Undeclared identifier Error in desaturate shader

Hello community!
I am very new to shader and google did not help me this time.
Is anyone here can point me into the problem please?
so here is the shader
I get an Undeclared identifier "_MainTex" at line 103 (on metal)

Code (CSharp):

1. Shader "Sprites/Desaturate"
2. {
3.     Properties
4.     {
5.         [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
6.         _Color ("Tint", Color) = (1,1,1,1)
7.         _Saturate ("Saturate", Range (0, 1)) = 1
8.         _Bright ("Brightness", Float) = 0
9.         [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
10.        
11.         // required for UI.Mask
12.         _StencilComp ("Stencil Comparison", Float) = 8
13.         _Stencil ("Stencil ID", Float) = 0
14.         _StencilOp ("Stencil Operation", Float) = 0
15.         _StencilWriteMask ("Stencil Write Mask", Float) = 255
16.         _StencilReadMask ("Stencil Read Mask", Float) = 255
17.         _ColorMask ("Color Mask", Float) = 15
18.  
19.         [Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
20.     }
21.  
22.     SubShader
23.     {
24.         Tags
25.         {
26.             "Queue"="Transparent"
27.             "IgnoreProjector"="True"
28.             "RenderType"="Transparent"
29.             "PreviewType"="Plane"
30.             "CanUseSpriteAtlas"="True"
31.         }
32.                 Stencil
33.         {
34.             Ref [_Stencil]
35.             Comp [_StencilComp]
36.             Pass [_StencilOp]
37.             ReadMask [_StencilReadMask]
38.             WriteMask [_StencilWriteMask]
39.          }
40.  
41.         Cull Off
42.         Lighting Off
43.         ZWrite Off
44.         Fog { Mode Off }
45.         Blend One OneMinusSrcAlpha
46.  
47.         Pass
48.         {
49. //            Stencil {
50. //                Ref 2
51. //                Comp Less
52. //                Pass keep
53. //                Fail decrWrap
54. //                ZFail keep
55. //            }
56.         CGPROGRAM
57.             #pragma vertex vert
58.             #pragma fragment frag
59.             #pragma multi_compile DUMMY PIXELSNAP_ON
60.             #include "UnityCG.cginc"
61.            
62.             struct appdata_t
63.             {
64.                 float4 vertex   : POSITION;
65.                 float4 color    : COLOR;
66. //                float saturation: FLOAT;
67.                 float2 texcoord : TEXCOORD0;
68.             };
69.  
70.             struct v2f
71.             {
72.                 float4 vertex   : SV_POSITION;
73.                 fixed4 color    : COLOR;
74.                 float saturation: FLOAT;
75.                 fixed brightness: FIXED;
76.                 half2 texcoord  : TEXCOORD0;
77.             };
78.            
79.             fixed4 _Color;
80.             fixed _Saturate;
81.             fixed _Bright;
82.            
83.             v2f vert(appdata_t IN)
84.             {
85.                 v2f OUT;
86.                 OUT.vertex = UnityObjectToClipPos(IN.vertex);
87.                 OUT.texcoord = IN.texcoord;
88.                 OUT.color = IN.color * _Color;
89.                 OUT.saturation = _Saturate;
90.                 OUT.brightness = _Bright;
91.                 #ifdef PIXELSNAP_ON
92.                 OUT.vertex = UnityPixelSnap (OUT.vertex);
93.                 #endif
94.  
95.                 return OUT;
96.             }
97.  
98.  
99.             fixed4 frag(v2f IN) : SV_Target
100.             {
101.                 float luminance = (tex2D(_MainTex, IN.texcoord).r + tex2D(_MainTex, IN.texcoord).g + tex2D(_MainTex, IN.texcoord).b)/3.0;
102.                 fixed4 colorSature = fixed4(lerp(luminance, tex2D(_MainTex, IN.texcoord).r, IN.saturation) + IN.brightness,    lerp(luminance, tex2D(_MainTex, IN.texcoord).g, IN.saturation) + IN.brightness, lerp(luminance, tex2D(_MainTex, IN.texcoord).b, IN.saturation) + IN.brightness, tex2D(_MainTex, IN.texcoord).a);
103.                 fixed4 c = colorSature * IN.color;
104.                 c.rgb *= c.a;
105.                 return c;
106.             }
107.         ENDCG
108.         }
109.     }
110. }
111.  

 

Hi guys I figured it out

it misses a line name of the variable in the pass of shader
sampler2D _MainTex;

 

I have the same problem, how did you fix it?

found it, need to add "sampler2D _MainTex;" somewhere. looked here:
https://answers.unity.com/questions/1261665/sprites-diffuse-shader-explained.html