UnityCG.cginc有一个叫ShadeVertexLightsFull的函数可以用来计算顶点光照。
源码如下：
// Used in Vertex pass: Calculates diffuse lighting from lightCount lights. Specifying true to spotLight is more expensive
// to calculate but lights are treated as spot lights otherwise they are treated as point lights.
float3 ShadeVertexLightsFull (float4 vertex, float3 normal, int lightCount, bool spotLight)
{
float3 viewpos = UnityObjectToViewPos (vertex);
float3 viewN = normalize (mul ((float3x3)UNITY_MATRIX_IT_MV, normal));
float3 lightColor = UNITY_LIGHTMODEL_AMBIENT.xyz;
for (int i = 0; i < lightCount; i++) {
float3 toLight = unity_LightPosition[i].xyz - viewpos.xyz * unity_LightPosition[i].w;
float lengthSq = dot(toLight, toLight);
// don't produce NaNs if some vertex position overlaps with the light
lengthSq = max(lengthSq, 0.000001);
toLight *= rsqrt(lengthSq);
float atten = 1.0 / (1.0 + lengthSq * unity_LightAtten[i].z);
if (spotLight)
{
float rho = max (0, dot(toLight, unity_SpotDirection[i].xyz));
float spotAtt = (rho - unity_LightAtten[i].x) * unity_LightAtten[i].y;
atten *= saturate(spotAtt);
}
float diff = max (0, dot (viewN, toLight));
lightColor += unity_LightColor[i].rgb * (diff * atten);
}
return lightColor;
}
代码中使用了unity_LightPosition等Unity的预制变量，这些变量只在Vertex Pass中可以使用。在Pass的Tags中设置"LightMode" = "Vertex"来激活Vertex Pass，否则这些预制变量不生效。
参考文献
https://docs.unity3d.com/Manual/SL-UnityShaderVariables.html