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

#ifdef USE_NORMALMAP

	uniform sampler2D normalMap;
	uniform vec2 normalScale;

			// Per-Pixel Tangent Space Normal Mapping
			// http://hacksoflife.blogspot.ch/2009/11/per-pixel-tangent-space-normal-mapping.html

	vec3 perturbNormal2Arb( vec3 eye_pos, vec3 surf_norm ) {

		vec3 q0 = dFdx( eye_pos.xyz );
		vec3 q1 = dFdy( eye_pos.xyz );
		vec2 st0 = dFdx( vUv.st );
		vec2 st1 = dFdy( vUv.st );

		vec3 S = normalize( q0 * st1.t - q1 * st0.t );
		vec3 T = normalize( -q0 * st1.s + q1 * st0.s );
		vec3 N = normalize( surf_norm );

		vec3 mapN = texture2D( normalMap, vUv ).xyz * 2.0 - 1.0;
		mapN.xy = normalScale * mapN.xy;
		mat3 tsn = mat3( S, T, N );
		return normalize( tsn * mapN );

	}

#endif