// Textures
// referenced by material_index/texture_type pairs.
std::map<std::pair<size_t,size_t>,int64_t> texture_uids;
const std::map<aiTextureType,std::string> prop_name_by_tt = {
    {aiTextureType_DIFFUSE,      "DiffuseColor"},
    {aiTextureType_SPECULAR,     "SpecularColor"},
    {aiTextureType_AMBIENT,      "AmbientColor"},
    {aiTextureType_EMISSIVE,     "EmissiveColor"},
    {aiTextureType_HEIGHT,       "Bump"},
    {aiTextureType_NORMALS,      "NormalMap"},
    {aiTextureType_SHININESS,    "ShininessExponent"},
    {aiTextureType_OPACITY,      "TransparentColor"},
    {aiTextureType_DISPLACEMENT, "DisplacementColor"},
    //{aiTextureType_LIGHTMAP, "???"},
    {aiTextureType_REFLECTION,   "ReflectionColor"},
    **{aiTextureType_BASE_COLOR, "BaseColor" },
    {aiTextureType_NORMAL_CAMERA, "NormalCamera" },
    {aiTextureType_EMISSION_COLOR, "EmissionColor" },
    // copycd:: add.
    {aiTextureType_METALNESS,   "MetalnessMap" }**
    //{aiTextureType_UNKNOWN, ""}
};