How to mimic multiple material blending in a shader?

I was wondering how how if a renderer has multiple materials on the same object these materials blended together and how this can be done in a shader?

I would like to get the effect of blending four (or more) of the same material in a single shader. I am working from the "Vegetation" example found in the Unity manual and found that I got a result I liked if I used four or more copies of the same material on a renderer.

I have tried adding multiple passes trying to duplicate the second pass with different blend effects, I have tried turning off ZTest on and using previous texture values on the third or fourth passes with different blending but no luck so far

Shader "Vegetation" {
Properties {
_Color ("Main Color", Color) = (.5, .5, .5, .5)
_MainTex ("Base (RGB) Alpha (A)", 2D) = "white" {}
_Cutoff ("Base Alpha cutoff", Range (0,.9)) = .5
}
SubShader {
// Set up basic lighting
Material {
Diffuse [_Color]
Ambient [_Color]
}
Lighting On

// Render both front and back facing polygons.
Cull Off

// first pass:
// render any pixels that are more than [_Cutoff] opaque
Pass {
AlphaTest Greater [_Cutoff]
SetTexture [_MainTex] {
combine texture * primary, texture
}
}

// Second pass:
// render in the semitransparent details.
Pass {
// Dont write to the depth buffer
ZWrite off
// Don't write pixels we have already written.
ZTest Less
// Only render pixels less or equal to the value
AlphaTest LEqual [_Cutoff]

// Set up alpha blending
Blend SrcAlpha OneMinusSrcAlpha

SetTexture [_MainTex] {
combine texture * primary, texture
}
}

// below is where I try to make another pass
//trying to duplicate the previous pass
Pass {
// Dont write to the depth buffer
ZWrite off
// Do write alpha pixels again.

// Only render pixels less or equal to the value
AlphaTest LEqual [_Cutoff]

// It seems like it has to be alpha blending....
Blend SrcAlpha OneMinusSrcAlpha

// try to use the previous texture
SetTexture [_MainTex] {
combine previous * primary, previous
}
}
}
}

 

Try other blend modes, like "Blend One One". Ztest won't have any effect in what you're doing in this case. You don't want to do "combine previous" by itself in a separate pass, since there is no previous; that only applies when there's more than one texture you're combining in the same pass. Take a look on the wiki and examine some of the shaders there, like some of the multi-layer terrain shaders, to get an idea how to combine stuff.

--Eric

 

Thanks Eric,

After making the changes you suggested I found that in the third pass I found that I could get the result I was looking for by changing the pass SetTexture to "SetTexture [_MainTex] { Combine texture * primary, texture * primary }"

And also by adding the tag "Tags {Queue=Transparent}" to the sub shader

Cheers