Ray-Traced Reflections

A summarised guide on utilising ray-traced reflections to capture dynamic reflections which are sourced out of the camera view for an accurately simulated environment. Performance optimisations will be presented which includes roughness control, distance limiting, number of bounces, samples per pixel, and shadow control.

Realistic Reflections

• Ray-traced reflections simulate accurate environment reflections that can support multiple bounces to create inter-reflection for reflective surfaces.

• When compared with Screen Space Reflections (SSR), planar reflections, or even reflection probes, ray-traced reflections captures the entire scene dynamically and is not limited to static captures or objects within the current camera view to be visible in the reflection. These differences are easily noted in the comparison below:

  Image 1: Scene rendered with Screen Space Reflections

  Image 2: Scene rendered with Ray-Traced Reflections

• These can be enabled by ticking the Type box and setting the dropdown to Ray Tracing under the Reflections settings of the Post Process Volume in the Details pane.

• Due to the reflection simulation not being limited to what is within the camera view, the computational cost is exceptionally high depending on the number of reflections to compute. Key optimisations exist to find a balance between accuracy and performance.

Optimisation

• There are a multitude of settings which help balance the quality versus performance of ray-traced reflections. The exact value of each setting will depend on the requirements and components of your scene. These settings are found under the Ray Tracing Reflections section of the Post Process Volume in the Details pane:

  • Max Roughness: Sets the maximum roughness value that ray-traced reflections will be visible before falling back to raster methods that are less expensive. Reflection contribution is smoothly faded when close to the roughness threshold and this parameter behaves similarly to SSR’s Max Roughness setting. Lower values fall back to other methods more quickly. Scene roughness can be viewed by selecting the Roughness option under the Buffer Visualization settings of the editor View Mode.

  • Max Bounces: Sets the maximum number of bounces that ray-traced reflections uses. More bounces create inter-reflection but comes at a higher cost. Set to 1 bounce by default.

  • Samples Per Pixel: Sets the number of samples to use per pixel for ray-traced reflections. Additional samples decrease performance while increasing quality and accuracy. Set to 1 sample per pixel by default.

  • Shadows: Sets how shadows should be reflected. Choose between:

    • Hard Shadows which has no soft shadows

    • Area Shadows to have soft shadowing like ray-traced shadows

    • Disable to disable shadowing in ray-traced reflections

• RT reflections can be expensive when rendering multiple bounces with reflections inside of reflections. Without multiple bounces, the intra-reflected material will appear black. To use Reflection Capture Actors as the last bounce in RT reflections, press the Backtick key to open the console and then input the following command:

    r.RayTracing.Reflections.ReflectionCaptures 1
  

  For example, you can have a single bounce of RT reflection, then use a reflection capture for the second bounce, saving performance. If you use two bounces of RT reflection, then the third bounce would be the reflection capture. This example is shown below:

  Image 3: Single Bounce RT Reflection with no Reflection Capture Fallback

  Image 4: Two Bounces RT Reflection with no Reflection Capture Fallback

  Image 5: Single Bounce RT Reflection with Reflection Capture Fallback