float2 blurVec = (previous.xy - projPos.xy) * 0.5f;
float2 v = blurVec / NumberOfSamples;

half4 colour;
for(int i = 0; i < NumberOfSamples; ++i)
{
  float2 uv = input.uv + (v * i);
  colour += tex2D(_MainTex, uv);
}

colour /= NumberOfSamples;