diff --git a/imageviewer-qt4.cpp b/imageviewer-qt4.cpp
index 972da2e..b94d70b 100644
--- a/imageviewer-qt4.cpp
+++ b/imageviewer-qt4.cpp
@@ -293,20 +293,21 @@ void ImageViewer::adjustBrightness(int b) {
 void ImageViewer::adjustContrast(int c) {
   int h, s, old_l;
   int new_l = 0;
-  int delta = c - 255;
+  double alpha = c / 255.0;
+  int average_intensity = getAverageIntensity();
   for(int x=0; x<image->width(); x++) {
     for(int y=0; y<image->height(); y++) {
-//       QColor color = QColor::fromRgb(original_image->pixel(x, y));
-//       color.getHsl(&h, &s, &old_l);
-//       new_l = old_l + delta;
-//       if(new_l > 255) new_l = 255;
-//       if(new_l < 0) new_l = 0;
-//       color.setHsl(h, s, new_l);
-//       image->setPixel(x, y, color.rgba());
+      QColor color = QColor::fromRgb(original_image->pixel(x, y));
+      color.getHsl(&h, &s, &old_l);
+      new_l = average_intensity + ((int) round((old_l - average_intensity) * alpha));
+      if(new_l > 255) new_l = 255;
+      if(new_l < 0) new_l = 0;
+      color.setHsl(h, s, new_l);
+      image->setPixel(x, y, color.rgba());
     }
   }
   updateImageDisplay();
-  logFile << "[TODO!] Contrast adjusted to: " << old_l << " + " << delta << " -> (" << h << ", " << s << ", " << new_l << ")" << std::endl;
+  logFile << "[TODO!] Contrast adjusted to: " << old_l << " * " << alpha << " -> (" << h << ", " << s << ", " << new_l << ")" << std::endl;
   renewLogging();
 }
 
diff --git a/imageviewer-qt4.h b/imageviewer-qt4.h
index 243758b..7075eb5 100644
--- a/imageviewer-qt4.h
+++ b/imageviewer-qt4.h
@@ -49,7 +49,7 @@
 #include <QSlider>
 #include <QColor>
 
-#include <cmath>
+#include <math.h>
 #include "fstream"
 #include <iostream>