How to write "combine texture * primary" by fragment progam?

Unity Engine
1

Hi there. I’m learning about Mobile/Particles/Additive shader

Shader "Mobile/Particles/Additive" {
Properties {
	_MainTex ("Particle Texture", 2D) = "white" {}
}

Category {
	Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" }
	Blend SrcAlpha One
	Cull Off Lighting Off ZWrite Off Fog { Color (0,0,0,0) }
	
	BindChannels {
		Bind "Color", color
		Bind "Vertex", vertex
		Bind "TexCoord", texcoord
	}
	
	SubShader {
		Pass {
			SetTexture [_MainTex] {
				combine texture * primary
			}
		}
	}
}
}

And I don’t really understand the code “combine texture * primary”. What is the primary? Is it the color in depth buffer? How to write this shader by fragment program?

My code is

Shader "PngRecorder/PRParticle" {
	Properties {
		_MainTex ("Base (RGB)", 2D) = "white" {}
	}
	SubShader {
		Tags {"Queue" = "Transparent"  "RenderType"="Transparent" }
		Pass{
			Blend SrcAlpha One
			ZWrite Off       
			
			CGPROGRAM
			#pragma vertex vert_img
			#pragma fragment frag
			#pragma fragmentoption ARB_precision_hint_fastest
			#include "UnityCG.cginc"
			
			uniform sampler2D _MainTex;
			
			fixed4 frag(v2f_img i) : COLOR{
				float4 col = tex2D(_MainTex,i.uv);
				return col;
			}
			
			ENDCG
		}
	} 
	FallBack "Diffuse"
}

But it has different effects. What’s the problem ?

2

Primary is the per-vertex colour in fixed function shaders. It can come from two sources: lighting results or mesh data.

To use per-vertex mesh colours in a fragment program, follow the example Visualizing Vertex Colors.

3

Thank you Daniel Brauer ! I understand now.