glTF export with alpha masking not right

Support Materials and Textures
1

I brought the classic “Bistro” glTF into Blender, and re-exported it as a .glb file. There’s some strangeness around alpha masking. Running a test program for glTF which uses Rend3 / WGPU, I get this:

bistro-failure
bistro-failure1280×720 1.52 MB

Take a good look at the window curtains and the ivy near the archway. You can see the checkerboard pattern where something had an alpha channel but was treated as Opaque on export. I

The relevant glTF for the curtains is

	{
		"name":"MASTER_Curtains",
		"normalTexture":{
			"index":104
		},
		"pbrMetallicRoughness":{
			"baseColorTexture":{
				"index":105
			},
			"metallicRoughnessTexture":{
				"index":106
			}
		}
	},


with default alpha handling, which is, I think, Opaque/



The curtain image looks like this:

blendercurtains
blendercurtains827×663 283 KB

The material menu for the curtains:

blendercurtainspane
blendercurtainspane921×1028 476 KB

Note that “Alpha” is “Channel packed”, which is puzzling. This .blend file was created by importing the .glTF file (actually a .glb file). What’s the appropriate setting for something that needs to have alpha clipping in .glTF?

2

I think this is a problem of that test program because it seems it not only used transparency for the material image but “transfers” this into the alpha channel of the produced images.

Also:
Because extension gltf and glb are “nothing else than” the ascii and binary version of “the same” Graphics Library Transmission Format.. why not convert this more direclty with some (gltf) ascii to (glb) binary converter. Using any 3D modeling software for this might be problematic because the GLTF material is converted to some application dependent material system and then again into the GLTF material system..

3

I know there’s a problem with the test program, but I’m not sure if glTF behavior is well defined for a a base color texture object with an alpha channel but marked as opaque. Is that valid glTF? If not, Blender should not emit it.