What does lightposition.w mean?

Hello,
In UnityCG.cginc, there exist many LightPosition.w. But I don't know the reason why to multiply LightPosition.w.

For example, there is
float3 toLight = unity_LightPosition.xyz - viewpos.xyz * unity_LightPosition.w;
in the following code.

Code (CSharp):

1. float3 ShadeVertexLightsFull (float4 vertex, float3 normal, int lightCount, bool spotLight)
2. {
3.     float3 viewpos = UnityObjectToViewPos (vertex);
4.     float3 viewN = normalize (mul ((float3x3)UNITY_MATRIX_IT_MV, normal));
5.  
6.     float3 lightColor = UNITY_LIGHTMODEL_AMBIENT.xyz;
7.     for (int i = 0; i < lightCount; i++) {
8.         float3 toLight = unity_LightPosition[i].xyz - viewpos.xyz * unity_LightPosition[i].w;
9.         float lengthSq = dot(toLight, toLight);
10.  
11.         // don't produce NaNs if some vertex position overlaps with the light
12.         lengthSq = max(lengthSq, 0.000001);
13.  
14.         toLight *= rsqrt(lengthSq);
15.  
16.         float atten = 1.0 / (1.0 + lengthSq * unity_LightAtten[i].z);
17.         if (spotLight)
18.         {
19.             float rho = max (0, dot(toLight, unity_SpotDirection[i].xyz));
20.             float spotAtt = (rho - unity_LightAtten[i].x) * unity_LightAtten[i].y;
21.             atten *= saturate(spotAtt);
22.         }
23.  
24.         float diff = max (0, dot (viewN, toLight));
25.         lightColor += unity_LightColor[i].rgb * (diff * atten);
26.     }
27.     return lightColor;
28. }

 

here it says,
View-space light position:
(-direction,0) for directional lights
(position,1) for point/spot lights.
https://docs.unity3d.com/Manual/SL-UnityShaderVariables.html

 

Oh, thanks. It is so kind of you!