Grabpass is different from a rendertexture in that it doesn’t require to re-render the scene. It blits the contents of the framebuffer to a temporary texture, which isn’t free but still much cheaper than rendering the scene multiple times.
Sure, you have more control with render to texture approaches, but for simple stuff grabpass should be fine.
I slapped toghether the refraction shader with gaussian blur. It’s a bit hackish, but gets the job done.
The kernel is hard coded for efficiency. If you know an efficient way for a dynamic kernel, let me know 
Depending on geometry it could be more efficient to do a single pass with two dimensional gaussian, at least with a small kernel. Not sure though. I did it splitted in horizontal+vertical as everyone else and I’m too lazy to try.
Third pass is just for distortion and tint color.
Shader "Custom/SimpleGrabPassBlur" {
Properties {
_Color ("Main Color", Color) = (1,1,1,1)
_BumpAmt ("Distortion", Range (0,128)) = 10
_MainTex ("Tint Color (RGB)", 2D) = "white" {}
_BumpMap ("Normalmap", 2D) = "bump" {}
_Size ("Size", Range(0, 20)) = 1
}
Category {
// We must be transparent, so other objects are drawn before this one.
Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Opaque" }
SubShader {
// Horizontal blur
GrabPass {
Tags { "LightMode" = "Always" }
}
Pass {
Tags { "LightMode" = "Always" }
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#pragma fragmentoption ARB_precision_hint_fastest
#include "UnityCG.cginc"
struct appdata_t {
float4 vertex : POSITION;
float2 texcoord: TEXCOORD0;
};
struct v2f {
float4 vertex : POSITION;
float4 uvgrab : TEXCOORD0;
};
v2f vert (appdata_t v) {
v2f o;
o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
#if UNITY_UV_STARTS_AT_TOP
float scale = -1.0;
#else
float scale = 1.0;
#endif
o.uvgrab.xy = (float2(o.vertex.x, o.vertex.y*scale) + o.vertex.w) * 0.5;
o.uvgrab.zw = o.vertex.zw;
return o;
}
sampler2D _GrabTexture;
float4 _GrabTexture_TexelSize;
float _Size;
half4 frag( v2f i ) : COLOR {
// half4 col = tex2Dproj( _GrabTexture, UNITY_PROJ_COORD(i.uvgrab));
// return col;
half4 sum = half4(0,0,0,0);
#define GRABPIXEL(weight,kernelx) tex2Dproj( _GrabTexture, UNITY_PROJ_COORD(float4(i.uvgrab.x + _GrabTexture_TexelSize.x * kernelx*_Size, i.uvgrab.y, i.uvgrab.z, i.uvgrab.w))) * weight
sum += GRABPIXEL(0.05, -4.0);
sum += GRABPIXEL(0.09, -3.0);
sum += GRABPIXEL(0.12, -2.0);
sum += GRABPIXEL(0.15, -1.0);
sum += GRABPIXEL(0.18, 0.0);
sum += GRABPIXEL(0.15, +1.0);
sum += GRABPIXEL(0.12, +2.0);
sum += GRABPIXEL(0.09, +3.0);
sum += GRABPIXEL(0.05, +4.0);
return sum;
}
ENDCG
}
// Vertical blur
GrabPass {
Tags { "LightMode" = "Always" }
}
Pass {
Tags { "LightMode" = "Always" }
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#pragma fragmentoption ARB_precision_hint_fastest
#include "UnityCG.cginc"
struct appdata_t {
float4 vertex : POSITION;
float2 texcoord: TEXCOORD0;
};
struct v2f {
float4 vertex : POSITION;
float4 uvgrab : TEXCOORD0;
};
v2f vert (appdata_t v) {
v2f o;
o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
#if UNITY_UV_STARTS_AT_TOP
float scale = -1.0;
#else
float scale = 1.0;
#endif
o.uvgrab.xy = (float2(o.vertex.x, o.vertex.y*scale) + o.vertex.w) * 0.5;
o.uvgrab.zw = o.vertex.zw;
return o;
}
sampler2D _GrabTexture;
float4 _GrabTexture_TexelSize;
float _Size;
half4 frag( v2f i ) : COLOR {
// half4 col = tex2Dproj( _GrabTexture, UNITY_PROJ_COORD(i.uvgrab));
// return col;
half4 sum = half4(0,0,0,0);
#define GRABPIXEL(weight,kernely) tex2Dproj( _GrabTexture, UNITY_PROJ_COORD(float4(i.uvgrab.x, i.uvgrab.y + _GrabTexture_TexelSize.y * kernely*_Size, i.uvgrab.z, i.uvgrab.w))) * weight
//G(X) = (1/(sqrt(2*PI*deviation*deviation))) * exp(-(x*x / (2*deviation*deviation)))
sum += GRABPIXEL(0.05, -4.0);
sum += GRABPIXEL(0.09, -3.0);
sum += GRABPIXEL(0.12, -2.0);
sum += GRABPIXEL(0.15, -1.0);
sum += GRABPIXEL(0.18, 0.0);
sum += GRABPIXEL(0.15, +1.0);
sum += GRABPIXEL(0.12, +2.0);
sum += GRABPIXEL(0.09, +3.0);
sum += GRABPIXEL(0.05, +4.0);
return sum;
}
ENDCG
}
// Distortion
GrabPass {
Tags { "LightMode" = "Always" }
}
Pass {
Tags { "LightMode" = "Always" }
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#pragma fragmentoption ARB_precision_hint_fastest
#include "UnityCG.cginc"
struct appdata_t {
float4 vertex : POSITION;
float2 texcoord: TEXCOORD0;
};
struct v2f {
float4 vertex : POSITION;
float4 uvgrab : TEXCOORD0;
float2 uvbump : TEXCOORD1;
float2 uvmain : TEXCOORD2;
};
float _BumpAmt;
float4 _BumpMap_ST;
float4 _MainTex_ST;
v2f vert (appdata_t v) {
v2f o;
o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
#if UNITY_UV_STARTS_AT_TOP
float scale = -1.0;
#else
float scale = 1.0;
#endif
o.uvgrab.xy = (float2(o.vertex.x, o.vertex.y*scale) + o.vertex.w) * 0.5;
o.uvgrab.zw = o.vertex.zw;
o.uvbump = TRANSFORM_TEX( v.texcoord, _BumpMap );
o.uvmain = TRANSFORM_TEX( v.texcoord, _MainTex );
return o;
}
fixed4 _Color;
sampler2D _GrabTexture;
float4 _GrabTexture_TexelSize;
sampler2D _BumpMap;
sampler2D _MainTex;
half4 frag( v2f i ) : COLOR {
// calculate perturbed coordinates
half2 bump = UnpackNormal(tex2D( _BumpMap, i.uvbump )).rg; // we could optimize this by just reading the x y without reconstructing the Z
float2 offset = bump * _BumpAmt * _GrabTexture_TexelSize.xy;
i.uvgrab.xy = offset * i.uvgrab.z + i.uvgrab.xy;
half4 col = tex2Dproj( _GrabTexture, UNITY_PROJ_COORD(i.uvgrab));
half4 tint = tex2D( _MainTex, i.uvmain ) * _Color;
return col * tint;
}
ENDCG
}
}
}
}
Here’s how it looks.
Cheers.
