Volumetric lighting is one of the most evocative techniques in 3D computer graphics. Often referred to by artists and developers as god rays, light beams, or light shafts, volumetric lighting simulates how light scatters as it passes through a participating medium like fog, smoke, or dust within a three-dimensional space. This is essential in both static 3D renders and real-time applications like video games or VR, where depth perception and immersion hinge on subtle lighting cues. Proper implementation requires attention to shaders, performance, and the rendering engine’s pipeline.
Rendering volumetric lights starts with understanding the relationship between your light source, camera, and fog volume. While it varies across software, the general principle is consistent: volumetric lighting is achieved by calculating how light interacts with a voxel-based representation of fog or particles. Here’s a broad workflow for offline and real-time rendering engines:
Start with a spot light or point light. Make sure it's aimed toward a volume filled with fog or dust particles. This fog volume is typically defined by density, scattering coefficient, and anisotropy, factors that determine how light diffuses through it. In Blender, for instance, this would involve using a volume scatter shader inside a cube and placing a light source like a spotlight with high power values aimed through it.
Anisotropy controls the directionality of scattering. A value of 0 scatters light evenly, whereas positive values (toward 1) concentrate light forward, and negative values scatter it backward. The right setting can drastically influence how beams of light appear (sharp and focused, or soft and diffuse). Higher density values create thicker fog, which can obscure the light or exaggerate its presence, depending on how far the camera is from the volume.
For video-games, accurate volumetric lighting depends on well-defined shadows within the volume. This is done through shadow mapping. Without proper shadow data, beams will appear flat or artifact-prone. In offline rendering, ray tracing handles these interactions naturally. In real-time engines like Unity or Unreal, the use of shadow maps in combination with froxel-based lighting (fragmented voxel data) ensures performance and accuracy.
Most engines simulate additional effects like god rays or bloom using post-processing. Unity’s HDRP and URP pipelines both offer volumetric fog and light settings in their post-processing stack. These blend with the rest of your lighting setup, boosting realism with minimal manual tweaking.
Raytraced volumetric light offers unmatched realism, but with a cost: it’s resource intensive. Performance optimization is crucial, especially in animation pipelines or real-time scenarios. Here are the best practices for efficient volumetric lighting in high-resolution rendering:
Gobo lights (light sources with patterned textures) can be used to break up the beam for visual interest. They’re especially useful in creating believable volumetric effects without having to simulate complex scattering.
In animation, caching volumetric lighting as separate render passes allows reuse and speeds up iteration. Some render engines support precomputed voxel or shadow volumes that can be reused for multiple frames, reducing flickering and ghosting artifacts.
Analytical lighting calculates lighting interactions using mathematically simplified models, often resulting in faster renders. Real-time solutions, on the other hand, balance performance with approximation.
This approach uses equations to estimate scattering within a volume. It doesn’t simulate every photon, but rather approximates the light cone based on direction, density, and attenuation.
In engines like Unity or Unreal Engine, real-time volumetric lighting relies on optimized voxel representations (froxel grids) and depth-based light scattering techniques. Unity’s HDRP uses a froxel-based volumetric lighting system. It processes lights into 3D tiles and combines them per frame, allowing multiple dynamic lights without significant frame drops. URP, by contrast, offers a lighter implementation that’s suitable for mobile or less demanding applications, with fewer features and less accurate fog interaction. If you're using Unity, enabling volumetric lighting requires tweaks in several places:
In other engines like Roblox, volumetric lighting is tied to lighting quality and resource constraints.You can achieve depth without impacting framerate or causing lag through numerous techniques and connecting the effect to the camera. One of the most common ways is to use a mesh or a “fog mesh” for an easy way to get the same effects. However, the mesh method is recommended to only be used for showcases as it can cause lag.
Volumetric lighting can be implemented across various pipelines whether you're working in a game engine, custom renderer, or real-time visualization tool. While feature sets vary, the core setup remains largely consistent. Here’s how to get volumetric lighting in URP and other pipelines:
Additive particle systems with alpha textures can cheaply simulate volumetric effects. Most engines or rendering frameworks offer tools or plugins to simplify this process. Support for depth textures, fog volumes, and screen-space effects is key for achieving quality results without excessive performance cost.
Used in film, volumetric light tells the viewer where to look. In games, it can guide players, signal threats, or set emotional tone. In product visualizations or architecture, it simply makes the space feel real. So the next time you hit render and feel your scene is missing something, consider adding a shaft of light.
