How can I import Blender specular value via glTF Assimp import?

[wedesoft]

Question

I created a textured sample cube in Blender and exported it as glTF. I am using Assimp from LWJGL 3.3.2 and it is working quite well and I managed to access faces, vertices, normals, as well as texture coordinates and the texture itself. However I am struggling to understand the material properties interface. I managed to access the roughness and metallic factor but I can't see a way to access the specular value.

(import '[org.lwjgl.assimp Assimp AIMaterial AIMaterialProperty])
(import '[org.lwjgl PointerBuffer])

(def scene (Assimp/aiImportFile "etc/cube.gltf" Assimp/aiProcess_Triangulate))

(.mNumMaterials scene)
; 2
(def material (AIMaterial/create ^long (.get (.mMaterials scene) 0)))

(def properties (map #(AIMaterialProperty/create (.get (.mProperties material) %)) (range (.mNumProperties material))))

(map (fn [prop] (.dataString (.mKey prop))) properties)
; ("?mat.name" "$clr.diffuse" "$clr.base" "$tex.file" "$tex.uvwsrc" "$tex.mappingname" "$tex.mappingid" "$tex.mapmodeu"
; "$tex.mapmodev" "$tex.mappingfiltermag" "$tex.mappingfiltermin" "$tex.file" "$tex.uvwsrc" "$tex.mappingname"
; "$tex.mappingid" "$tex.mapmodeu" "$tex.mapmodev" "$tex.mappingfiltermag" "$tex.mappingfiltermin" 
; "$mat.metallicFactor" "$mat.roughnessFactor" "$mat.shininess" "$clr.emissive" "$mat.twosided" "$mat.opacity" 
; "$mat.gltf.alphaMode" "$mat.gltf.alphaCutoff" "$mat.shadingm")

(def p (PointerBuffer/allocateDirect 1))
(Assimp/aiGetMaterialProperty material Assimp/AI_MATKEY_ROUGHNESS_FACTOR 0 0 p)
; 0
(.getFloat (.mData (AIMaterialProperty/create ^long (.get p 0))))
; 0.3

(def p (PointerBuffer/allocateDirect 1))
(Assimp/aiGetMaterialProperty material Assimp/AI_MATKEY_METALLIC_FACTOR 0 0 p)
; 0
(.getFloat (.mData (AIMaterialProperty/create ^long (.get p 0))))
; 0.1

(def p (PointerBuffer/allocateDirect 1))
(Assimp/aiGetMaterialProperty material Assimp/AI_MATKEY_SPECULAR_FACTOR 0 0 p)
; -1

(def color (AIColor4D/create))
(Assimp/aiGetMaterialColor material Assimp/AI_MATKEY_COLOR_DIFFUSE Assimp/aiTextureType_NONE 0 color)
; 0
(Assimp/aiGetMaterialColor material Assimp/AI_MATKEY_COLOR_SPECULAR Assimp/aiTextureType_NONE 0 color)
; -1

In Blender the specular value is 0.2 and I can see the value 0.4 under "specularColorFactor" in the glTF file:

{
  "asset":{
    "generator":"Khronos glTF Blender I/O v3.6.27",
    "version":"2.0"
  },
  "extensionsUsed":[
    "KHR_materials_specular"
  ],
  "scene":0,
  "scenes":[
    {
      "name":"Scene",
      "nodes":[
        0
      ]
    }
  ],
  "nodes":[
    {
      "mesh":0,
      "name":"Cube"
    }
  ],
  "materials":[
    {
      "doubleSided":true,
      "extensions":{
        "KHR_materials_specular":{
          "specularColorFactor":[
            0.4000000059604644,
            0.4000000059604644,
            0.4000000059604644
          ]
        }
      },
      "name":"Material.002",
      "pbrMetallicRoughness":{
        "baseColorTexture":{
          "index":0
        },
        "metallicFactor":0.10000000149011612,
        "roughnessFactor":0.30000001192092896
      }
    }
  ],
  ...
}

I have tried to import the Blender file instead but that doesn't seem to expose texture coordinates and normal vectors (and maybe other entries).