Volumetric Clouds do not Ray/Path Trace correctly when SkyAtmosphere planet center is used

Development Rendering
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1

It seems that the Ray Tracing / Path Tracing source is unable to sample the volumetric material correctly if the sky’s origin is too far away from the camera’s position.

In this case the Ray traced reflections do not work correctly if the sky atmosphere “Ground Radius” is set to default (6360 km) and the transform mode is set to “Planet Center at Component Transform”

The reflections do work and look correct with the other two transform modes.

This seems to work with a planet radius less than 345 km. The clouds still seem to reflect incorrectly as if the reflection point is traced from the planet origin rather than the reflection point.

IncorrectReflections2.PNG illustrates a planet radius of 345 km

IncorrectCloudReflections.PNG illustrates 6360 km radius

Please advise…

2

Steps to Reproduce
Volumetric Cloud Reflections when using Lumen and Hardware Ray Tracing do not render correctly when I set the SkyAtmosphere Translation mode to “Planet Center at Component Transform”

Clouds DO reflect if I use other transform modes or make the planet much smaller than earth (345 km max)

  • Use default “Simulation Scene”
  • Lumen reflection mode with hardware ray tracing must be on
  • Ensure these modes are set in the PPV as well
  • Set Sky Atmosphere Transform Mode =“Planet Center at Component Transform”
    • Ensure component transform is 0,0,0
  • Create a reflective mirror surface primitive
    • Transform 0,0,636000000 (effectively placing at North pole)
  • Clouds should render correctly in sky but will not reflect on reflective surface
  • Enable Path Tracing mode. Clouds no longer render at all.
  • Reduce SkyAtmosphere.GroundRadius to 345.0
  • Transform reflective primitive to 0,0,34500000
  • Clouds begin to reflect but seem to do so from the perspective of the planet center
3

Hello there,

I believe what is happening is that the SkyLight is providing the reflection information, but the sky capture is based on the ASkyLight actor position, which is still the origin.

If this is moved to the same transform as the mirror surface, this should work.

I hope that helps.

Best regards,

Chris

4

That was indeed the error. Updating the SkyLight position to match the pawn position seemed to fix the issue.

5

That’s great news.

If you have any further questions, please feel free to ask. Otherwise, would you mind if I close this case for now?

Best regards,

Chris

6

Please do.

7

I’m glad I could help.

I’ll close this case out for now.

Best regards,

Chris