I'm trying to write a UserInterface blur material to use on the contents of a RetainerBox. However it seems like GetPostProcessInputSize(0) and View.ViewSizeAndSceneTexelSize don't work.

What should I use?

Here's my shader:

Code:
float4 res = 0;
// !!Doesn't work!!
float2 invSize = GetPostProcessInputSize(0).zw;

// !!Doesn't work either!!
float2 invSize = View.ViewSizeAndSceneTexelSize.xy / View.RenderTargetSize;


float weights[] =
{
  0.01, 0.02, 0.04, 0.02, 0.01,
  0.02, 0.04, 0.08, 0.04, 0.02,
  0.04, 0.08, 0.16, 0.08, 0.04,
  0.02, 0.04, 0.08, 0.04, 0.02,
  0.01, 0.02, 0.04, 0.02, 0.01
};
float offsets[] = { -2, -1, 0, 1, 2 };

UV *= 0.5;
for (int i = 0; i < 5; ++i)
{
  float v = UV.y + offsets[i] * invSize.y;
  int temp = i * 5;
  for (int j = 0; j < 5; ++j)
  {
    float u = UV.x + offsets[j] * invSize.x;
    float2 uvShifted = UV + float2(u, v);
    float weight = weights[temp + j];
    float4 tex = Texture2DSample(Texture, TexSampler, uvShifted);
    res += tex * weight;
  }
}

return res;