danini-the-panini/mittsu-opengl

#ifdef USE_BUMPMAP

    uniform sampler2D bumpMap;
    uniform float bumpScale;

    // Derivative maps - bump mapping unparametrized surfaces by Morten Mikkelsen
    // http://mmikkelsen3d.blogspot.sk/2011/07/derivative-maps.html

    // Evaluate the derivative of the height w.r.t. screen-space using forward differencing (listing 2)

    vec2 dHdxy_fwd() {

        vec2 dSTdx = dFdx( vUv );
        vec2 dSTdy = dFdy( vUv );

        float Hll = bumpScale * texture( bumpMap, vUv ).x;
        float dBx = bumpScale * texture( bumpMap, vUv + dSTdx ).x - Hll;
        float dBy = bumpScale * texture( bumpMap, vUv + dSTdy ).x - Hll;

        return vec2( dBx, dBy );

    }

    vec3 perturbNormalArb( vec3 surf_pos, vec3 surf_norm, vec2 dHdxy ) {

        vec3 vSigmaX = dFdx( surf_pos );
        vec3 vSigmaY = dFdy( surf_pos );
        vec3 vN = surf_norm;        // normalized

        vec3 R1 = cross( vSigmaY, vN );
        vec3 R2 = cross( vN, vSigmaX );

        float fDet = dot( vSigmaX, R1 );

        vec3 vGrad = sign( fDet ) * ( dHdxy.x * R1 + dHdxy.y * R2 );
        return normalize( abs( fDet ) * surf_norm - vGrad );

    }

#endif