Shadow Receiving Issue with GLB from Minecraft Education Edition

Based on your description, the issue appears to be related to the material type used in your GLB file. The key insight is that your model is using the KHR_materials_pbrSpecularGlossiness extension which creates a GLTFMeshStandardSGMaterial in Three.js, while the re-exported version from Blender uses the standard MeshStandardMaterial and correctly receives shadows.

Possible Solutions

1. Convert the materials after loading:
   You could traverse the loaded model and replace the GLTFMeshStandardSGMaterial with standard MeshStandardMaterial instances while preserving the texture information:

   JAVASCRIPTCopied
   gltfLoader.load('your-model.glb', (gltf) => {
     const model = gltf.scene;
     
     model.traverse((node) => {
       if (node.isMesh) {
         // Ensure shadows are enabled
         node.castShadow = true;
         node.receiveShadow = true;
         
         // If using the problematic material type
         if (node.material && node.material.type === 'GLTFMeshStandardSGMaterial') {
           // Create a new standard material
           const newMaterial = new THREE.MeshStandardMaterial({
             map: node.material.map,
             normalMap: node.material.normalMap,
             roughnessMap: node.material.roughnessMap,
             metalnessMap: node.material.metalnessMap,
             color: node.material.color,
             roughness: node.material.roughness,
             metalness: node.material.metalness,
             transparent: node.material.transparent,
             opacity: node.material.opacity
           });
           
           // Replace the material
           node.material = newMaterial;
         }
       }
     });
     
     scene.add(model);
   });

2. Use a GLTF converter tool:
   Instead of using Blender (which seems to break your textures), you could try using a dedicated GLTF converter tool that can convert from the Specular-Glossiness workflow to the Metallic-Roughness workflow while preserving textures.

3. Update your Three.js version:
   If you're using an older version of Three.js, consider updating to the latest version, as there have been improvements in how the library handles various GLTF extensions.

4. Check material properties:
   Ensure that the material's shadowSide property is set correctly. You might need to explicitly set it to THREE.FrontSide or THREE.DoubleSide.

The issue is likely related to how the KHR_materials_pbrSpecularGlossiness extension is implemented in Three.js, as this is an older extension that has been largely replaced by the standard PBR metallic-roughness workflow in newer GLTF specifications.