Why do the transparent objects in my 3D model appear opaque?

This usually happens because each 3D program has different ways of handling transparency in materials or creating materials of different types. Some of these methods may not be compatible when exporting to the GLB and GLTF formats, which are supported in VTPro, hence they appear as opaque afterwards.

To avoid this, it's important to ensure using materials or parameters that are compatible with these formats. Here are the supported features: https://docs.blender.org/manual/en/latest/addons/import_export/scene_gltf2.html

In the case of Blender (and similar in other programs), you need to use materials of the type "Principled BSDF", and most importantly, change the Blend Mode to "Alpha Blend", so that transparency takes effect when exporting to GLB/GLTF.

Attached is an image showing the main parameters affecting transparency: