Support for KHR_materials_variants

[javagl]

tl;dr:

KHR_materials_variants is a ratified glTF extension that allows defining different material variants for meshes. The extension defines a set of 'variant names' that can be activated. For each mesh primitive, the extension defines the material that should be used when a certain variant is activated.

It should be possible to extract the information about material variants from a glTF asset, and select the active material variant in CesiumJS, switching the material/textures at runtime.

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Related:

• An older issue that describes the goal of dynamically changing textures: Support ability to dynamically change a 3D model's texture #5094
  This already summarizes some possible approaches, and has some references to the KHR_materials_variants extension. Some of the discussion there is a bit out-dated, though (as it refers to older versions and implementations), and was not targeted at explicitly supporting KHR_materials_variants
• A recent issue in cesium-native: Support for KHR_materials_variants in cesium-native and implementations cesium-native#676
  This referred to support of KHR_materials_variants in cesium-native on the level of the glTF files, and on the level of tilesets (more on that below).

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The first step would be to transport the information about the KHR_materials_variants extension through the Model.fromGltf loading process. This may already raise some tricky low-level questions (e.g. whether the textures should be loaded immediately, or only when a certain material variant is activated), but should be doable

Based on that, one could expose the information about the material variants on an API level. Clients should be able to do basic things like

const materialVariantNames = model.getMaterialVariantNames();
model.setActiveMaterialVariant(materialVariantNames[i]);

in a sandcastle. (Eventually, there might even be a UI for that)

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Support for material variants on a tileset level:

The issue in cesium-native describes a concept for storing material variants information on the level of a tileset. The summary is:

• The tileset JSON contains metadata that includes an array of strings with the TILESET_MATERIALS_VARIANTS_NAMES semantic
• These strings are the material variant names that are expected to be present in all glTF assets that are used as the tile content in this tileset
• Setting a certain "active" material variant on the level of the tileset will activate this variant in all glTF tile contents that are currently loaded for that tileset