Set Material alphaMode properties to glTF/GLB 2 export

[Anderson Luiz Nichele]

When I add texture from a PNG file, we don’t have the background transparency after export to glTF/GLB 2.0.

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I edited directly the glTF file and add the key value "alphaMode": "BLEND" to material and after that, the texture is ok, with transparency.

The material definition:

  "materials": [
  ...
    {
    "alphaMode": "BLEND",
      "pbrMetallicRoughness": {
        "metallicFactor": 0,
        "roughnessFactor": 1,
        "baseColorTexture": {
          "index": 0
        }
      },
      "name": "Material_Texture_2"
    }
  ],

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The Assimp.Material does not provide a property to set the Material alphaMode.

Below, the way I create the material with texture:

We can check the glTF Alpha Coverage Specification for more details.

[Nicholas Woodfield]

It is important to note that those properties are for convenience, all of them use the (public) AddProperty/GetProperty/HasProperty methods and are just key-value pairs.

On the 5.0 branch I think this is the last thing I wanted to sort out before doing a RC release…I hope to make some time this week since I’m getting requests for getting that on nuget. I’ve been buy with other projects.

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The GLTF PBR keys can be found here:

https://github.com/assimp/assimp/blob/master/include/assimp/pbrmaterial.h

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What is confusing (to me at least) in the 5.0 version of native Assimp is they define GLTF-specific keys here, and then “generic” PBR texture keys in (search for PBR Material):

https://github.com/assimp/assimp/blob/master/include/assimp/material.h

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I don’t know why they chose to have it this way.

[Anderson Luiz Nichele]

Thanks for the first line you wrote, it gave me a light to follow!

for other people who need this option, follow the workaroud done:

        Dictionary<String, MaterialProperty> m_properties = (Dictionary<String, MaterialProperty>)GetInstanceField(materialTexture, "m_properties");

        MaterialProperty matProp = new MaterialProperty("$mat.gltf.alphaMode", "BLEND");

        m_properties.Add(matProp.FullyQualifiedName, matProp);

        FieldInfo mProperties = typeof(Assimp.Material).GetField("m_properties", BindingFlags.Instance | BindingFlags.NonPublic);
        mProperties.SetValue((object)materialTexture, (object)m_properties);

                                ...

        private static object GetInstanceField<T>(T instance, string fieldName)
        {
            BindingFlags bindFlags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static;
            FieldInfo field = typeof(T).GetField(fieldName, bindFlags);
            return field.GetValue(instance);
        }

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[Nicholas Woodfield]

I was thinking along the lines of:

materialTexture.AddProperty(new MaterialProperty("$mat.gltf.alphaMode", "BLEND"));

All the methods for accessing the material property dictionary are public APIs, no reflection is necessary.