ShaderLab Unity : Render the vertex into UV space using Surface Shader - unity3d

Question

I am following this tutorial on baking the shader into a texture Map here It works great. I just have some problem applying it on a standard shader.
so this is an unwrap shader they made using a vertex/fragment(I think these are responsible to render the vertex into uv space):
v2f vert (appdata v)
{
v2f o;
v.vertex = float4(v.uv.xy, 0.0, 1.0);
o.vertex = mul(UNITY_MATRIX_P, v.vertex);
//o.vertex = UnityObjectToClipPos(v.vertex);
o.uv = v.uv;
return o;
}
fixed4 frag (v2f IN) : SV_Target
{
//here is where you would put the shader code you want to bake
//for now I will just do a noise
float f = fbm(IN.uv + fbm(5*IN.uv, 4), 4);
fixed4 color = fixed4(f,f,1,1);
return color;
}
I wanted to do that but on a surface shader. Can you guys help me?

Answer 1

You have two options. If its not too late already...
Option 1: Alter the generated code from your surface shader
Option 2: Use vertex modifier and counteract UnityObjectToClipPos(v.vertex);
Unity surface shaders are also just Vertex and Fragmentshaders like your snippet up above. It is a long collection of different passes and predefined functions to add all kinds of effects like shadows, fog etc.
If you create a shader as a surface shader you tell unity which predefined functions to use with pragmas like "surface surf Standard fullforwardshadows"
You can always look at the generated code if you click on you compiled shader there is a button. There you see a long shader with vertex and fragment portions. Its a little cluttered with #ifelse and the code that is run on your machine is only a small portion in the end. It is possible to declutter it by hand and only change the specific parts that you want to alter i.e. add a custom vertex shader.
Since this is time consuming and error prone unity has a lot of customisability of the standard shaders.
Option 2:
One of these is the vertex modifier pragma with vertex:vert you can assign a custom vertex function to be called in the vertex shader to modify the vertex position. (See Normal Extrusion in: here) Unfortunately unity wraps a
UnityObjectToClipPos(v.vertex); directly after your vertex modifier function. So if you do a transformation from object-space to uv-space unity then automatically converts the uv-coordinates to clip-space as if it were object-space coordinates resulting in nonsense. What you can do is to counteract this with mul(transpose(UNITY_MATRIX_IT_MV), v.vertex);
The resulting multiplication then is: PVM*t(t(inv(MV))) = P
That way you cheat the vertex modifier to act as an override. Keep in mind thought that position is also used for other operations shadows etc. If you want to have more control and skip this unecessary double multiplication you can do
Option 2:
You can always go into the generated code from your surface shader and alter the vertex functions. You can i.e. remove the call to UnityObjectToClipPos(v.vertex); manually.
Note: One problem i ran into a lot with UV-Space manipulation is that the clipspace depends on your graphics api used. Wether it is OpenGL like or Directx like you might need to set your z coords to -1.0 or 0.0 in float4(v.uv.xy, 0.0, 1.0);. This also might change depending on your build target.
Edit:
Here is an example vertex modifier for a surface shader:
#pragma surface surf Standard fullforwardshadows vertex:vert
void vert (inout appdata_full v) {
v.vertex = float4(v.texcoord.xy, 0.0, 1.0);
//or float4(v.texcoord.xy, -1.0, 1.0);
v.vertex = mul(transpose(UNITY_MATRIX_IT_MV), v.vertex);
}