[UE5.8] Subsurface Profile shading model produces no SSS effect on Mobile (Multi-pass Deferred Shading) — works correctly on Desktop

General Issues and Bug Reporting
, , , ,
1

Summary

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

  1. Create a brand-new UE 5.8 project.
  2. In Project Settings → Engine → Rendering → Mobile, set Mobile Shading = “Deferred Shading”.
  3. Create a new Material:
    • Shading Model = Subsurface Profile
    • Opacity = 1 (constant)
    • BaseColor = any skin-tone constant color
  4. Create a brand-new Subsurface Profile asset (default Skin preset, Burley Normalized, default parameters), assign it to the material’s Subsurface Profile slot.
  5. Apply the material to a Sphere static mesh in a new level with a Directional Light + Sky Light.
  6. 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 change
  • stat gpu on 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 ShadingModel on 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.

Please select what you are reporting on:

Verse

What Type of Bug are you experiencing?

Other

Steps to Reproduce

  1. Create a new UE 5.8 project.
  2. In Project Settings → Engine → Rendering → Mobile, set “Mobile Shading” = Deferred Shading.
  3. Create a new Material: Shading Model = Subsurface Profile, Opacity = 1 (constant), BaseColor = skin tone constant color.
  4. Create a new Subsurface Profile asset (default Skin preset, default parameters) and assign it to the material’s Subsurface Profile slot.
  5. Apply the material to a Sphere static mesh in a new level, lit with a Directional Light + Sky Light.
  6. View the sphere in the PC/Desktop viewport — observe SSS transmission effect at the edges.
  7. Switch the Material Editor / level preview to Mobile (Preview Platform = Generic Android Vulkan, or Mobile Preview in PIE) — observe the sphere again.
  8. Run “stat gpu” in both Desktop and Mobile preview and compare the “SubsurfaceScattering” line.
  9. Run “viewmode ShadingModel” in Mobile preview and observe the sphere/character.

Expected Result

Subsurface scattering should render on Mobile (Deferred Shading) the same way it does on Desktop — a soft, color-tinted light transmission effect visible at the edges of the mesh where light passes through, consistent with the Subsurface Profile shading model documentation. The “SubsurfaceScattering” pass in stat gpu should show real timing data on Mobile (similar to Desktop), and “viewmode ShadingModel” should correctly classify the material as Subsurface Profile rather than rendering it as black/unclassified.

Observed Result

On Desktop, SSS renders correctly with visible light transmission at the mesh edges.

On Mobile (Deferred Shading), the material shows zero subsurface scattering — the surface appears flat/opaque, visually indistinguishable from Default Lit, even with the exact same material and Subsurface Profile asset.

“stat gpu” on Mobile shows the “SubsurfaceScattering” entry is present in the GPU Graphics Queue list, but with no Avg/Max/Min timing data (effectively not executed), while the same stat on Desktop shows ~0.27ms real cost with valid timing data.

“viewmode ShadingModel” on Mobile renders the Subsurface Profile material as completely black (no shading model classification written to GBuffer), while Hair and Eye shading models on the same character render correctly in their respective debug colors in the same view.

r.SSS.Scale and r.SSS.Checkerboard have no visible effect when changed on Mobile. r.AllowStaticLighting is already False (not the cause). Issue reproduces identically with Substrate both enabled and disabled at the project level, and with Anti-Aliasing Method = TAA (not TSR, so not related to the known TSR+Substrate SSS issue).

This points to the ShadingModelID for Subsurface Profile materials not being correctly written to the Mobile GBuffer in UE 5.8’s Mobile Deferred rendering path, even though the SubsurfaceScattering pass itself is registered in the render graph.

Platform(s)

Mobile: Android, Vulkan (Deferred Shading) — issue reproduces here
Desktop: Windows, PC preview (SM6) — works correctly, used as comparison baseline