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

export default /* glsl */`
float faceDirection = gl_FrontFacing ? 1.0 : - 1.0;

#ifdef FLAT_SHADED

	vec3 fdx = dFdx( vViewPosition );
	vec3 fdy = dFdy( vViewPosition );
	vec3 normal = normalize( cross( fdx, fdy ) );

#else

	vec3 normal = normalize( vNormal );

	#ifdef DOUBLE_SIDED

		normal *= faceDirection;

	#endif

#endif

#ifdef USE_NORMALMAP_TANGENTSPACE

	#ifdef USE_TANGENT

		mat3 tbn = mat3( normalize( vTangent ), normalize( vBitangent ), normal );

	#else

		mat3 tbn = getTangentFrame( - vViewPosition, normal, vNormalMapUv );

	#endif

	#if defined( DOUBLE_SIDED ) && ! defined( FLAT_SHADED )

		tbn[0] *= faceDirection;
		tbn[1] *= faceDirection;

	#endif

#endif

#ifdef USE_CLEARCOAT_NORMALMAP

	#ifdef USE_TANGENT

		mat3 tbn2 = mat3( normalize( vTangent ), normalize( vBitangent ), normal );

	#else

		mat3 tbn2 = getTangentFrame( - vViewPosition, normal, vClearcoatNormalMapUv );

	#endif

	#if defined( DOUBLE_SIDED ) && ! defined( FLAT_SHADED )

		tbn2[0] *= faceDirection;
		tbn2[1] *= faceDirection;

	#endif

#endif

// non perturbed normal for clearcoat among others

vec3 geometryNormal = normal;

`;