Summary
On Unreal Engine 5.8, materials using the Subsurface Profile shading model render with zero subsurface scattering effect when previewed/run on Mobile (Deferred Shading / Multi-pass Deferred), while the exact same material and Subsurface Profile asset work correctly on Desktop (PC/SM6).
This is a 100% clean repro from a brand-new project, brand-new material, and brand-new Subsurface Profile asset — not related to any asset migration from earlier engine versions.
Engine Version
5.8 (release)
Platform
Android, Vulkan (Mobile Deferred Shading)
Desktop comparison: Windows, SM6/PC preview
Steps to Reproduce
- Create a brand-new UE 5.8 project.
- In Project Settings → Engine → Rendering → Mobile, set Mobile Shading = “Deferred Shading”.
- Create a new Material:
- Shading Model = Subsurface Profile
- Opacity = 1 (constant)
- BaseColor = any skin-tone constant color
- Create a brand-new Subsurface Profile asset (default Skin preset, Burley Normalized, default parameters), assign it to the material’s Subsurface Profile slot.
- Apply the material to a Sphere static mesh in a new level with a Directional Light + Sky Light.
- Compare:
- PC/Desktop viewport preview → SSS scattering visible (soft red transmission at light-facing edges)
- Mobile Preview (Android Vulkan) → No SSS effect whatsoever; surface renders as flat/opaque, indistinguishable from Default Lit.
Expected Result
Subsurface scattering should render consistently (allowing for quality/performance differences) on both Desktop and Mobile Deferred Shading, as documented in the Subsurface Profile Shading Model documentation.
Actual Result
SSS effect is completely absent on Mobile. Further diagnostics below confirm the SSS pass is not being driven by any data on this platform.
Diagnostic Steps Already Performed
r.SSS.Scale 10→ no visible change on Mobile (cvar appears to have zero effect)r.SSS.Checkerboard 0→ no changestat gpuon Mobile → “SubsurfaceScattering” entry is present in the GPU Graphics Queue list, but shows no Avg/Max/Min timing data (effectively 0 cost / not executed), whereas the same stat on Desktop shows ~0.27ms average cost with real timing data.viewmode ShadingModelon Mobile (both in-editor Mobile Preview and in the Material Editor with Preview Platform = “Generic Android Vulkan”) → the character/sphere using Subsurface Profile renders completely black (no shading model classification), while Hair and Eye shading models on the same character render correctly in their respective debug colors.- Confirmed
r.AllowStaticLighting = False(not the cause — this is a separate known issue, UE5 Mobile Deferred – Subsurface Shading Mode disabled when r.AllowStaticLighting=True with dynamic lights). - Confirmed
r.Mobile.SupportGPUScene = 1,r.SubsurfaceScattering = 1. - Confirmed the Subsurface Profile asset itself is valid and correctly assigned (not an empty/None reference) on the Material Instance used.
- Tested with both Substrate enabled and disabled at the project level — issue persists in both cases.
- Anti-Aliasing Method = TAA (not TSR), so this is not related to the known TSR+Substrate SSS interaction issue.
Conclusion from diagnostics
The ShadingModelID for Subsurface Profile materials does not appear to be correctly written to the Mobile GBuffer in UE 5.8’s Mobile Deferred (Multi-pass Deferred) rendering path, even though the SubsurfaceScattering pass itself is registered in the render graph. This looks like a regression introduced alongside the 5.8 mobile rendering changes (multi-pass deferred rendering as the new default mobile path, per the 5.8 release notes).
Additional Notes
This appears to affect any project using Subsurface Profile materials (e.g., character skin shading) on Mobile Deferred in 5.8, which is a significant regression for mobile character rendering quality, especially since 5.8 otherwise improved mobile anti-aliasing and AO.
Screenshots and stat gpu comparison available on request.