three.js/src/renderers/shaders/ShaderChunk/normal_fragment_maps.glsl.js

export default /* glsl */`

#ifdef OBJECTSPACE_NORMALMAP

	normal = texture2D( normalMap, vUv ).xyz * 2.0 - 1.0; // overrides both flatShading and attribute normals

	#ifdef FLIP_SIDED

		normal = - normal;

	#endif

	#ifdef DOUBLE_SIDED

		normal = normal * ( float( gl_FrontFacing ) * 2.0 - 1.0 );

	#endif

	normal = normalize( normalMatrix * normal );

#elif defined( TANGENTSPACE_NORMALMAP )

	vec3 mapN = texture2D( normalMap, vUv ).xyz * 2.0 - 1.0;
	mapN.xy *= normalScale;

	#ifdef USE_TANGENT

		mat3 vTBN = mat3( tangent, bitangent, normal );
		normal = normalize( vTBN * mapN );

	#else

		normal = perturbNormal2Arb( -vViewPosition, normal, mapN );

	#endif

#elif defined( USE_BUMPMAP )

	normal = perturbNormalArb( -vViewPosition, normal, dHdxy_fwd() );

#endif
`;