Index: trunk/psLib/src/imageops/psImageCovariance.c
===================================================================
--- trunk/psLib/src/imageops/psImageCovariance.c	(revision 28151)
+++ trunk/psLib/src/imageops/psImageCovariance.c	(revision 28152)
@@ -576,4 +576,5 @@
         for (int x = xMinOut; x <= xMaxOut; x++) {
             float xIn = x * scale + 0.5 - xMinIn + 1; // Position on input (not the kernel)
+
             double value;                                     // Value on output
             if (!psImageInterpolate(&value, NULL, NULL, xIn, yIn, interp)) {
@@ -586,4 +587,10 @@
     psFree(interp);
 
+    // Problem: the interpolation has introduced power into the covariance matrix that shouldn't be there.  We
+    // can't scale the sum of the matrix because that would throw off the central value and affect the noise
+    // calculation for this image.  But we can't scale just by the central value because that would throw off
+    // the noise calculation for convolutions of this image.  We choose to scale by the sum of the non-central
+    // elements.  This is almost having the best of both worlds.
+
     double inSum = 0.0;                 // Sum of covariance
     for (int y = yMinIn; y <= yMaxIn; y++) {
@@ -593,8 +600,10 @@
     }
 
-    float norm = inSum / PS_SQR(scale) / outSum; // Renormalisation (to remove errors in interp.)
+    float norm = (inSum - in->kernel[0][0]) / (outSum - out->kernel[0][0]) / PS_SQR(scale); // Renormalisation
     for (int y = yMinOut; y <= yMaxOut; y++) {
         for (int x = xMinOut; x <= xMaxOut; x++) {
-            out->kernel[y][x] *= norm;
+            if (x != 0 && y != 0) {
+                out->kernel[y][x] *= norm;
+            }
         }
     }