diff --git a/src/nvimage/CMakeLists.txt b/src/nvimage/CMakeLists.txt
index 9db199c..0d180f1 100644
--- a/src/nvimage/CMakeLists.txt
+++ b/src/nvimage/CMakeLists.txt
@@ -7,6 +7,7 @@ SET(IMAGE_SRCS
     DirectDrawSurface.h DirectDrawSurface.cpp
     Filter.h Filter.cpp
     FloatImage.h FloatImage.cpp
+	ErrorMetric.h ErrorMetric.cpp
     Image.h Image.cpp
     ImageIO.h ImageIO.cpp
     NormalMap.h NormalMap.cpp
diff --git a/src/nvimage/Filter.cpp b/src/nvimage/Filter.cpp
index 21a2609..6bcfded 100644
--- a/src/nvimage/Filter.cpp
+++ b/src/nvimage/Filter.cpp
@@ -265,6 +265,19 @@ void KaiserFilter::setParameters(float alpha, float stretch)
     this->stretch = stretch;
 }
 
+GaussianFilter::GaussianFilter(float w) : Filter(w) { setParameters(1); }
+
+float GaussianFilter::evaluate(float x) const
+{
+    // variance = sigma^2
+    return (1.0f / sqrtf(2 * PI * variance)) * expf(-x*x / (2 * variance));
+}
+
+void GaussianFilter::setParameters(float variance)
+{
+    this->variance = variance;
+}
+
 
 
 /// Ctor.
diff --git a/src/nvimage/Filter.h b/src/nvimage/Filter.h
index 407839b..bb9ca7d 100644
--- a/src/nvimage/Filter.h
+++ b/src/nvimage/Filter.h
@@ -116,6 +116,19 @@ namespace nv
         float stretch;
     };
 
+    // Gaussian filter.
+    class GaussianFilter : public Filter
+    {
+    public:
+        GaussianFilter(float w);
+        virtual float evaluate(float x) const;
+
+        void setParameters(float variance);
+
+    private:
+        float variance;
+    };
+
 
 
     /// A 1D kernel. Used to precompute filter weights.
diff --git a/src/nvmath/Color.h b/src/nvmath/Color.h
index 02c3b70..f42a0a1 100644
--- a/src/nvmath/Color.h
+++ b/src/nvmath/Color.h
@@ -178,10 +178,10 @@ namespace nv
 
     inline float perceptualColorDistance(Vector3::Arg c0, Vector3::Arg c1)
     {
-        float rmean = (c0.r + c1.r) * 0.5f;
-        float r = c1.r - c0.r;
-        float g = c1.g - c0.g;
-        float b = c1.b - c0.b;
+        float rmean = (c0.x + c1.x) * 0.5f;
+        float r = c1.x - c0.x;
+        float g = c1.y - c0.y;
+        float b = c1.z - c0.z;
         return sqrtf((2 + rmean)*r*r + 4*g*g + (3 - rmean)*b*b);
     }