danini-the-panini/mittsu-opengl

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

    }

#endif