How to use a directional light in a blinn shader instead of point light? - opengl

Question

So I am using a blinn shader program on some of my models, that uses a DIRECTIONAL light as opposed to a point light. I started out this original code that uses a point light:
VERTEX:
varying vec3 Half;
varying vec3 Light;
varying vec3 Normal;
varying vec4 Ambient;
void main()
{
// Vertex location in modelview coordinates
vec3 P = vec3(gl_ModelViewMatrix * gl_Vertex);
Light = vec3(gl_LightSource[0].position) - P;
Normal = gl_NormalMatrix * gl_Normal;
Half = gl_LightSource[0].halfVector.xyz;
Ambient = gl_FrontMaterial.emission + gl_FrontLightProduct[0].ambient + gl_LightModel.ambient*gl_FrontMaterial.ambient;
gl_TexCoord[0] = gl_MultiTexCoord0;
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}
FRAGMENT:
varying vec3 Half;
varying vec3 Light;
varying vec3 Normal;
varying vec4 Ambient;
uniform sampler2D tex;
vec4 blinn()
{
vec3 N = normalize(Normal);
vec3 L = normalize(Light);
vec4 color = Ambient;
float Id = dot(L,N);
if (Id>0.0)
{
color += Id*gl_FrontLightProduct[0].diffuse;
vec3 H = normalize(Half);
float Is = dot(H,L); // Specular is cosine of reflected and view vectors
if (Is>0.0) color += pow(Is,gl_FrontMaterial.shininess)*gl_FrontLightProduct[0].specular;
}
return color;
}
void main()
{
gl_FragColor = blinn() * texture2D(tex,gl_TexCoord[0].xy);
}
However, as stated above, instead of a point light I want I want a directional light, such that no matter where in the scene the model is the direction of the light is the same. So I make the following changes:
Instead of:
varying vec3 Light;
and
vec3 P = vec3(gl_ModelViewMatrix * gl_Vertex);
Light = vec3(gl_LightSource[0].position) - P;
I get rid of the above lines of code and instead in my fragment shader have:
uniform vec4 lightDir;
and
vec3 L = normalize(lightDir.xyz);
I pass the direction of the light as a uniform outside of a my shader program, and this works good: the model is lighted from a single direction no matter its location in the world! HOWEVER, now the lighting changes dramatically and unrealistically depending on the user's view, which makes sense since I got rid of ("-P ") in the light calculation in the original code. I've already tried adding that to the code (by moving lightDir back to vertex shader and passing it along again in a varying) to what I have and it just doesn't fix the problem. I'm afraid I just don't understand what is going well enough to figure this out, I understand that the " - P" for the Light vec3 is necessary: to make specular/reflection work, but I don't know how to make it work for a directional light. How do I make take the original above code, and make it so the light is treated as a directional light as opposed to a point light?