Path tracing

Path tracing is a ray tracing algorithm that sends rays from the Camera and, when a ray hits a reflective or refractive surface, recurses the process until it reaches a light source. The series of rays from the Camera to the Light form a "path".

It enables HDRP to compute many different effects (such as hard or soft shadows, mirror or glossy reflections and refractions, and indirect illumination) in one single unified process.

A notable downside to path tracing is noise. However, noise vanishes as more paths accumulate, and eventually converges toward a clean image.

Noisy image with Maximum Samples set to 1

Clean image with Maximum Samples set to 256

The current implementation for path tracing in the High Definition Render Pipeline (HDRP) accumulates paths for every pixel up to a maximum count unless the Camera moves. If the Camera moves, HDRP restarts the path accumulation. Path tracing supports Lit, LayeredLit, and Unlit materials, and area, point, directional, and environment lights.

Set up path tracing

Path tracing shares the general requirements and setup as other ray tracing effects, so for information on hardware requirements and set up, see getting started with ray tracing. You must carry out this setup before you can add path tracing to your Scene.

Add path tracing to your Scene

Path tracing uses the Volume framework, so to enable this feature, and modify its properties, you must add a Path Tracing override to a Volume in your Scene. To do this:

  1. In the Scene or Hierarchy view, select a GameObject that contains a Volume component to view it in the Inspector.
  2. In the Inspector, select Add Override > Ray Tracing > Path Tracing.
  3. In the Inspector for the Path Tracing Volume Override, check the Enable option. If you do not see the Enable option, make sure your HDRP Project supports ray tracing. For information on setting up ray tracing in HDRP, see getting started with ray tracing. This switches HDRP to path-traced rendering and you should initially see a noisy image that converges towards a clean result.
  4. If the image does not converge over time, select the drop-down next to the effect toggle and enable Always Refresh.

Properties

Property Description
Maximum Samples Set the number of frames to accumulate for the final image. There is a progress bar at the bottom of the Scene view which indicates the current accumulation with respect to this value.
Minimum Depth Set the minimum number of light bounces in each path.
Maximum Depth Set the maximum number of light bounces in each path. You can not set this to be lower than Minimum Depth.
Note: You can set this and Minimum Depth to 1 if you only want to direct lighting. You can set them both to 2 if you only want to visualize indirect lighting (which is only visible on the second bounce).
Maximum Intensity Set a value to clamp the intensity of the light value each bounce returns. This avoids very bright, isolated pixels in the final result.
Note: This property makes the final image dimmer, so if the result looks dark, increase the value of this property.

Minimum Depth set to 1, Maximum Depth set to 2: direct and indirect lighting (1 bounce)

Minimum Depth set to 1, Maximum Depth set to 1: direct lighting only

Minimum Depth set to 2, Maximum Depth set to 2: indirect lighting only (1 bounce)

Path tracing and automatic histogram exposure

Path tracing creates noise that changes the minimum and maximum values that HDRP uses for automatic, histogram-based exposure. You can visualize this when you use the RGB Histogram to debug the exposure in your scene.

This is especially visible in the first few un-converged frames that have the highest level of noise. However, this does not affect the exposure of the final converged frame.

If there is any noise that affects the exposure in the final converged frame, adjust the following properties in the Automatic Histogram override to set the exposure to your desired range:

  • Limit Min

  • Limit Max

Limitations

This section contains information on the limitations of HDRP's path tracing implementation. Mainly, this is a list of features that HDRP supports in its rasterized render pipeline, but not in its path-traced render pipeline.

Unsupported features of path tracing

There is no support for path tracing on platforms other than DX12 for now.

HDRP path tracing in Unity 2020.2 has the following limitations:

  • If a Mesh in your scene has a Material assigned that does not have the HDRenderPipeline tag, the mesh will not appear in your scene. For more information, see Ray tracing and Meshes.
  • Does not support 3D Text and TextMeshPro.
  • Does not support Shader Graph nodes that use derivatives (for example, a normal map that derives from a texture).
  • Does not support Shader Graphs that use Custom Interpolators.
  • Does not support decals.
  • Does not support tessellation.
  • Does not support Tube and Disc-shaped Area Lights.
  • Does not support Translucent Opaque Materials.
  • Does not support several of HDRP's Materials. This includes Fabric, Eye, StackLit, Hair, Decal.
  • Does not support per-pixel displacement (parallax occlusion mapping, height map, depth offset).
  • Does not support MSAA.
  • For renderers that have LODs, the ray tracing acceleration structure only includes the highest level LOD and ignores the lower LODs.
  • Does not support Graphics.DrawMesh.
  • Does not support Streaming Virtual Texturing.