How to overlay a texture on a polygon not based on UVs?

Looking for a nudge in the right direction.

First of all, I'm not sure what graphics/shader terms I should be using to describe what I'm trying to do. What I would like to is overlay a texture onto an arbitrarily shaped UI polygon that has vertices that will change positions at runtime without distorting the texture. So it would have to be done without UVs.

I'm not sure if that is very clear but here is an illustration of what I would like to accomplish:


On the right is the texture, on the left is the polygon that I would like the texture applied to.

If I can answer any questions that would provide further clarification, please ask. Thank you

 

Alrighty, so the term that I was looking for is "screen space".

I've modified the Default UI shader to apply the texture based on the screen space position of the pixels. This works with tileable textures.



The problem as you can see in the above screenshot is the texture isn't moving with the object. How can I add/subtract the object's position from IN.screenPos in the shader below to compensate for that?

Code (CSharp):

1.  
2. Shader "UI/ScreenSpaceTexture"
3. {
4.     Properties
5.     {
6.         [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
7.         _Color ("Tint", Color) = (1,1,1,1)
8.        
9.         _StencilComp ("Stencil Comparison", Float) = 8
10.         _Stencil ("Stencil ID", Float) = 0
11.         _StencilOp ("Stencil Operation", Float) = 0
12.         _StencilWriteMask ("Stencil Write Mask", Float) = 255
13.         _StencilReadMask ("Stencil Read Mask", Float) = 255
14.  
15.         _ColorMask ("Color Mask", Float) = 15
16.        
17.         _Tiling ("Tiling", Float) = 0
18.     }
19.  
20.     SubShader
21.     {
22.         Tags
23.         {
24.             "Queue"="Transparent"
25.             "IgnoreProjector"="True"
26.             "RenderType"="Transparent"
27.             "PreviewType"="Plane"
28.             "CanUseSpriteAtlas"="True"
29.         }
30.        
31.         Stencil
32.         {
33.             Ref [_Stencil]
34.             Comp [_StencilComp]
35.             Pass [_StencilOp]
36.             ReadMask [_StencilReadMask]
37.             WriteMask [_StencilWriteMask]
38.         }
39.  
40.         Cull Off
41.         Lighting Off
42.         ZWrite Off
43.         ZTest [unity_GUIZTestMode]
44.         Blend SrcAlpha OneMinusSrcAlpha
45.         ColorMask [_ColorMask]
46.  
47.         Pass
48.         {
49.         CGPROGRAM
50.             #pragma vertex vert
51.             #pragma fragment frag
52.             #include "UnityCG.cginc"
53.            
54.             struct appdata_t
55.             {
56.                 float4 vertex   : POSITION;
57.                 float4 color    : COLOR;
58.                 float2 texcoord : TEXCOORD0;
59.             };
60.  
61.             struct v2f
62.             {
63.                 float4 vertex   : SV_POSITION;
64.                 fixed4 color    : COLOR;
65.                 half2 screenPos : TEXCOORD1;
66.             };
67.            
68.             fixed4 _Color;
69.             fixed _Tiling;
70.  
71.             v2f vert(appdata_t IN)
72.             {
73.                 v2f OUT;
74.                 OUT.vertex = UnityObjectToClipPos(IN.vertex);
75.                 OUT.screenPos = ComputeScreenPos(IN.vertex);
76. #ifdef UNITY_HALF_TEXEL_OFFSET
77.                 OUT.vertex.xy += (_ScreenParams.zw-1.0)*float2(-1,1);
78. #endif
79.                 OUT.color = IN.color * _Color;
80.                 return OUT;
81.             }
82.              
83.             sampler2D _MainTex;
84.  
85.             fixed4 frag(v2f IN) : SV_Target
86.             {
87.                 half4 color = tex2D(_MainTex, IN.screenPos * _Tiling * .001) * IN.color;
88.                 clip (color.a - 0.01);
89.                 return color;
90.             }
91.         ENDCG
92.         }
93.     }
94. }
95.  

If there is anything I can do to improve this modification, please let me know.