First version with own implementation of SSIM.

tools/auto-tester/src/ImageComparer.cpp

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+//
+//  ImageComparer.cpp
+//
+//  Created by Nissim Hadar on 18 Nov 2017.
+//  Copyright 2013 High Fidelity, Inc.
+//
+//  Distributed under the Apache License, Version 2.0.
+//  See the accompanying file LICENSE or http://www.apache.org/licenses/LICENSE-2.0.html
+//
+#include "ImageComparer.h"
+
+#include <glm/glm.hpp>
+
+// Computes SSIM - see https://en.wikipedia.org/wiki/Structural_similarity
+// The value is computed for the luminence component and the maximum value is returned
+double ImageComparer::compareImages(QImage resultImage, QImage expectedImage) const {
+    const double K1{ 0.01 };
+    const double K2{ 0.03 };
+
+    // Make sure the image is 8 bits per colour
+    QImage::Format format = expectedImage.format();
+    if (format != QImage::Format::Format_RGB32) {
+        throw -1;
+    }
+
+    const int L = 255; // (2^number of bits per pixel) - 1
+    const double c1 = pow((K1 * L), 2);
+    const double c2 = pow((K2 * L), 2);
+
+    // Coefficients for luminosity calculation
+    const double RED_COEFFICIENT = 0.212655f;
+    const double GREEN_COEFFICIENT = 0.715158f;
+    const double BLUE_COEFFICIENT = 0.072187f;
+
+    // First go over all full 8x8 blocks
+    // This is done in 3 loops
+    //      1) Read the pixels into a linear array (an optimization)
+    //      2) Calculate mean
+    //      3) Calculate variance and covariance
+    //
+    // p - pixel in expected image
+    // q - pixel in result image
+    //
+    const int WIN_SIZE = 8;
+    int x{ 0 };             // column index (start of block)
+    int y{ 0 };             // row index (start of block
+    double ssimMax{ 0.0 };
+    int w = expectedImage.width();
+    int h = expectedImage.height();
+    while (x < expectedImage.width()) {
+        int lastX = x + WIN_SIZE;
+        if (lastX > expectedImage.width()) {
+            x -= (lastX - expectedImage.width());
+        }
+
+        while (y < expectedImage.height()) {
+            int lastY = y + WIN_SIZE;
+            if (lastY > expectedImage.height()) {
+                y -= (lastY - expectedImage.height());
+            }
+
+            // Collect pixels
+            int i{ 0 };
+            double p[WIN_SIZE * WIN_SIZE];
+            double q[WIN_SIZE * WIN_SIZE];
+            for (int xx = 0; xx < WIN_SIZE; ++xx) {
+                for (int yy = 0; yy < WIN_SIZE; ++yy) {
+                    // Get pixels
+                    QRgb pixelP = expectedImage.pixel(QPoint(xx, yy));
+                    QRgb pixelQ = resultImage.pixel(QPoint(xx, yy));
+
+                    // Convert to vec3
+                    p[i] = 
+                        RED_COEFFICIENT * qRed(pixelP) + GREEN_COEFFICIENT * qGreen(pixelP) + BLUE_COEFFICIENT * qBlue(pixelP);
+
+                    q[i] = 
+                        RED_COEFFICIENT * qRed(pixelQ) + GREEN_COEFFICIENT * qGreen(pixelQ) + BLUE_COEFFICIENT * qBlue(pixelQ);
+
+                    ++i;
+                }
+            }
+  
+            // Calculate mean
+            double mP{ 0.0 };    // average value of expected pixel
+            double mQ{ 0.0 };    // average value of result pixel
+            for (int j = 0; j < WIN_SIZE * WIN_SIZE; ++j) {
+                mP += p[j];
+                mQ += q[j];
+            }
+            mP /= (WIN_SIZE * WIN_SIZE);
+            mQ /= (WIN_SIZE * WIN_SIZE);
+
+            // Calculate variance and covariance
+            double sigsqP{ 0.0 };
+            double sigsqQ{ 0.0 };
+            double sigPQ{ 0.0 };
+            for (int j = 0; j < WIN_SIZE * WIN_SIZE; ++j) {
+                sigsqP += (p[j] - mP) * (p[j] - mP);
+                sigsqQ += (q[j] - mQ) * (q[j] - mQ);
+
+                sigPQ += (p[j] - mP) * (q[j] - mQ);
+            }
+            sigsqP /= (WIN_SIZE * WIN_SIZE);
+            sigsqQ /= (WIN_SIZE * WIN_SIZE);
+            sigPQ /= (WIN_SIZE * WIN_SIZE);
+
+            double numerator = (2.0f * mP * mQ + c1) * (2.0f * sigPQ + c2);
+            double denominator = (mP * mP + mQ * mQ + c1) * (sigsqP + sigsqQ + c2);
+
+            double ssim = numerator / denominator;
+
+            if (ssim > ssimMax) {
+                ssimMax = ssim;
+            }
+
+            y += WIN_SIZE;
+        }
+        x += WIN_SIZE;
+    }
+
+    return ssimMax;
+};