Unwanted Distortion on Screen Space Shader

Unity Engine
1

Hi,

I’m trying to use a shader to render a repeating texture in a screen-space way.
See the screenshot for what I’m trying to achieve.

1293848--59549--$distortedcube.png

But as you can see, there is some distortion, the pixelly texture is being bent.
This is using the standard Unity cube model.

  • Is there a way I can fix this?
  • What am I doing wrong?

The shader I am using, for reference:

Shader "Ben/ScreenSpaceBasic" {
  Properties {
    _MainTex ("Texture", 2D) = "white" { }
    _Color ("Main Color", Color) = (1,1,1,0.5)
  }
  SubShader {
    Tags { "RenderType" = "Opaque" }
    Cull Off
    Lighting Off
    
    Pass {
      CGPROGRAM
      #pragma vertex vert
      #pragma fragment frag
      
      #include "UnityCG.cginc"

      uniform sampler2D _MainTex;
      uniform float4    _MainTex_ST;
      uniform float4    _Color;

      struct VertexInput {
        float4 vertex : POSITION;
        float3 normal : NORMAL;
        float4 texcoord : TEXCOORD0;
      };
      
      struct v2f {
        float4 pos : SV_POSITION;
        float2 uv  : TEXCOORD0;
        float2 uv_screen : TEXCOORD1;
      };

      v2f vert(VertexInput v) {
        v2f o;
        o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
        o.uv  = TRANSFORM_TEX(v.texcoord, _MainTex);

        float4 clipSpace = o.pos;
        clipSpace.xy    /= clipSpace.w;
        clipSpace.xy     = 0.5*(clipSpace.xy+1.0);
        o.uv_screen      = clipSpace.xy;

        o.uv_screen *= _MainTex_ST.xy + _MainTex_ST.zw;

        return o;
      }

      half4 frag(v2f i) : COLOR {
        half4 texcol = tex2D(_MainTex, i.uv_screen);
        return texcol * _Color;
      }
      ENDCG
    }
  }
  Fallback "VertexLit"
}
2

The divide by w needs to be in the pixel shader. Either move the code there or use tex2Dproj.