I saw some discussions saying gltf does not export bumpmap but if I look into my exported gltf file (separate files mode) there is a bumpmap file with a bumpscale (and also normal map). After loading into three.js I get a meshstandard material with bumpmap object and scale value and also normalmap object. But they seem to have no effect on the rendered object. What is exactly allowed to do to export relief effect from Blender to Three.js with gltf ?
