When merging GLTF models that have a Transmission value, the transparency effect is lost

syuilo · April 6, 2026, 9:10am

When merging GLTF models that have a Transmission value, the transparency effect is lost.

I’m not sure if this is a bug, an intended feature, or a limitation, but I’m reporting it just in case.

Please check the repro for details:

In a model where several sticks are submerged in oil, the oil has a Transmission value set to 1 in Blender and renders as translucent, allowing the sticks to be seen through the oil.

When first loaded into Babylon, it renders correctly, but after merging the model using MergeMeshes (with the multiMaterials option set to true), the oil becomes opaque.

Note: These sticks are instance meshes, and in this example, some of them disappear because they cannot be merged—please disregard this.

Thank you for this great framework!

Evgeni_Popov · April 6, 2026, 1:22pm

Thanks for reporting, here’s the fix:

github.com/BabylonJS/Babylon.js

[Loaders] Fix transmission effect lost after MergeMeshes with MultiMaterial (#18240)

master ← Popov72:fix/transmission-merge-meshes

opened 01:21PM - 06 Apr 26 UTC

Popov72

+112 -25

## Description The `TransmissionHelper` in the `KHR_materials_transmission` g…lTF extension did not handle `MultiMaterial` (created by `Mesh.MergeMeshes` with `multiMultiMaterials=true`). When meshes with transmission were merged, the transparency effect was lost because the helper incorrectly classified the merged mesh. ### Root cause 1. `_addMesh` checked `isMatchingMaterialType(mesh.material)` which returns `false` for `MultiMaterial` — the merged mesh was treated as opaque 2. The code then crashed trying to create a PBR adapter for a non-PBR `MultiMaterial` 3. The merged mesh ended up in the opaque render target, causing transmissive submeshes to see themselves as opaque ### Fix - Added `_classifyMeshMaterials()` helper that handles both single materials and `MultiMaterial` by checking each sub-material individually. Returns `'transparent'`, `'opaque'`, or `'mixed'`. - For **mixed** `MultiMaterial` meshes (both opaque and translucent sub-materials), the mesh stays in the opaque render target's render list, but translucent submeshes are temporarily excluded during the opaque RT render. This ensures opaque submeshes (e.g., sticks) are visible through the transmissive surface (e.g., oil) while the transmissive submeshes themselves don't render into the opaque texture. - Also fixed two pre-existing bugs: - `_addMesh` fell through to `_getOrCreateMaterialAdapter` for non-matching material types (crash risk) - `_setupRenderTargets` used `return` instead of `continue` in its loop (premature exit) ### Testing Verified visually in the local Playground using the reporter's reproduction: - Before fix: oil becomes completely opaque after merge - After fix: oil remains translucent with sticks visible through it ### Forum post https://forum.babylonjs.com/t/when-merging-gltf-models-that-have-a-transmission-value-the-transparency-effect-is-lost/63068

syuilo · April 6, 2026, 2:31pm

Thank you for the quick fix. I can’t wait for the release!

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When merging GLTF models that have a Transmission value, the transparency effect is lost

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