Help with corner pin vertex shader

I'm new to shader programming and am having some problems. What I want to do is to be able to reposition all corners of a mesh one by one.

I have a shader that is working to some degree, but I'm having some problems. This might be more of a math question than a shader programming question, but perhaps someone knows a better way to achieve this.

Here is the shader that I am using right now, based on another thread found on this forum:

Code (CSharp):

1. Shader "Distort/Experiment"
2. {
3.     Properties{
4.         _MainTex("Base (RGB)", 2D) = "white" {}
5.     _DLx("Down Left x", Range(-20.0,20.0)) = 0
6.         _DLy("Down Left y", Range(-20.0,20.0)) = 0
7.         _DRx("Down Right x", Range(-2.0,2.0)) = 0
8.         _DRy("Down Right y", Range(-2.0,2.0)) = 0
9.         _TLx("Top Left x", Range(-2.0,2.0)) = 0
10.         _TLy("Top Left y", Range(-2.0,2.0)) = 0
11.         _TRx("Top Right x", Range(-2.0,2.0)) = 0
12.         _TRy("Top Right y", Range(-2.0,2.0)) = 0
13.     }
14.         SubShader{
15.         Pass{
16.         //ZTest Always Cull Off ZWrite Off
17.         Fog{ Mode off }
18.         CGPROGRAM
19. #pragma vertex vert
20. #pragma fragment frag
21. #pragma fragmentoption ARB_precision_hint_fastest
22. #include "UnityCG.cginc"
23.         uniform sampler2D _MainTex;
24.     struct v2f {
25.         float4 pos : SV_POSITION;
26.         float2 uv : TEXCOORD0;
27.     };
28.     float _DRx;
29.     float _DRy;
30.     float _DLx;
31.     float _DLy;
32.     float _TLx;
33.     float _TLy;
34.     float _TRx;
35.     float _TRy;
36.     v2f vert(appdata_img v)
37.     {
38.         v2f o;
39.         o.pos = UnityObjectToClipPos(v.vertex);
40.         o.uv = v.texcoord;
41.  
42.         o.pos.x += _DLx*o.uv.x*o.uv.y;
43.         o.pos.y += _DLy*o.uv.x*o.uv.y;
44.         o.pos.x += _DRx*(1 - o.uv.x)*o.uv.y;
45.         o.pos.y += _DRy*(1 - o.uv.x)*o.uv.y;
46.         o.pos.x += _TLx*o.uv.x*(1 - o.uv.y);
47.         o.pos.y += _TLy*o.uv.x*(1 - o.uv.y);
48.         o.pos.x += _TRx*(1 - o.uv.x)*(1 - o.uv.y);
49.         o.pos.y += _TRy*(1 - o.uv.x)*(1 - o.uv.y);
50.         return o;
51.     }
52.     float4 frag(v2f i) : SV_Target
53.     {
54.         return tex2D(_MainTex, i.uv);
55.     }
56.         ENDCG
57.     }
58.     }
59.         Fallback off
60. }

Here's an image that highlights the main problem:


I didn't notice this at first, but as you can see, this gives me a curved line in the texture, but I need the offset to be linear.

When all corners are in play, this is not as obvious, but the problem is still there:


I've been trying to figure out another approach to this, but I'm feeling a bit stuck. Any ideas?
Thanks,
Hannes

 

Hi @Spree_User
Did you ever find a solution to this?

 

The solution is perspective correct texture mapping rather than bilinear texture mapping, the later of which is what this shader appears to be producing.
http://www.reedbeta.com/blog/quadrilateral-interpolation-part-1/
http://www.reedbeta.com/blog/quadrilateral-interpolation-part-2/