Why changing the normal affects the viewDir?

Questions & Answers
1

First, I would like to let clear that this shader makes no sense. It’s just the problem of my real shader isolated.

Shader "Test" {
	SubShader
	{
		CGPROGRAM
		#pragma surface surf Lambert

		struct Input
		{
			float3 viewDir;
    	};

		void surf (Input IN, inout SurfaceOutput o)
		{
			o.Albedo = IN.viewDir;
			//o.Normal = half3(0, 0, 0);
		}

		ENDCG
	}
	
	FallBack "Mobile/Diffuse"
}

Apply this shader to an object and move the camera/view port around. You will see the object changing color. This happen because the RGB is mapped to the relative camera/view port position. That’s fine.

Uncomment the commented line and the colors will change. Of course we would expect a lighting change, but what happens is a hue change, what makes no sense for me.

So, why changing the normal affects the viewDir?

2

Well…changing the normals changes the direction a surface is looking at… If it wasn’t like that, then for example fresnel shaders with normal maps wouldn’t work correctly.