参考链接：
https://blog.csdn.net/akof1314/article/details/50428200
1.Mask.shader
1 // Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)
2
3 Shader "Custom/Mask"
4 {
5 Properties
6 {
7 [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
8 _Color ("Tint", Color) = (1,1,1,1)
9
10 _StencilComp ("Stencil Comparison", Float) = 8
11 _Stencil ("Stencil ID", Float) = 0
12 _StencilOp ("Stencil Operation", Float) = 0
13 _StencilWriteMask ("Stencil Write Mask", Float) = 255
14 _StencilReadMask ("Stencil Read Mask", Float) = 255
15
16 _ColorMask ("Color Mask", Float) = 15
17
18 [Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
19
20 _MaskTex ("Mask Texture", 2D) = "white" {}
21 }
22
23 SubShader
24 {
25 Tags
26 {
27 "Queue"="Transparent"
28 "IgnoreProjector"="True"
29 "RenderType"="Transparent"
30 "PreviewType"="Plane"
31 "CanUseSpriteAtlas"="True"
32 }
33
34 Stencil
35 {
36 Ref [_Stencil]
37 Comp [_StencilComp]
38 Pass [_StencilOp]
39 ReadMask [_StencilReadMask]
40 WriteMask [_StencilWriteMask]
41 }
42
43 Cull Off
44 Lighting Off
45 ZWrite Off
46 ZTest [unity_GUIZTestMode]
47 Blend SrcAlpha OneMinusSrcAlpha
48 ColorMask [_ColorMask]
49
50 Pass
51 {
52 Name "Default"
53 CGPROGRAM
54 #pragma vertex vert
55 #pragma fragment frag
56 #pragma target 2.0
57
58 #include "UnityCG.cginc"
59 #include "UnityUI.cginc"
60
61 #pragma multi_compile __ UNITY_UI_CLIP_RECT
62 #pragma multi_compile __ UNITY_UI_ALPHACLIP
63
64 struct appdata_t
65 {
66 float4 vertex : POSITION;
67 float4 color : COLOR;
68 float2 texcoord : TEXCOORD0;
69 UNITY_VERTEX_INPUT_INSTANCE_ID
70 };
71
72 struct v2f
73 {
74 float4 vertex : SV_POSITION;
75 fixed4 color : COLOR;
76 float2 texcoord : TEXCOORD0;
77 float4 worldPosition : TEXCOORD1;
78 UNITY_VERTEX_OUTPUT_STEREO
79 };
80
81 sampler2D _MainTex;
82 fixed4 _Color;
83 fixed4 _TextureSampleAdd;
84 float4 _ClipRect;
85 float4 _MainTex_ST;
86
87 sampler2D _MaskTex;
88
89 v2f vert(appdata_t v)
90 {
91 v2f OUT;
92 UNITY_SETUP_INSTANCE_ID(v);
93 UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
94 OUT.worldPosition = v.vertex;
95 OUT.vertex = UnityObjectToClipPos(OUT.worldPosition);
96
97 OUT.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
98
99 OUT.color = v.color * _Color;
100 return OUT;
101 }
102
103 fixed4 frag(v2f IN) : SV_Target
104 {
105 half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;
106 color.a *= tex2D(_MaskTex, IN.texcoord).a;
107
108 #ifdef UNITY_UI_CLIP_RECT
109 color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
110 #endif
111
112 #ifdef UNITY_UI_ALPHACLIP
113 clip (color.a - 0.001);
114 #endif
115
116 return color;
117 }
118 ENDCG
119 }
120 }
121 }
核心代码：
1 color.a *= tex2D(_MaskTex, IN.texcoord).a;
即遮罩图alpha=0的地方不会显示
遮罩图和效果图(遮罩图白色的地方实际上是透明的)：
2.UV问题
上面的结果表面上看是没什么问题的，但如果把图片打进图集就会发现不对了。原因是，shader里面uv坐标不再是单个图片的坐标了，而是图集里面该图片的坐标，使得shader计算错误，最终渲染错乱。解决方法是，将单个图片的纹理坐标保存到uv1中。
SetVertUV1.cs
1 using UnityEngine;
2 using UnityEngine.UI;
3
4 public class SetVertUV1 : BaseMeshEffect
5 {
6 public override void ModifyMesh(VertexHelper vh)
7 {
8 UIVertex uiVertex = new UIVertex();
9 for (int i = 0; i < vh.currentVertCount; i++)
10 {
11 vh.PopulateUIVertex(ref uiVertex, i);
12 int uvX = i >> 1;
13 int uvY = uvX ^ (i & 1);
14 uiVertex.uv1 = new Vector2(uvX, uvY);
15 vh.SetUIVertex(uiVertex, i);
16 }
17 }
18 }
即将4个顶点的uv1依次设置为(0,0),(0,1),(1,1),(1,0)
MaskUV1.shader
1 // Unity built-in shader source. Copyright (c) 2016 Unity Technologies. MIT license (see license.txt)
2
3 Shader "Custom/MaskUV1"
4 {
5 Properties
6 {
7 [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
8 _Color ("Tint", Color) = (1,1,1,1)
9
10 _StencilComp ("Stencil Comparison", Float) = 8
11 _Stencil ("Stencil ID", Float) = 0
12 _StencilOp ("Stencil Operation", Float) = 0
13 _StencilWriteMask ("Stencil Write Mask", Float) = 255
14 _StencilReadMask ("Stencil Read Mask", Float) = 255
15
16 _ColorMask ("Color Mask", Float) = 15
17
18 [Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
19
20 _MaskTex ("Mask Texture", 2D) = "white" {}
21 }
22
23 SubShader
24 {
25 Tags
26 {
27 "Queue"="Transparent"
28 "IgnoreProjector"="True"
29 "RenderType"="Transparent"
30 "PreviewType"="Plane"
31 "CanUseSpriteAtlas"="True"
32 }
33
34 Stencil
35 {
36 Ref [_Stencil]
37 Comp [_StencilComp]
38 Pass [_StencilOp]
39 ReadMask [_StencilReadMask]
40 WriteMask [_StencilWriteMask]
41 }
42
43 Cull Off
44 Lighting Off
45 ZWrite Off
46 ZTest [unity_GUIZTestMode]
47 Blend SrcAlpha OneMinusSrcAlpha
48 ColorMask [_ColorMask]
49
50 Pass
51 {
52 Name "Default"
53 CGPROGRAM
54 #pragma vertex vert
55 #pragma fragment frag
56 #pragma target 2.0
57
58 #include "UnityCG.cginc"
59 #include "UnityUI.cginc"
60
61 #pragma multi_compile __ UNITY_UI_CLIP_RECT
62 #pragma multi_compile __ UNITY_UI_ALPHACLIP
63
64 struct appdata_t
65 {
66 float4 vertex : POSITION;
67 float4 color : COLOR;
68 float2 texcoord : TEXCOORD0;
69 float2 texcoord1 : TEXCOORD1;
70 UNITY_VERTEX_INPUT_INSTANCE_ID
71 };
72
73 struct v2f
74 {
75 float4 vertex : SV_POSITION;
76 fixed4 color : COLOR;
77 float2 texcoord : TEXCOORD0;
78 float4 worldPosition : TEXCOORD1;
79 float2 texcoord1 : TEXCOORD2;
80 UNITY_VERTEX_OUTPUT_STEREO
81 };
82
83 sampler2D _MainTex;
84 fixed4 _Color;
85 fixed4 _TextureSampleAdd;
86 float4 _ClipRect;
87 float4 _MainTex_ST;
88
89 sampler2D _MaskTex;
90
91 v2f vert(appdata_t v)
92 {
93 v2f OUT;
94 UNITY_SETUP_INSTANCE_ID(v);
95 UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(OUT);
96 OUT.worldPosition = v.vertex;
97 OUT.vertex = UnityObjectToClipPos(OUT.worldPosition);
98
99 OUT.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
100
101 OUT.color = v.color * _Color;
102 OUT.texcoord1 = v.texcoord1;
103 return OUT;
104 }
105
106 fixed4 frag(v2f IN) : SV_Target
107 {
108 half4 color = (tex2D(_MainTex, IN.texcoord) + _TextureSampleAdd) * IN.color;
109 color.a *= tex2D(_MaskTex, IN.texcoord1).a;
110
111 #ifdef UNITY_UI_CLIP_RECT
112 color.a *= UnityGet2DClipping(IN.worldPosition.xy, _ClipRect);
113 #endif
114
115 #ifdef UNITY_UI_ALPHACLIP
116 clip (color.a - 0.001);
117 #endif
118
119 return color;
120 }
121 ENDCG
122 }
123 }
124 }