Help with corner pin vertex shader

Unity Engine
1

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:

Shader "Distort/Experiment"
{
    Properties{
        _MainTex("Base (RGB)", 2D) = "white" {}
    _DLx("Down Left x", Range(-20.0,20.0)) = 0
        _DLy("Down Left y", Range(-20.0,20.0)) = 0
        _DRx("Down Right x", Range(-2.0,2.0)) = 0
        _DRy("Down Right y", Range(-2.0,2.0)) = 0
        _TLx("Top Left x", Range(-2.0,2.0)) = 0
        _TLy("Top Left y", Range(-2.0,2.0)) = 0
        _TRx("Top Right x", Range(-2.0,2.0)) = 0
        _TRy("Top Right y", Range(-2.0,2.0)) = 0
    }
        SubShader{
        Pass{
        //ZTest Always Cull Off ZWrite Off
        Fog{ Mode off }
        CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#pragma fragmentoption ARB_precision_hint_fastest
#include "UnityCG.cginc"
        uniform sampler2D _MainTex;
    struct v2f {
        float4 pos : SV_POSITION;
        float2 uv : TEXCOORD0;
    };
    float _DRx;
    float _DRy;
    float _DLx;
    float _DLy;
    float _TLx;
    float _TLy;
    float _TRx;
    float _TRy;
    v2f vert(appdata_img v)
    {
        v2f o;
        o.pos = UnityObjectToClipPos(v.vertex);
        o.uv = v.texcoord;
  
        o.pos.x += _DLx*o.uv.x*o.uv.y;
        o.pos.y += _DLy*o.uv.x*o.uv.y;
        o.pos.x += _DRx*(1 - o.uv.x)*o.uv.y;
        o.pos.y += _DRy*(1 - o.uv.x)*o.uv.y;
        o.pos.x += _TLx*o.uv.x*(1 - o.uv.y);
        o.pos.y += _TLy*o.uv.x*(1 - o.uv.y);
        o.pos.x += _TRx*(1 - o.uv.x)*(1 - o.uv.y);
        o.pos.y += _TRy*(1 - o.uv.x)*(1 - o.uv.y);
        return o;
    }
    float4 frag(v2f i) : SV_Target
    {
        return tex2D(_MainTex, i.uv);
    }
        ENDCG
    }
    }
        Fallback off
}

Here’s an image that highlights the main problem:

1468×567 288 KB

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:

1446×700 223 KB

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

2

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

3

The solution is perspective correct texture mapping rather than bilinear texture mapping, the later of which is what this shader appears to be producing.