounced light, has to be approximated with what Microsoft's DirectX team calls "a wide variety of clever techniques".

Screen-space reflections show the limit: they reuse the image already drawn, so they cannot reflect what is hidden or off screen. Microsoft presented DirectX Raytracing in 2018 as a way to fill in that missing geometry, and NVIDIA described a hybrid approach for its first RTX cards: rasterization to determine what is visible, ray tracing for reflections, refractions and shadows.

Real-time ray tracing in games

For a long time, ray tracing was out of reach for games. Presenting its Turing architecture in 2018, NVIDIA explained that ray tracing without hardware acceleration takes thousands of software instruction slots per ray, which made it impossible on GPUs in real time. The milestones since then:

What RT cores do

RT cores are the units of an NVIDIA RTX GPU dedicated to the intersection step. According to NVIDIA, they accelerate BVH traversal and ray/triangle intersection testing, which leaves the rest of the chip free for shading, working out the color of whatever each ray hit. AMD's equivalent is the Ray Accelerator. Games reach this hardware through programming interfaces: DXR on Windows and Xbox, and Vulkan Ray Tracing, which Khronos designed to run either on general GPU compute or on dedicated ray tracing hardware.

What the ray tracing settings in a game do

Few games trace the whole image. Most offer ray tracing effect by effect, on top of a rasterized frame, and the names change from one game to the next. The effects NVIDIA, AMD and Microsoft describe are these:

• Reflections: water, glass, mirrors and glossy floors can show objects that are hidden or off screen, which screen-space reflections cannot. They were the first ray-traced effect in a released game: Battlefield V and its "DXR Reflections Quality" setting, in November 2018.
• Shadows: rays cast toward the lights decide what is in shadow, for what NVIDIA calls physically accurate shadows.
• Global illumination: light that bounces from one surface onto another, so a sunlit red wall tints the floor next to it and rooms are lit by more than their lamps.
• Ambient occlusion: the soft darkening in corners and where objects meet, computed from the real geometry rather than from the depth buffer. The guide to ambient occlusion covers the screen-space versions.
• Path tracing, or full ray tracing: all the lighting comes from traced light paths. NVIDIA introduced the Overdrive mode of Cyberpunk 2077, in 2023, as "full ray tracing, otherwise known as path tracing", where earlier techniques handled shadows, reflections and global illumination separately.

Why ray-traced effects need a denoiser

Even with RT cores, rays stay expensive: NVIDIA writes that shooting rays for every pixel on the screen is too computationally demanding, so games fire a handful of ray samples across the scene. The raw output is a noisy, speckled image with gaps. Denoisers clean it up by accumulating pixels over several frames, at the risk of ghosting, and by blending neighboring pixels, at the risk of blending away detail. DLSS Ray Reconstruction, on NVIDIA cards, replaces these hand-tuned denoisers with a trained AI network.

The cost in frame rate, and what DLSS and FSR do

Every ray-traced effect adds GPU work to every frame, so turning one on costs frames per second, by an amount that depends on the game, the effect and the card. NVIDIA's own figures for the path tracing mode of Cyberpunk 2077 show how heavy full ray tracing is: at 4K on a GeForce RTX 4090, the company says DLSS 3 boosts performance 4.6 times, to 103 frames per second.

Upscaling is how games win that time back: the GPU renders fewer pixels, then rebuilds a higher-resolution image.

• DLSS (NVIDIA): Super Resolution uses AI to output higher-resolution frames from a lower-resolution input. NVIDIA lists Super Resolution and Ray Reconstruction for the GeForce RTX 20 series and newer, Frame Generation, which creates extra frames with AI, for the RTX 40 and 50 series, and Multi Frame Generation for the RTX 50 series.
• FSR (AMD): its first versions are open source, published under the MIT license, and AMD says they run on a large variety of GPUs, not only the latest architectures. FSR 2 is a temporal upscaler, and FSR 3 adds frame generation. The current FSR Upscaling (formerly FSR 4) uses neural networks on Radeon RX 6000, 7000 and 9000 cards, ships only as prebuilt DLLs and falls back to FSR 3.1 on other GPUs.

Ray tracing in 3D rendering: Blender, Cycles and EEVEE

Offline rendering, for animation, product shots or architecture, has no frame rate to hold, so the renderer can trace many rays per pixel and follow light over many bounces: that is path tracing. Blender has one engine on each side of the line.

Cycles: a path tracer

The Blender manual describes Cycles as "Blender's physically-based path tracer for production rendering". It traces rays from the camera into the scene and onto the lights, and each path keeps bouncing, with rays sent according to the material's BSDF (how the surface scatters light) or toward known light sources. Each pixel averages many such paths, its samples: few samples leave noise, more samples clean it up. Cycles renders on the CPU, or on the GPU once a device is enabled in Edit > Preferences > System > Cycles Render Devices and the scene's Device is set to GPU Compute.

EEVEE: a rasterizer that can trace rays

EEVEE, the engine a new file starts on, "is based on rasterization and is not a path tracer", says its manual. Since Blender 4.2, when EEVEE was rewritten, it uses screen space ray tracing for every BSDF. Tick the checkbox in the header of Render Properties > Raytracing, off in a new file, and EEVEE generates rays from each BSDF and finds their intersections with the scene. Method sets how: Light Probe, the cheapest, uses the light probe spheres and planes placed in the scene; Screen-Trace traces rays against the depth buffer and falls back to the light probes when a ray leaves the view. With the panel off, EEVEE uses a faster pipeline based on pre-filtered light probes.

The limitations page of the manual spells out what screen space means: ray-triangle intersection is not supported, and the depth buffer stands in for the scene. Objects hidden behind other objects are ignored, and the effects disappear at the edge of the frame, which the Overscan option partly fixes.

In the viewport, Material Preview always runs on EEVEE. To see what Cycles traces, switch to Rendered shading, which uses the scene's render engine. Choosing an engine for the final image is covered in how to render in Blender.

Hardware for ray tracing

Cycles can trace rays on a CPU; dedicated hardware makes the job faster. In games, ray-traced settings are designed around GPUs that have it: the first release of ray tracing in Battlefield V required a GeForce RTX card. The list now includes AMD Radeon RX 6000 and newer, Intel Arc, the PlayStation 5 and Xbox Series X, and Macs with M3 chips. In Blender, Cycles reaches that hardware through its GPU backends:

• NVIDIA: OptiX takes advantage of the hardware ray tracing acceleration of RTX cards; CUDA does not use the RT cores. In Blender 5.2, OptiX requires driver 575 or newer. The guide to OptiX vs CUDA covers the setup.
• AMD: HIP runs on RDNA1 cards and newer; its hardware ray tracing, enabled in the Preferences, works on the Radeon RX 6000 series and newer.
• Intel: oneAPI on Arc A-Series and B-Series cards, with hardware acceleration for ray tracing and denoising.
• Apple: Metal on Apple Silicon, with GPU-accelerated ray tracing and denoising.

Video memory counts too, since the manual warns that GPUs have more limited memory for complex scenes: the guide to the best PC for 3D rendering weighs both.