fragment/vertex shader - distorting UVs and the Grabpass

Two things shown here, with a bit of fiddling needed to make it actually work, which shouldn't be a problem if you've looked at all the other bits.

In the appdata struct we're including the UVs now, with TEXCOORD0 attribute.

In the v2f function, we use TRANSFORM_TEX(v.uv, _MainTex) to get our uv coordinates into screenspace. Once we have these values, we use the sin function to distort both the X&Y, or U & V!, multiplying the input by some properties we defined earlier. This is passed to the Frag shader, which displays "a" texture.. 

What I've not mentioned is the GrabPass -it essentially takes a snapshot of the framebuffer and stores it as a texture. The Transparent Queue tag stops it from going crazy & rendering into infinity (think of when you record a tv and feed the recording into the tv..
The GrabPass is defined as a sampler2D, which must be labelled as _GrabTexture- after that it can be used in a tex2D function as usual.

 Shader "Dave/Unlit/vf_Mat"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _ScaleUVX("Scale UVX",Range(1,10))=1
        _ScaleUVY("Scale UVY",Range(1,10)) = 1

    }
    SubShader
    {
        Tags{"Queue"="Transparent"}//we push the object to render last (or later), so the grab pass doesn't recursively go to black
        GrabPass{}
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            
            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            sampler2D _GrabTexture;//must be spelt this way
            sampler2D _MainTex;
            float4 _MainTex_ST;
            float _ScaleUVX;
            float _ScaleUVY;


            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                o.uv.x = sin(o.uv.x*_ScaleUVX);
                o.uv.y = sin(o.uv.y*_ScaleUVY);
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                // sample the texture
                fixed4 col = tex2D(_GrabTexture, i.uv);
               
                return col;
            }
            ENDCG
        }
    }
}