alpha channel from light color

[yonek]

I want to write a shader that uses alpha from vertex colors that are calculated from light.
All I could get is a shader that uses the alpha from diffuse color but is a solid color. I isn't interpolated to the ambient alpha which is 0.0

[andeeee]

What effect do you want to achieve with this shader?

[yonek]

Imagine, a sphere with no texture and directional light in front of it facing towards the sphere. Now let the sphere have a vertex lit shader. I set the ambient color to (0,0,0,0) and diffuse color to (1,1,1,1).
This will result in a sphere that is white in the middle and getting darker toward the edges.
The color is nicely interpolated, as one can predict.
The problem is that the alpha isn't! it is 1.0 all over the sphere.
It seems that somewhere in the light formula vertices get alpha = 1.0.

[dreamora]

lights don't have alpha so you would have to interpolate the alpha from RGB ( r + b + g / 3 )

[yonek]

Did I mentioned that this should be a iPhone shader ?

[Daniel Brauer]

Indeed, fixed function vertex lighting always produces an alpha of 1.

And yes, you should mention if a shader is supposed to be for the iPhone. Most shaders can't be made to work on it (this one included).

[yonek]

That is not true. The diffuse light color alpha works! But still is constant for all vertices.

[Daniel Brauer]

[yonek]

Shader "VertexLitt" {
Properties {
_Color ("Main Color", Color) = (1,1,1,0)
_MainTex ("Base (RGB)", 2D) = "white" {}
}
SubShader {
Pass {
Material {
Diffuse [_Color]
}
Blend SrcAlpha OneMinusSrcAlpha
Lighting On
SetTexture [_MainTex] {
Combine texture * primary, primary
}
}
}
}


Now you can see that the Main Color affects alpha

[Daniel Brauer]

I see. I only tried changing the light alpha, which doesn't work. Unfortunately, all the other colours you put into the lighting equation will be uniform across the surface.