Terrain Tree Billboard have bright outline against dark background

I just ran into this problem when switching to linear lighting in unity 5.

The above solution didn't solve it, after much digging it turns out the background color of the render texture unity uses for tree billboards isn't black (it's (0.025,0.025,0.025)).

In gamma space this doesn't present itself as a problem. However in linear space, this is converted to gamma space at the end of the render pipeline bringing it to a much more visible grey (0.19,0.19,0.19)

To solve this I modified the TreeCreatorLeavesRendertex.shader to convert the billboard render texture to an approximation of gamma color space with

 c.rgb = sqrt(c.rgb);
 c.rgb = max(c.rgb,0.025); //Ensure our tree is never rendered darker than the background color

towards the end of the fragment shader

Then do the opposite operation in the BillboardTree.shader

 col.rgb = col.rgb * col.rgb;

Not a perfect solution, but will work for us until speedtree stops performing so poorly

[UPDATED 22.03.17] I've used this shader, plus:

1. TOD_TreeBillboardState. Means need to Colorize or not (Night or Day), set through Shader.SetGlobalFloat

2. TOD_TreeBillboardColor. Which Color to use for fill. Uses white-to-black Gradient, based on Night-time. Set through Shader.SetGlobalColor

Gradient t instructions:

 if (Cycle.Hour > 17f)
        t = Mathf.Lerp(0f, .5f, (Cycle.Hour - 17f) / 7f);
 else
        t = Mathf.Lerp(.5f, 1f, Cycle.Hour / 7f);

!All is Updating every frame, with day-night-cycle script. No performance problems.

 Shader "Hidden/TerrainEngine/BillboardTree" {
     Properties {
         _MainTex ("Base (RGB) Alpha (A)", 2D) = "white" {}
     }
     
     SubShader {
         Tags { "Queue" = "Transparent-1" "IgnoreProjector"="True" "RenderType"="TransparentCutout" }
         Pass {
             Tags { "RequireOption" = "SoftVegetation" }
 
             // Dont write to the depth buffer
             ZWrite off
         
             // Set up alpha blending
             Blend SrcAlpha OneMinusSrcAlpha
             Cull Front
             CGPROGRAM
             #pragma vertex vert 
             #pragma fragment frag
             #pragma multi_compile_fog
             #include "UnityCG.cginc"
             #include "TerrainEngine.cginc"
 
             Float TOD_TreeBillboardState;
             fixed4 TOD_TreeBillboardColor;
 
             struct v2f {
                 float4 pos : SV_POSITION;
                 fixed4 color : COLOR0;
                 float2 uv : TEXCOORD0;
                 UNITY_FOG_COORDS(1)
             };
 
             v2f vert (appdata_tree_billboard v) {
                 v2f o;
                 TerrainBillboardTree(v.vertex, v.texcoord1.xy, v.texcoord.y);    
                 o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
                 o.uv.x = v.texcoord.x;
                 o.uv.y = v.texcoord.y > 0;
                 o.color = v.color;
                 UNITY_TRANSFER_FOG(o,o.pos);
                 return o;
             }
 
             sampler2D _MainTex;
             fixed4 frag(v2f input) : SV_Target
             {
                 fixed4 col = tex2D( _MainTex, input.uv);
                 if (TOD_TreeBillboardState > .5)
                     if (col.r < .18 && col.a < .9)
                         col.rgb *= TOD_TreeBillboardColor.rgb;
                 col.rgb *= input.color.rgb;
                 clip(col.a);
                 UNITY_APPLY_FOG(input.fogCoord, col);
                 return col;
             }
             ENDCG            
         }
         Pass {
             Cull Front
             CGPROGRAM
             #pragma vertex vert 
             #pragma fragment frag
             #pragma multi_compile_fog
             #include "UnityCG.cginc"
             #include "TerrainEngine.cginc"
 
             struct v2f {
                 float4 pos : SV_POSITION;
                 fixed4 color : COLOR0;
                 float2 uv : TEXCOORD0;
                 UNITY_FOG_COORDS(1)
             };
 
             v2f vert (appdata_tree_billboard v) {
                 v2f o;
                 TerrainBillboardTree(v.vertex, v.texcoord1.xy, v.texcoord.y);    
                 o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
                 o.uv.x = v.texcoord.x;
                 o.uv.y = v.texcoord.y > 0;
                 o.color = v.color;
                 UNITY_TRANSFER_FOG(o,o.pos);
                 return o;
             }
 
             sampler2D _MainTex;
             fixed4 frag(v2f input) : SV_Target
             {
                 fixed4 col = tex2D( _MainTex, input.uv);
                 col.rgb *= input.color.rgb;
                 clip(col.a - .99);
                 UNITY_APPLY_FOG(input.fogCoord, col);
                 return col;
             }
             ENDCG            
         }
     }
     Fallback Off
 }

Another pretty simple solution for that is changing the alpha cutout directly in the shader. Simply go to "clip(col.a)" and add -0.9 to it. This will increase the cutout and removes the grey outline.

An easy way to fix this is to go to terrain settings and change the billboard to about halfway, and if it still appears, put it up all the way.