From 855683983c5cfa7e155f7b2224dcee9ccccc1a2a Mon Sep 17 00:00:00 2001
From: Pavel Krajcevski <pavel@cs.unc.edu>
Date: Fri, 11 Oct 2013 13:06:29 -0400
Subject: [PATCH] SSIM measurement is usually the smallest SSIm index, so
 report that instead of MSSIM, but if the user wants SSIM we can do that, too.

---
 Base/include/Image.h    |  2 +-
 Base/src/Image.cpp      | 22 ++++++++++++++++++----
 Base/test/TestImage.cpp |  5 ++++-
 3 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/Base/include/Image.h b/Base/include/Image.h
index cd81d9b..2550481 100644
--- a/Base/include/Image.h
+++ b/Base/include/Image.h
@@ -106,7 +106,7 @@ namespace FasTC {
     }
 
     double ComputePSNR(Image<PixelType> *other);
-    double ComputeMSSIM(Image<PixelType> *other);
+    double ComputeSSIM(Image<PixelType> *other, double *mssim = 0);
     
     // Function to allow derived classes to populate the pixel array.
     // This may involve decompressing a compressed image or otherwise
diff --git a/Base/src/Image.cpp b/Base/src/Image.cpp
index de8f2ac..07c6a9f 100644
--- a/Base/src/Image.cpp
+++ b/Base/src/Image.cpp
@@ -238,7 +238,7 @@ static Image<PixelType> FilterValid(const Image<PixelType> &img, uint32 size, do
 }
 
 template<typename PixelType>
-double Image<PixelType>::ComputeSSIM(Image<PixelType> *other) {
+double Image<PixelType>::ComputeSSIM(Image<PixelType> *other, double *mssim) {
   if(!other) {
     return -1.0;
   }
@@ -361,7 +361,11 @@ double Image<PixelType>::ComputeSSIM(Image<PixelType> *other) {
     }
   }
 
-  double mssim = 0.0;
+  double minSSIM = 1.0;
+  if(mssim) {
+    *mssim = 0.0;
+  }
+
   for(uint32 j = 0; j < h; j++) {
     for(uint32 i = 0; i < w; i++) {
       double m1sq = static_cast<float>(mu1_sq(i, j));
@@ -372,13 +376,23 @@ double Image<PixelType>::ComputeSSIM(Image<PixelType> *other) {
       double s2sq = static_cast<float>(sigma2_sq(i, j));
       double s1s2 = static_cast<float>(sigma12(i, j));
 
-      mssim +=
+      double ssim =
         ((2.0 * m1m2 + C1) * (2.0 * s1s2 + C2)) /
         ((m1sq + m2sq + C1) * (s1sq + s2sq + C2));
+
+      if(mssim) {
+        *mssim += ssim;
+      }
+
+      minSSIM = ::std::min(ssim, minSSIM);
     }
   }
 
-  return mssim / static_cast<double>(w * h);
+  if(mssim) {
+    *mssim /= static_cast<double>(w * h);
+  }
+
+  return minSSIM;
 }
 
 // !FIXME! These won't work for non-RGBA8 data.
diff --git a/Base/test/TestImage.cpp b/Base/test/TestImage.cpp
index baf5ed1..f1c2a68 100644
--- a/Base/test/TestImage.cpp
+++ b/Base/test/TestImage.cpp
@@ -164,5 +164,8 @@ TEST(Image, ComputeMSSIM) {
     }
   }
 
-  EXPECT_EQ(img.ComputeMSSIM(&img), 1.0);
+  double MSSIM;
+  double SSIM = img.ComputeSSIM(&img, &MSSIM);
+  EXPECT_EQ(SSIM, 1.0);
+  EXPECT_EQ(MSSIM, 1.0);
 }