How can I add a glossiness / specular texture to a GLTF model?

Question

QUESTION:

I have a specular/glossiness texture image for a model which is not yet being used by the GLTF model.

How can I use/add the texture to my model so that it is reflective/glossy where it should be ?

I don't know what to change inside the .gltf file to make use of the texture.

EDIT: Added more lines of the GLTF to get a better picture of the situation.

---

GLTF JSON:

"images": [
    {
        "name": "...",
        "uri": "..."
    },
    {
        "name": "...",
        "uri": "..."
    },
    {
        "name": "...",
        "uri": "..."
    },
    {
        "name": "...",
        "uri": "..."
    },
    {
        "name": "...",
        "uri": "..."
    }
],
"samplers": [
    {}
],
"textures": [
    {
        "name": "T_N",
        "sampler": 0,
        "source": 0
    },
    {
        "name": "Map #9",
        "sampler": 0,
        "source": 1
    },
    {
        "name": "T_D",
        "sampler": 0,
        "source": 2
    },
    {
        "name": "Map #6",
        "sampler": 0,
        "source": 3
    },
    {
        "name": "Specular",
        "sampler": 0,
        "source": 4
    }
],

"materials": [
    {
        "name": "Head",
        "alphaMode": "OPAQUE",
        "extras": {
            "fromFBX": {
                "shadingModel": "<unknown>",
                "isTruePBR": false
            }
        },
        "normalTexture": {
            "index": 0,
            "texCoord": 0
        },
        "emissiveTexture": {
            "index": 1,
            "texCoord": 0
        },
        "pbrMetallicRoughness": {
            "baseColorTexture": {
                "index": 2,
                "texCoord": 0
            },
            "baseColorFactor": [
                1.0,
                1.0,
                1.0,
                1.0
            ],
            "metallicFactor": 0.800000002980232,
            "roughnessFactor": 0.600000011920929
        }
    },
    {
        "name": "Body",
        "alphaMode": "BLEND",
        "extras": {
            "fromFBX": {
                "shadingModel": "<unknown>",
                "isTruePBR": true
            }
        },
        "pbrMetallicRoughness": {
            "baseColorTexture": {
                "index": 3,
                "texCoord": 0
            },
            "baseColorFactor": [
                1.0,
                1.0,
                1.0,
                1.0
            ],
            "metallicFactor": 0.80000002980232,
            "roughnessFactor": 0.600000011920929
        },
        "extensions": {
            "KHR_materials_pbrSpecularGlossiness": {
                "specularGlossinessTexture": {
                    "index": 4
                }
            }
        }
    }
],
"meshes": [
    {
        "name": "Head",
        "primitives": [
            {
                "material": 0,
                "mode": 4,
                "attributes": {
                    "COLOR_0": 3,
                    "NORMAL": 2,
                    "POSITION": 1,
                    "TEXCOORD_0": 4
                },
                "indices": 0,
                "extensions": {
                    "KHR_draco_mesh_compression": {
                        "bufferView": 0,
                        "attributes": {
                            "COLOR_0": 2,
                            "NORMAL": 1,
                            "POSITION": 0,
                            "TEXCOORD_0": 3
                        }
                    }
                }
            }
        ]
    },
    {
        "name": "Body",
        "primitives": [
            {
                "material": 1,
                "mode": 4,
                "attributes": {
                    "COLOR_0": 8,
                    "NORMAL": 7,
                    "POSITION": 6,
                    "TEXCOORD_0": 9
                },
                "indices": 5,
                "extensions": {
                    "KHR_draco_mesh_compression": {
                        "bufferView": 1,
                        "attributes": {
                            "COLOR_0": 2,
                            "NORMAL": 1,
                            "POSITION": 0,
                            "TEXCOORD_0": 3
                        }
                    }
                }
            }
        ]
    }
],

---

ERROR:

Here is the error I am currenlty getting with the above modified GLTF when trying to load the model:

{"message":"Cannot read property 'getMaterialType' of undefined","name":"TypeError"} 

Answer 1

There are some good examples in the glTF-Sample-Models repository. Here is the same model using two different PBR workflows:

• spec/gloss PBR material
• metal/rough PBR material

Your example JSON above is using metal/rough, so you'll have to "convert" it to spec/gloss by replacing the metallic and roughness factors with equivalent spec/gloss properties.

I'd suggest taking a look at the specification for more info:

• glTF specification
• spec/gloss extension

You may also find it easier to do this in a tool like Substance Painter or Blender, which can import and export glTF.