// Z range (relative to camera) must be 3..7

varying vec3 n, rn, rv;
varying float z;

#vertex

void main() {
    // vertex position
    vec4 v = gl_ModelViewMatrix * gl_Vertex;
    gl_Position = gl_ProjectionMatrix * v;

    // normal vector
    n = normalize(gl_NormalMatrix * gl_Normal);

    // generate some varyings
    rn = gl_Normal;
    rv = gl_Vertex.xyz;
    z = -0.25 * (v.z + 3.0);
}

#fragment

uniform sampler2D tex;

vec3 smpl(in vec3 en, in vec2 tc) {
    return 0.5 * dot(rn, en) * (texture2D(tex, 0.5 * tc + vec2(0.5, 0.5)).xyz - vec3(0.5, 0.5, 0.5));
}

void main() {
    vec3 nn;
    nn = normalize(normalize(n)
       + smpl(vec3(1.0, 0.0, 0.0), rv.yz)
       + smpl(vec3(0.0, 1.0, 0.0), rv.xz)
       + smpl(vec3(0.0, 0.0, 1.0), rv.xy));
    gl_FragColor = vec4(0.5 * nn + vec3(0.5, 0.5, 0.5), z);
}