Unity uses a map to control the transparency of the material's main map

There's a need recently ： The transparency of the material's main map is controlled by the color value of a map .

Make a note first ： The map used to control the transparency of the main map , Let's call it “Alpha Mapping ”; The main map is called “ Main map ”.

First , What we need to solve is to Alpha The color value of the map is converted to transparency ; After many tests , Finally, the following methods are adopted ：1） Compare pixels r、g、b;2） hypothesis r Maximum , be float val = r* The weight +g+b;3）alpha = val/( The weight +2)

Go straight to the code ：

Shader "Custom/MainTexAlphaCtrl"
{
	Properties
	{
		_MainTex("Texture", 2D) = "white" {}// Main map texture 
		_AlphaTex("AlphaTexture",2D) = "white"{}//Alpha texture 
		_BaseColor("Base Color",Color) = (1, 1, 1, 1)
		_Weight("Weight",float) = 0//Alpha Weight when calculating 
		[Toggle(_Reverse)] _Reverse("Reverse", float) = 0// Whether to reverse , Used for transparency calculation of the main map 
	}
		SubShader
		{
		Tags { "RenderType" = "Transparent"}
			LOD 100
			Cull Off
			// If you use Alpha blend , Need to turn off deep buffer writing 
			ZWrite Off
			// Add mixed command 
			Blend SrcAlpha OneMinusSrcAlpha

			Pass
			{
				CGPROGRAM
				#pragma vertex vert
				#pragma fragment frag
				// make fog work
				#pragma multi_compile_fog

				#include "UnityCG.cginc"

				struct appdata
				{
					float4 vertex : POSITION;
					float2 uv : TEXCOORD0;
				};

				struct v2f
				{
					float2 uv : TEXCOORD0;
					float4 vertex : SV_POSITION;
				};

				sampler2D _MainTex;
				sampler2D _AlphaTex;
				float4 _MainTex_ST;
				float4 _BaseColor;
				float _Weight;
				float _Reverse;

				// principle ：
				// 1.  precondition ：1） Single channel rendering ,Unity Built in parameters unity_StereoEyeIndex As a sign of rendering left and right eyes ,
				//                               unity_StereoEyeIndex = 0： Render the left eye ;=1： Render the right eye .
				//                         2） Texture contains left and right binocular pictures , If the texture is monocular , This method is not applicable to .
				// 2.  When rendering the left eye , Textured 0~0.5f( contain 0.5f) Range ; When rendering the right eye , Textured 0.5f( It doesn't contain 0.5f)~1 Range .
				v2f vert(appdata v)
				{
					v2f o;
					o.vertex = UnityObjectToClipPos(v.vertex);
					o.uv = TRANSFORM_TEX(v.uv, _MainTex);
					return o;
				}

				// Convert color values to transparency 
				float color2Alpha(fixed3 col)
				{
					float a = 0;
					float val = 0;
					// Compare r、g、b Three values , Get the maximum , If r Maximum , be val = 4*r+g+b;
					if (col.r > col.g)
					{
						if (col.r > col.b)
						{
							val = _Weight * col.r + col.g + col.b;
						}
						else
						{
							val = _Weight * col.b + col.g + col.r;
						}
					}
					else
					{
						if (col.g > col.b)
						{
							val = _Weight * col.g + col.r + col.b;
						}
						else
						{
							val = _Weight * col.b + col.g + col.r;
						}
					}
					// then val Divide （ The weight +2）, Get the final alpha value 
					return val / (_Weight+2);
					//return (col.r + col.g + col.b)/3;
				}

				fixed4 frag(v2f i) : SV_Target
				{
					// sample the texture
					fixed4 main_col = tex2D(_MainTex, i.uv) * _BaseColor;
					fixed3 alpha_col = tex2D(_AlphaTex, i.uv).rgb;
					if (_Reverse == 0)
					{
						return fixed4(main_col.rgb, main_col.a * color2Alpha(alpha_col));
					}
					else
					{
						return fixed4(main_col.rgb, main_col.a * (1 - color2Alpha(alpha_col)));
					}
				}
				ENDCG
			}
		}
}