Refraction Shader. Strange Artifacts

Hi! I'm trying to create a lens effect in front of a texture using vertex and fragment programs. The shader program is small and quite naive; I get some artifacts that I really can't understand why they are there or how to fix them;

Demo: http://johannes.gotlen.se/blog/2010/11/refraction/

Artifact-example:

// Per pixel refraction. // Uses a normal map and vertex normals to distort the image behind Shader "FX/Glass/RefractionMap" { Properties { _BumpAmt ("Glass Thickness", Range (-5,5)) = 0.2 _DiscRadius("Disc Radius", Float) = 1 _MainTexWidth ("Width", Float) = 512 _MainTexHeight("Height", Float) = 512 _MainTex ("Background", 2D) = "white" {} _BumpMap ("NormalInfo", 2D) = "bump" {} }

Category { // We must be transparent, so other objects are drawn before this one. Tags { "Queue"="Transparent" "RenderType"="Opaque" } SubShader {

     // This pass grabs the screen behind the object into a texture.
     // We can access the result in the next pass as _GrabTexture
     GrabPass {                          
         Name "BASE"
         Tags { "LightMode" = "Always" }
     }
     // Main pass: Take the texture grabbed above and use the bumpmap to perturb it
     // on to the screen
     Pass {
         Name "BASE"
         Tags { "LightMode" = "Always" }

CGPROGRAM #pragma exclude_renderers gles #pragma vertex vert #pragma fragment frag #pragma fragmentoption ARB_precision_hint_fastest #include "UnityCG.cginc"

struct appdata_t { half4 vertex : POSITION; half2 texcoord: TEXCOORD0; half4 normal: NORMAL; };

struct v2f { half4 vertex : POSITION; half3 normal : COLOR0; half2 uvmain : TEXCOORD0; }; const half _MainTexWidth; const half _MainTexHeight; const half _BumpAmt; const half4 _BumpMap_ST; const half4 _MainTex_ST; const half _DiscRadius; static const half2 pixelSize = 1h / half2(_MainTexWidth,_MainTexHeight);

v2f vert (appdata_t v) { v2f o; o.vertex = mul(UNITY_MATRIX_MVP, v.vertex); o.normal = mul(UNITY_MATRIX_IT_MV, v.normal); o.normal = o.normal * 0.5h + 0.5h; o.uvmain = TRANSFORM_TEX( v.texcoord, _MainTex ); return o; }

sampler2D _BumpMap; sampler2D _MainTex; static const half2 poisson[11] = { half2(-0.326212h, -0.40581h), half2(-0.840144h, -0.07358h), half2(-0.695914h, 0.457137h), half2(-0.203345h, 0.620716h), half2(0.96234h, -0.194983h), half2(0.473434h, -0.480026h), half2(0.519456h, 0.767022h), half2(0.185461h, -0.893124h), half2(0.507431h, 0.064425h), half2(0.89642h, 0.412458h), half2(-0.32194h, -0.932615h) };

half4 SiGrowablePoissonDisc12FilterRGB(sampler2D tSource, half2 texCoord, half2 pixelSize, half discRadius) {

 // Center tap
 half4 cOut = tex2D(tSource, texCoord);
 for(short tap = 0; tap </pre>

Thanks!