How to change RGB without changing alpha in sprite frag shader?

Question

I'm adding fog to a custom sprite vert+surf shader, rendered with a SpriteRenderer. It's as easy as removing #pragma nofog, but the result applies the fog color to transparent portions of my sprites, resulting in semi-visible boxes around them.

alt text

I traced this to Unity's UNITY_FOG_LERP_COLOR, called at the end of the Unity generated frag shader (frag_surf):

 #define UNITY_FOG_LERP_COLOR(col,fogCol,fogFac) col.rgb = lerp((fogCol).rgb, (col).rgb, saturate(fogFac))

If I change it to:

 col.r = 1;

Now it only sets the red channel to 1. It shouldn't affect other colors, or the alpha, right? But the result still adds semi-visible red boxes around my sprites (pictured above). Does Unity somehow embed alpha information into the R channel? How do I change RGB in the frag shader without affecting alpha?

I don't want to use clip. Multiplying col.rgb by col.a after applying the fog looks close to correct, but adds a faint dark outline.

My shader:

 Shader "Northway/SpritesDiffuseWave" {
     Properties {
         [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
     }
 
     SubShader {
         Tags {
             "Queue"="Geometry"
             "IgnoreProjector"="True"
             "RenderType"="Transparent"
             "PreviewType"="Plane"
             "CanUseSpriteAtlas"="True"
         }
 
         Cull Off
         Lighting Off
         ZWrite Off
         Blend One OneMinusSrcAlpha
 
         CGPROGRAM
         #pragma surface surf SimpleLambert vertex:vert nolightmap nodynlightmap keepalpha noinstancing //nofog 
         
         #include "UnitySprites.cginc"
 
         #undef UNITY_FOG_LERP_COLOR
         #define UNITY_FOG_LERP_COLOR(col,fogCol,fogFac) col.r = 1;
         
         struct Input {
             float2 uv_MainTex;
             fixed4 color;
         };
         
         half4 LightingSimpleLambert(SurfaceOutput s, half3 lightDir, half atten) {
             half4 c;
             c.rgb = s.Albedo * _LightColor0.rgb * atten * 0.60;
             c.a = s.Alpha;
             return c;
         }
  
         void vert (inout appdata_full v, out Input o) {
             UNITY_INITIALIZE_OUTPUT(Input, o);
 
             //... vertex shader code
         }
 
         void surf (Input IN, inout SurfaceOutput o) {
             fixed4 color = tex2D(_MainTex, IN.uv_MainTex) * IN.color;
             o.Albedo = color.rgb * color.a;
             o.Alpha = color.a;
         }
 
         ENDCG
     }
 
     Fallback "Transparent/VertexLit"
 }

Answer 1

Best Answer

Answer by sarahnorthway · Sep 19, 2018 at 09:34 PM

I was on the right track with multiplying rgb by alpha; I needed to multiply the FOG rgb by alpha before I lerp it in. Presumably col.rgb already has the alpha information baked in, but fogCol.rgb does not.

 #undef UNITY_FOG_LERP_COLOR
 #define UNITY_FOG_LERP_COLOR(col,fogCol,fogFac) col.rgb = lerp((fogCol).rgb * col.a, (col).rgb, saturate(fogFac))

This works with #pragma keepalpha or alpha:premul but not alpha:blend.

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How to change RGB without changing alpha in sprite frag shader?

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