How to use multiple UV maps with ShaderLab (Unity)

Question

I want to layer one texture over top of another on the same mesh, but map the textures to different UV maps.

The example for Detail Maps in the Surface Shader Examples looks almost identical to what I want, but it seems that it uses the same UV map as the albedo texture, multiplied by 2.

I will admit I am very new to ShaderLab, but I am willing to put the work in and research: I just haven't seemed to be able to find the right documentation for what I want to do, or at least I do not understand how to use it.

I have achieved this result in blender this way, though I understand obviously there isn't a 1:1 conversion here.

I also have it working with ShaderGraph, which is neat, but I need it to work using the internal Unity shader for the project I am working on, so unfortunately ShaderGraph is out.

My understanding is that I would declare the UV maps under the Input struct like

struct Input {
    float2 uv_MainTex : TEXCOORD0;
    float2 uv2_Detail : TEXCOORD1;
};

and then grab the textures from the Properties input by doing

sampler2D _MainTex;
sampler2D _Detail;

and then use the inputs similarly to the _MainTex call which is

fixed4 c = tex2d(_MainTex, IN.uv_Detail).rgb * _Color;

I have also seen this answer but it doesn't really seem to solve the problem for me, it still doesn't seem to access my second UV map.

I do know that there is some confusion with how UV maps are used in HLSL vs. in Unity. Generally speaking I'm not concerned about overwriting UVs with lightmaps or stuff like that, and if that is an issue I can just add dummy maps when I export from blender to put my custom UV map as the 4th index.

My question is: how do I make one image map use a certain UV Channel and a second image use a second UV channel? Currently my setup is basically just a standard Lit Shader that I added the properties for the detail map to.

Right now it is only displaying the _MainTex on UV0.

Code in full here.

Answer 1

Reviewing the Surface Shader documentation:

Surface Shader input structure

The input structure Input generally has any texture coordinates needed by the shader. Texture coordinates must be named “uv” followed by texture name (or start it with “uv2” to use second texture coordinate set).

So:

struct Input{
    float2 uv_MainTex : TEXCOORD0;
    float2 uv2_Detail : TEXCOORD1;
};