Why does my PostProcess material's alpha output get clobbered?

Default tonemapping passes don't preserve alpha. Set the material's Blendable Location to Before Tonemapping or use After Tonemapping with the project's Enable Alpha Channel Support set to Linear Color Space Only.

Where's Enable Alpha Channel Support?

Project Settings → Rendering → PostProcessing → Enable Alpha Channel Support. Pick Linear Color Space Only or LDR. Costs a bit of perf but lets PP materials output meaningful alpha.

What's the right Blendable Location for compositing?

Before Tonemapping for visual effects that should be tonemapped along with the scene. After Tonemapping for UI overlays that need exact LDR values.

Fix: Unreal PostProcess Material Loses Alpha

Quick answer: Project Settings → Rendering → Enable Alpha Channel Support: Linear Color Space Only. Material’s Blendable Location: Before Tonemapping.

Compositing scene over a green screen. PP material outputs alpha. Final image alpha is fully opaque. Tonemap pass dropped it.

The Symptom

Movie Render Queue or capture component shows zero/one alpha; you can’t composite over a background.

The Fix

Project Settings → Rendering → PostProcessing:
  Enable Alpha Channel Support:    Linear Color Space Only

Material (PP_AlphaCompose):
  Material Domain:                 PostProcess
  Blendable Location:              Before Tonemapping
  Output Velocity:                 false
  Output Pin: Emissive Color (RGB) + Alpha (A)

Restart editor after changing the rendering setting; alpha-aware passes get rebuilt. Material now outputs meaningful A in the final composite.

Movie Render Queue

For final composites, use MRQ’s Anti-Aliasing → Output Alpha. Enables alpha-tracking through TAA accumulation, otherwise alpha gets noisy near edges.

Verifying

Capture an EXR or PNG. Open in compositing tool. Alpha matches the material’s output. Without the fix: alpha = 1 everywhere.

“Alpha enabled in settings. Pre-tonemap location. Composite works.”

Related Issues

For RenderTarget2D clearing, see render target. For Anim BP state alias, see state alias.

Alpha support on. Pre-tonemap. Composite ready.