Convert images to black-and-white format with alpha channel

Hello, my shader is not working properly with an alpha channel, what went wrong?

Code (CSharp):

1. Shader "Custom/BWDiffuse" {
2.     Properties{
3.         _MainTex("Base (RGB)", 2D) = "white" {}
4.         _bwBlend("Blend", Range(0, 1)) = 0
5.     }
6.         SubShader{
7.         Pass{ Blend SrcAlpha OneMinusSrcAlpha
8.         CGPROGRAM
9. #pragma vertex vert_img
10. #pragma fragment frag
11.  
12. #include "UnityCG.cginc"
13.  
14.         uniform sampler2D _MainTex;
15.     uniform float _bwBlend;
16.  
17.     float4 frag(v2f_img i) : COLOR{
18.         float4 c = tex2D(_MainTex, i.uv);
19.  
20.         float lum = c.r*.3 + c.g*.59 + c.b*.11;
21.         float3 bw = float3(lum, lum, lum);
22.  
23.         float4 result = c;
24.         result.rgb = lerp(c.rgb, bw, 1 - _bwBlend);
25.         return result;
26.     }
27.         ENDCG
28.     }
29.     }
30. }

 

why not just use float3 bw =Luminance(c.rgb);?

 

This don't significantly, the result does not change.

 

adding test scene, showing the bug.

 

You need to tell the shader not to write to the depth buffer by using ZWrite Off. You should also set some tags, like "Queue"="Transparent", "RenderType"="Transparent", and "IgnoreProjector"="True". Check it out here: http://docs.unity3d.com/Manual/SL-SubShaderTags.html

Here's your shader back. This works in your test scene.

 

Thank you