fix PSNR metric to return the correct DB

Author: Cristy

magick/compare.c

@@ -1430,31 +1430,31 @@ static MagickBooleanType GetPeakSignalToNoiseRatio(const Image *image,
   if ((channel & RedChannel) != 0)
     {
       if (fabs(distortion[RedChannel]) >= MagickEpsilon)
-        distortion[RedChannel]=(-10.0*MagickLog10(distortion[RedChannel]))/
+        distortion[RedChannel]=(10.0*MagickLog10(distortion[RedChannel]))/
           48.1647;
     }
   if ((channel & GreenChannel) != 0)
     {
       if (fabs(distortion[GreenChannel]) >= MagickEpsilon)
-        distortion[GreenChannel]=(-10.0*MagickLog10(distortion[GreenChannel]))/
+        distortion[GreenChannel]=(10.0*MagickLog10(distortion[GreenChannel]))/
           48.1647;
     }
   if ((channel & BlueChannel) != 0)
     {
       if (fabs(distortion[BlueChannel]) >= MagickEpsilon)
-        distortion[BlueChannel]=(-10.0*MagickLog10(distortion[BlueChannel]))/
+        distortion[BlueChannel]=(10.0*MagickLog10(distortion[BlueChannel]))/
           48.1647;
     }
   if (((channel & OpacityChannel) != 0) && (image->matte != MagickFalse))
     {
       if (fabs(distortion[OpacityChannel]) >= MagickEpsilon)
-        distortion[OpacityChannel]=(-10.0*
+        distortion[OpacityChannel]=(10.0*
           MagickLog10(distortion[OpacityChannel]))/48.1647;
     }
   if (((channel & IndexChannel) != 0) && (image->colorspace == CMYKColorspace))
     {
       if (fabs(distortion[BlackChannel]) >= MagickEpsilon)
-        distortion[BlackChannel]=(-10.0*MagickLog10(distortion[BlackChannel]))/
+        distortion[BlackChannel]=(10.0*MagickLog10(distortion[BlackChannel]))/
           48.1647;
     }
   distortion[CompositeChannels]=0.0;

wand/compare.c

@@ -1227,11 +1227,21 @@ WandExport MagickBooleanType CompareImageCommand(ImageInfo *image_info,
     }
   if (fabs(distortion) > CompareEpsilon)
     similar=MagickFalse;
-  if (metric == NormalizedCrossCorrelationErrorMetric)
+  switch (metric)
+  {
+    case NormalizedCrossCorrelationErrorMetric:
     {
       distortion=1.0-distortion;
       similarity_metric=1.0-similarity_metric;
     }
+    case PeakSignalToNoiseRatioMetric:
+    {
+      distortion=fabs(distortion);
+      similarity_metric+=1.0;
+    }
+    default:
+      break;
+  }
   if (difference_image == (Image *) NULL)
     status=0;
   else