Calculate the refraction direction for an incident vector
### Declaration
```glsl
float refract(float I, float N, float eta)
vec2 refract(vec2 I, vec2 N, float eta)
vec3 refract(vec3 I, vec3 N, float eta)
vec4 refract(vec4 I, vec4 N, float eta)
```
### Parameters
```I``` specifies the incident vector.
```N``` specifies the normal vector.
```eta``` specifies the ratio of indices of refraction.
### Description
For a given incident vector ```I```, surface normal ```N``` and ratio of indices of refraction, ```eta```, refract returns the refraction vector, ```R```.
```R``` is calculated as:
```glsl
k = 1.0 - eta * eta * (1.0 - dot(N, I) * dot(N, I));
if (k <0.0)
R = genType(0.0); // or genDType(0.0)
else
R = eta * I - (eta * dot(N, I) + sqrt(k)) * N;
```
The input parameters ```I``` and ```N``` should be normalized in order to achieve the desired result.
