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MTL file (along with any textures) are exported from your software and everything is uploaded with the. If you're using the OBJ format and have materials defined, you should make sure that a.
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Blender has an option called " External Data → Automatically Pack Into. Most software that exports FBX files (for example Maya or 3ds Max) will have an option called "Embed media" in the FBX export options.
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If you're using textures, you need to make sure they are uploaded together with your 3D file, or in the case of some formats such as FBX and Blender, actually embedded in the file. There are a few common reasons this might happen. Sometimes you might have colors, materials, or textures applied to your model in your 3D software, but they don't come through correctly on Sketchfab.
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