[Regression] glTF export sets alphaMode to OPAQUE in Painter 12.1.2, while 10.1.2 exports BLEND correctly

I found a reproducible regression in glTF transparency export between Substance 3D Painter 10.1.2 (2024) and 12.1.2 (2026).

I have already submitted this issue using Painter's built-in Bug Report function.

Bug Report Reference: ADB-46817980-K2B5

Environment

• Adobe Substance 3D Painter Steam Edition

• Windows

• Painter 10.1.2 (2024): works correctly

• Painter 12.1.2 (2026): issue occurs

Problem

I created a very simple FBX containing only two polygon planes and tested the exact same FBX and textures in both Painter versions.

For transparency, I used:

• Opacity channel

• pbr-metal-rough-with-alpha-blending shader

• Base Color textures

• Opacity textures

Transparency is displayed correctly inside Painter in both versions.

However, the glTF export result is different.

Painter 10.1.2

The exported glTF contains:

"alphaMode": "BLEND"

Transparency works correctly in external glTF viewers.

Painter 12.1.2

The exported glTF contains:

"alphaMode": "OPAQUE"

Transparency does not work in external glTF viewers.

The exported Base Color PNG still contains the correct alpha channel.

If I manually change:

"alphaMode": "OPAQUE"

to:

"alphaMode": "BLEND"

the transparency immediately works correctly.

Therefore, the texture export itself appears to be correct. The issue seems to be the alphaMode value generated by the glTF exporter.

Steps to reproduce

1. Create a new project using the attached test_plane.fbx.

2. Add an Opacity channel.

3. Use pbr-metal-rough-with-alpha-blending.

4. Apply the provided Base Color and Opacity textures.

5. Confirm that transparency works correctly in the Painter viewport.

6. Export using the standard glTF PBR Metal Roughness preset.

7. Check the exported material's alphaMode.

Version comparison

Same FBX and same textures:

Painter 10.1.2

• Transparency in Painter: OK

• glTF: alphaMode = BLEND

• Transparency after export: OK

Painter 12.1.2

• Transparency in Painter: OK

• glTF: alphaMode = OPAQUE

• Transparency after export: NG

I also noticed that the Painter 12.1 release notes mention:

[GLTF Export] Alpha blending is always enabled at export

so I wonder whether this could be a regression related to the alpha mode detection changes.

I have reproduction files available, including the FBX, textures, Painter project data, and glTF outputs from both versions.

Can anyone reproduce this in Painter 12.1.x?

I would also appreciate confirmation from the Substance 3D Painter team whether this is an intended behavior change or a glTF export regression.

[Moved to Substance 3D Painter by a moderator］