 
 	
 	// directional light source
-	vec3 P = (modelview * position).xyz;
+	vec3 Pos = (modelview * position).xyz;
 	vec4 lightPosition = modelview*(directionArray[0]);
-	vec3 _k2 = normalize(-P);
+	vec3 _k2 = normalize(-Pos);
 	vec3 _k1 = normalize(lightPosition.xyz);
 	vec3 V = _k1 - _k2;
 	float u = V.x; float v = V.y; float w = V.z;

	vec3 camTangent = normalize((modelview*vec4(tangent,0.0)).xyz);

	// compute vector-field rotation
	float phi = computeRotationAngle(tangent)
			
	// compute Fresnel and Gemometric Factor
	float F = getFressnelFactor(_k1, camNormal);

// wave constants
const float lambda_min = 380.0*pow(10.0, -9.0);
const float lambda_max = 700.0*pow(10.0, -9.0);
const float dx = 2.5*pow(10.0, -6.0);

// error constants
const float eps_pq = 0.0001; 
