Define material indexes for triangles in QQuick3DGeometry

Dmitry87

I'm trying to load a model to a Quick 3D scene using QQuick3DGeometry
in the scene I can define (See Sub-mesh Example):

Model {
    source: "qrc:/meshes/distortedcube.mesh"
    //geometry: MeshData
    
    PrincipledMaterial {
        id: frontTop_material
        baseColor: "red"
        cullMode: Material.NoCulling
        lighting: PrincipledMaterial.NoLighting
    }

    PrincipledMaterial {
        id: frontBottom_material
        baseColor: "green"
        cullMode: Material.NoCulling
        lighting: PrincipledMaterial.NoLighting
    }

    materials: [
        frontTop_material,
        frontBottom_material
    ]
}

One part of the model is painted with frontTop_material and for the other frontBottom_material is used.
I created similar scene where mesh data is generated by QQuick3DGeometry and it is painted on the screen, but I can't see a way how to assing different materials to different triangles. How can I do that?
There is a set of attributes, which i can set via QQuick3DGeometry::addAttribute to every vertex, but none of them is for defining texture id.

johngod

I'm not sure if you want to apply diferente colors or different textures.
If you're trying to use textures, for a simple model like a sphere, a texture would be applied like this:

Model {
                id: planetEarth
                source: "#Sphere"
                materials: [
                    PrincipledMaterial {
                        baseColorMap: Texture {
                            source: "qrc:/images/earth.png"
                            scaleU: 1.0
                            scaleV: 1.0
                        }
                    }
                ]
            }

The example you posted distortedcube, AFAIK the model is generated with the balsam tool, (from a obj file, or other similiar 3d format file), hence you can define diferente materials for each sub mesh, be either a texture or a color. See https://doc.qt.io/qt-5/qtquick3d-tool-balsam.html

Note that I don´t think you can have submeshs with QQuick3DGeometry, so you can have only one texture for one model (but I maybe wrong). With QQuick3DGeometry, you should use QQuick3DGeometry::addAttribute with TexCoordSemantic to define your texture positions in your model. So if you want to load several textures I think you have to break your model a part.
If you have one model and want to apply different colors to different triangles, I would try to use Quick3DGeometry::addAttribute with ColorSemantic.

See also the custom geometry example https://doc-snapshots.qt.io/qt6-dev/qtquick3d-customgeometry-example.html