Why does my blur shader implementation produce this strange result?

Question

I tried to implement the blur shader shown here.

Instead of having a simple 2D texture I use Unity's GrabPass function to capture what's behind the plane to have a transparent effect. Then I apply this on a 3D plane:

struct v2f {
    float4 pos : SV_POSITION;
    float4 screenPos : TEXCOORD3;
    float3 uv;
};

v2f vert(appdata_full v)
{
    v2f o;
    o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
    o.screenPos = ComputeScreenPos(o.pos); 
    o.uv = UNITY_PROJ_COORD(o.screenPos);
    return o;
}

// In the frag part there is the shader blur computation but based on XYZ, with Z = 0

Then I apply the blur and this leads to this when I zoom in:

Is there a way to avoid this effect?

Answer 1

I concur with Nathan Reed's assessment that the issue is probably caused by spacing your blur kernel too widely.

Use additional rendering passes, downsample the image, decrease your blur radius, or some combination thereof.