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What is Lumen in Unreal Engine 5?

Unreal Engine 5 Lumen

Lumen is the new global illumination and reflection system created within Unreal Engine 5 to support the new generations of gaming consoles.

Lumen harnesses a fully dynamic indirect lighting pipeline which works in unison with any environments geometries, materials and light properties to provide optimized artist workflow. The optimized workflow is achieved through the full dynamic indirect lighting pipeline providing instant lighting builds and reflection captures. Lumen removes the need for reflection cubemaps, since it fully replaces other methods and is able to render geometrically precise reflections.

See below a full walkthrough and Lumen overview from Paulo Souza of Epic Games:

Lumen VS Ray-Tracing

A fundamental part to understand about Lumen is that its built on-top of Ray-Tracing and is basically a optimized and hybrid version of Ray-Tracing with the core aim of running on any level GPU or console. Cutting out the definite need of RTX cards for developers, Lumen is designed to harness several different Ray-Tracing methods within it’s pipeline. The 3 main Ray-Tracing methods are:

  • Signed Distance Fields (SDFs)
  • Mesh Distance Fields (MDFs)
  • Screen-Tracing (SSRT)

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Signed Distance Fields – Are a extremely fastest method of tracing a environment but unfortunately provides detail lacking results. Lumen harnesses this tracing method as a means to create a exoskeleton of the scene providing the walls, floor and large (but simple) texture blocks.

Mesh Distance Fields – Provide a much higher detailed traces but only hold sparse details near the surface of the objects. MDFs create a close 2m of “detailed tracing” area from the camera perspective, whilst the wider scene is done by a lower detail “Global tracing” method. MDF’s within the 2m “Detailed Tracing” area creates low poly traces of objects to aid with lighting and reflection calculations.

Screen-Tracing- The Lumen software actually starts with Screen Space Ray-Tracing, which is conducted against geometry’s  visible on screen and works to calculate where the screen space shadows (SSAO) are cast.

Surface Cache – To further speed up the process, Lumen uses Surface Cache. Surface cache captures the geometric properties of objects from all angles and stores them in a offline cache.

Lumen captures occurs as the player moves around the environment, in higher resolution as you move closer to objects and in lower resolution as you move further away.

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