Using the Material Expression custom node, copy and paste this code:

int tIndex = 14;
UV = ViewportUVToSceneTextureUV(UV,14);
int x;
int z;
float SumR = 1e+2;
float SumTotal;
float normalizedRatio = float((Size + 1) * (Size + 1));
float2 texelScale = float2(1.0/ScreenResolution.y,1.0/ScreenResolution.x);
float3 singleA = float3(0.0,0.0,0.0);
float3 singleB = singleA;
float3 singleC = singleA;
float3 singleD = singleA;
float3 DoubleA = singleA;
float3 DoubleB = singleA;
float3 DoubleC = singleA;
float3 DoubleD = singleA;
float3 sceneCol;
float4 sceneText = SceneTextureLookup(UV.xy,tIndex,false);
float4 Color;
for (z = -Size; z <= 0; ++z) {
for (x = -Size; x <= 0; ++x) {
sceneCol = SceneTextureLookup(UV + float2(x,z) * texelScale,tIndex,false).rgb;
singleA += sceneCol;
DoubleA += sceneCol * sceneCol;
}
}
for (z = -Size; z <= 0; ++z) {
for (x = 0; x <= Size; ++x) {
sceneCol = SceneTextureLookup(UV + float2(x,z) * texelScale,tIndex,false).rgb;
singleB += sceneCol;
DoubleB += sceneCol * sceneCol;
}
}
for (z = 0; z <= Size; ++z) {
for (x = 0; x <= Size; ++x) {
sceneCol = SceneTextureLookup(UV + float2(x,z) * texelScale,tIndex,false).rgb;
singleC += sceneCol;
DoubleC += sceneCol * sceneCol;
}
}
for (z = 0; z <= Size; ++z) {
for (x = -Size; x <= 0; ++x) {
sceneCol = SceneTextureLookup(UV + float2(x,z) * texelScale,tIndex,false).rgb;
singleD += sceneCol;
DoubleD += sceneCol * sceneCol;
}
}
singleA /= normalizedRatio;
singleB /= normalizedRatio;
singleC /= normalizedRatio;
singleD /= normalizedRatio;
DoubleA = abs(DoubleA / normalizedRatio - singleA * singleA);
DoubleB = abs(DoubleB / normalizedRatio - singleB * singleB);
DoubleC = abs(DoubleC / normalizedRatio - singleC * singleC);
DoubleD = abs(DoubleD / normalizedRatio - singleD * singleD);
SumTotal = DoubleA.r + DoubleA.g + DoubleA.b;
if (SumTotal < SumR) {
SumR = SumTotal;
Color = float4(singleA, sceneText.a);
}
SumTotal = DoubleB.r + DoubleB.g + DoubleB.b;
if (SumTotal < SumR) {
SumR = SumTotal;
Color = float4(singleB, sceneText.a);
}
SumTotal = DoubleC.r + DoubleC.g + DoubleC.b;
if (SumTotal < SumR) {
SumR = SumTotal;
Color = float4(singleC, sceneText.a);
}
SumTotal = DoubleD.r + DoubleD.g + DoubleD.b;
if (SumTotal < SumR) {
SumR = SumTotal;
Color = float4(singleD, sceneText.a);
}
return Color;