[UE 5.8] Thin Translucent Shading Model ignores Transmittance Color on Mobile Deferred (Multi-pass) — iOS/Metal packaged build only, works on Forward Shading and in Editor Mobile Preview

General Issues and Bug Reporting (Unreal Engine)
1

Summary

On Unreal Engine 5.8, materials using the Thin Translucent shading model no longer apply the Transmittance Color when rendered with Mobile Deferred Shading (the new default Multi-pass Deferred path) in a packaged iOS/Metal build. The surface’s own shading (specular highlights, reflections, base lighting) renders correctly, but the background behind the surface is not tinted by the Transmittance Color at all — visually equivalent to Transmittance Color being pure white / the feature being disabled.

This is a regression relative to UE 5.6, where the same material and project settings produced correct thin-translucent tinting on the same target hardware.

Engine Version: 5.8 (release)

Platform: iOS/iPadOS, Metal, Mobile Deferred Shading (Multi-pass) — tested on iPad Pro (M4). Desktop/PC preview and Mobile Forward Shading on the same device are used as working comparison baselines.

This may be related to other Mobile Deferred (Multi-pass) regressions already reported for 5.8, such as the Subsurface Profile SSS regression on Android/Vulkan (forums.unrealengine.com, thread “UE5.8 Subsurface Profile shading model produces no SSS effect on Mobile (Multi-pass Deferred Shading)”), which similarly points to shading-model-dependent output not being correctly propagated through the new mobile multi-pass deferred path.

Ruled out as causes: incorrect Lighting Mode (Surface ForwardShading is confirmed set for this material), half-precision shaders (project confirmed set to Full Precision), and stale shader cache (issue persists after a clean DDC wipe and recook).

This points to the Transmittance Color output — applied via dual-source blending in the mobile base pass — being dropped or not correctly wired specifically in the on-device Metal pipeline for Mobile Deferred Multi-pass, despite the equivalent editor preview path handling it correctly.

What Type of Bug are you experiencing?

Rendering (Graphics / Niagara)

Steps to Reproduce

  1. Create a new (or use an existing) UE 5.8 project targeting iOS.
  2. In Project Settings → Engine → Rendering → Mobile, set Mobile Shading Path = Deferred Shading (Multi-pass Deferred, the 5.8 default).
  3. Create a Material with Shading Model = Thin Translucent, Blend Mode = Translucent, Lighting Mode = Surface ForwardShading.
  4. Set the Transmittance Color to a clearly saturated color (e.g. pure red) and set Opacity so the surface is only partially opaque.
  5. Confirm Project Settings → Rendering → Full Precision (no half-precision/FP16 shaders) is enabled.
  6. Apply the material to a plane or sphere positioned in front of other scene geometry, lit by a Directional Light.
  7. Preview in-editor using Mobile Preview (ES3.1) with Deferred Shading selected → observe the material.
  8. Package the project for iOS and deploy/run on a physical device (iPad Pro M4) → observe the same material.
  9. For comparison, switch Mobile Shading Path to Forward Shading, repackage, and deploy the same material to the same device.
  10. For further comparison, open/package an equivalent project in UE 5.6 with Mobile Deferred Shading enabled.

Expected Result

The Thin Translucent material should tint the background scene behind it with the assigned Transmittance Color consistently across: Mobile Deferred packaged device build, Mobile Deferred Editor Preview, Mobile Forward Shading, and UE 5.6 Mobile Deferred — allowing only for expected quality/performance differences between paths.

Observed Result

  • Editor Mobile Preview (ES3.1), Mobile Deferred selected: Transmittance Color renders correctly; the background is tinted as expected.
  • Packaged iOS/Metal build, Mobile Deferred (Multi-pass), iPad Pro M4: Transmittance Color has no visible effect. The surface’s own shading (specular/reflections/lighting) still renders correctly, but the background is not tinted at all, as if Transmittance Color were pure white or disabled.
  • Packaged iOS/Metal build, Mobile Forward Shading, same device, same material, no changes: Transmittance Color renders correctly.
  • UE 5.6, same project/material, Mobile Deferred Shading: Transmittance Color renders correctly (used as the regression baseline).

Affects Versions

5.8

Platform(s)

iOS