Understand 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 various effects (such as hard or soft shadows, mirror or glossy reflections and refractions, and indirect illumination) in a single unified process.
A notable downside to path tracing is noise. Noise vanishes as more paths accumulate and converges toward a clean image. For more information about path tracing limitations in HDRP, see Unsupported features of path tracing.
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, Stacklit, AxF, and Unlit materials, and area, point, directional, and environment lights.
To troubleshoot this effect, HDRP provides a Path Tracing Debug Mode and a Ray Tracing Acceleration Structure Debug Mode in Lighting Full Screen Debug Mode.
Understand how path tracing affects material properties
Path tracing changes how the following Material properties behave in your scene:
- How transmissive objects absorb light
- How light refracts in transmissive objects
- Subsurface scattering
This is because path tracing in HDRP implements more precise light transport simulations than rasterization. To do this, path tracing computes all lighting effects and how light interacts with Materials at the same time. This changes the appearance of Materials in path-traced scenes. For example, in the images below, the Material appears darker.
The images below display the difference between transparent, double-sided materials in a rasterized and a path-traced scene:
GameObjects without path tracing (rasterized).
GameObjects with path tracing enabled.
Path tracing and double-sided materials
When you use path tracing, the Double-Sided property (menu: Inspector > Surface Options > Double-Sided) allows transparent materials to accumulate correctly. If you disable Double-sided property, rays which exit the GameObject will not behave correctly.
The following images display the same GameObjects with a single-sided Material and a double-sided material:
GameObjects with a single-sided Material and path tracing enabled
GameObjects with a double-sided Material and path tracing enabled
How path tracing affects refraction models
Path tracing changes the way refraction models on a Lit Material behave.
To change the refraction model a Lit Material uses, in the Transparency Inputs section, select a model from the Refraction model dropdown, displayed in the following image:
The following table describes how each refraction model behaves when you enable path tracing:
| Refraction model | Path tracing behavior | Compatible Surface sides |
|---|---|---|
| Box and Sphere | A surface type that represents thick objects like a paperweight or a crystal ball. When you enable path tracing, the Box and Sphere models behave in the same way. Rays can intersect the GameObjects in the scene and HDRP doesb't need to approximate transparent surfaces | This refraction model is compatible with a double-sided Material that has its Normal mode set to None. |
| Thin | A thin surface type with infinitesimal thickness. Select this for thin, window-like surfaces. When you enable path tracing, the behavior of the Thin refraction model behaves the same as in rasterization. | This refraction model is compatible with a double-sided Material that has its Normal mode set to Flip or Mirror. |
| None | A thin, refractive surface hardcoded to be smooth to simulate alpha blending. When you enable path tracing, the behavior of the None refraction model behaves the same as in rasterization. | This refraction model is compatible with a double-sided Material that has its Normal mode set to Flip or Mirror. |
From left to right, a GameObject with Sphere, Box, and Thin mode without path tracing (rasterized).
From left to right, a GameObject with Sphere, Box, and Thin mode with path tracing enabled.
Path tracing and subsurface scattering
For subsurface scattering's Transmission property (A) to work correctly with path tracing, you need to do the following:
- Open the Surface Options window.
- Enable the Double-Sided property (B).
The following example images display a sheet of fabric lit from below by a point light. The first image shows a single-sided surface, and the second shows a double-sided surface:
A single-sided surface with Transmission disabled.
A double-sided surface with Transmission enabled.
Hair
Path tracing gives human hair a volumetric look. To do this, path tracing calculates the multiple scattering events that happen in a full head of hair. It is particularly effective for lighter hair tones.
You can only use path tracing with hair you create with the Hair Master Stack. The Hair Master Stack provides two hair Material Types. Path tracing works with the Physical Type.
Tip: The second Material Type, Approximate, does not work with path tracing. You can learn more about it in The Approximate Material Type.
If you create hair using ribbons, it won’t work with path tracing in Unity. For path tracing to work with hair, you must use cards or tube geometry. For more information, see Geometry type.
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
Path tracing and Decals
In order to efficiently support the path tracing of decals, Decal Projectors are added to the Ray Tracing Light Cluster. In the case of many decals in a small volume, it might be necessary to increase the maximum number of lights per cell.
Additionally, the path tracer treats all decals as clustered decals. This might require increasing the "Maximum Lights per Cell (Ray Tracing)" (in the HDRP Quality settings, under lighting) and the size of the decal atlas (in the HDRP Quality settings, under Rendering), as more decals will be added to these data-structures. Emission from decals is currently not supported.