How to get Vertex Color in a cg shader?

Vertex Colors behave a little weird in Unity's CG code. If you were using Cg from OpenGL, for example, it would be enough to just mark the input variable with the : COLOR semantic. That's supposed to tell the Cg compiler that you want it to store the vertex color in that variable. But in Unity, you don't have the same control over what data is passed to the shader, because that kind of stuff is wrapped inside the Mesh class. I was tearing out my hair over this for half a day a couple of months ago, until I read this site closely:

http://unity3d.com/support/documentation/Components/SL-VertexProgramInputs.html

Look at the text right above where it describes the 6 vertex attributes that you can pass. Notice that it says "If you want to access different vertex data, you have to declare vertex structure yourself. The structure members must be from the following list:"

As I said, it took me half a day to realize that when they write "must be", they mean business, and it really does mean MUST BE. So, to get the vertex colors, you HAVE to declare a struct for the input value, and the variable MUST be called "color". Here's an example Cg vertex program that correctly grabs the vertex colors in a way Unity accepts (it's just a vertex/pixel shader pass-through):

Shader "Custom/ExampleVertexColorShader" { Properties { } SubShader { Tags { "RenderType"="Opaque"} pass { CGPROGRAM #pragma vertex wfiVertCol #pragma fragment passThrough #include "UnityCG.cginc"

         struct VertOut
         {
             float4 position : POSITION;
             float4 color : COLOR;
         };

         struct VertIn
         {
             float4 vertex : POSITION;
             float4 color : COLOR;
         };

         VertOut wfiVertCol(VertIn input, float3 normal : NORMAL)
         {
             VertOut output;
             output.position = mul(UNITY_MATRIX_MVP,input.vertex);
             output.color = input.color;
             return output;
         }

         struct FragOut
         {
             float4 color : COLOR;
         };

         FragOut passThrough(float4 color : COLOR)
         {
             FragOut output;
             output.color = color;
             return output;
         }
         ENDCG

     }
 }
 FallBack "Diffuse"

Again, Unity is really totally anal about this: The variable MUST be called "color", and it MUST be defined in a struct, or it won't store the vertex color in it.