Optimization and minimal Pixel or Fragment shader logic

Hi
In my attempts at optimizing a Vertex Fragment shader I have moved all the logic in my Fragment shader (except for texture calculations) to my Vertex shader. So my Fragment shader is simply

Code (csharp):

1.  
2. float4 frag(vertexOutput input) : COLOR
3. {
4.  
5. //Texture Maps if using textures
6. float4 tex = tex2D(_MainTex, input.tex.xy * _MainTex_ST.xy + _MainTex_ST.zw);
7.  
8. returnfloat4(tex.xyz * input.col * _Color.xyz, 1.0);
9. //return input.col;
10. }
11.  

My question >
Is there ever a case when your Fragment shader MUST contain more logic than the above?

 

Sure, whenever you need to do something at a higher frequency than the vertices, ie: something based off of a texture mask, or specular highlights, or normal mapping, etc. etc.

 

If you wanna go even faster
in this line:
float4 tex = tex2D(_MainTex, input.tex.xy* _MainTex_ST.xy+ _MainTex_ST.zw);

do the * + of MainTex_ST on the vertex unless you're doing some crazy stuff the emulates 3D on a camera facing quad, you should be fine

Cos atm your causin a dependant read, not super bad on desktop, but not great for GPU (especially if your doing multiple texture fetches)