To create a shader program that implements the Phong reflection model using a directional light source, we'll use the same vertex shader as in the previous recipe, except with a single light source.  Within the phongModel function, replace the calculation of the s vector with the following: 

vec3 s;
if( Light.Position.w == 0.0 )
  s = normalize( Light.Position.xyz );
else
  s = normalize( Light.Position.xyz - position );