Why does this simple additive shader draw as non-additive?

Unity Engine
1

I have this simple shader:

Shader "Swirly Glow Outline" 
{
	Properties 
	{
		_Color ("Main Color", Color) = (1,1,1,1)
		_MainTex ("Base (RGB)", 2D) = "white" {}
	}
	
	SubShader 
	{
		Tags 
		{ 
			"RenderType" = "Transparent" 
			"Queue" = "Transparent" 
		}
		Lighting Off
		ZWrite Off
		Cull Off 
		Blend One One 
		
		CGPROGRAM
			#pragma target 3.0
			#pragma surface surf BlinnPhong vertex:vert alpha
			#include "UnityCG.cginc" 
		   
			sampler2D _MainTex;
			float4 _Color;
			
			void vert (inout appdata_full v)
			{
				v.texcoord = float4(sin(_Time.x), _Time.x, 0, 0)*1.5;
			 }
		   
			struct Input
			{
				float2 uv_MainTex;
				float4 color:COLOR;
				float4 screenPos;
			};
		   
			void surf (Input IN, inout SurfaceOutput o)
			{	
				IN.uv_MainTex += IN.screenPos*4;
				half tex = tex2D(_MainTex, IN.uv_MainTex).r;
				o.Albedo = _Color.rgb;
				o.Alpha = 1;
			}
		ENDCG
	}
	Fallback "Particles/Additive"
}

which should result in additive blended, but for some reason shows up as normal blending, like so:

The yellow outline shape should be additive, but it’s not. I’m probably missing something really obvious, but I’m damned if I can see it. Can anyone shed some light on this?

2

I think the surface shader auto-generates the blend thing due to the pragmas you used. See here: How to set up additive blending on a surface shader? - Questions & Answers - Unity Discussions

3

Ah, thanks Imaginary Human. It seems the alpha pragma specifically overrides the blending mode. That’s a bit of a nuisance.

4

Try decal:add. I’m not sure how decal:blend differs from the alpha directive, but the result of decal:add is an additively blended surface shader.