When IBL is disabled, materials using KHR_materials_transmission don't do any texture lookup in the opaque pass. So, all transmissive materials appear pitch black when IBL is off.
To reproduce this, turn off image-based lighting and view any of the transmission samples: AttenuationTest, TransmissionTest, MosquitoInAmber, DragonAttenuation, etc.
When IBL is disabled, materials using KHR_materials_transmission don't do any texture lookup in the opaque pass. So, all transmissive materials appear pitch black when IBL is off.
Spotted in https://github.com/KhronosGroup/glTF-Sample-Viewer/pull/544#issuecomment-2177363153
To reproduce this, turn off image-based lighting and view any of the transmission samples: AttenuationTest, TransmissionTest, MosquitoInAmber, DragonAttenuation, etc.