diff --git a/imageviewer-qt4.cpp b/imageviewer-qt4.cpp
index 5076d84..c060fda 100644
--- a/imageviewer-qt4.cpp
+++ b/imageviewer-qt4.cpp
@@ -487,8 +487,8 @@ void ImageViewer::applyBasicFilter() {
   for(int i=0; i<filter_size; i++) {
     for(int j=0; j<filter_size; j++) {
       int current_data = data[i*filter_size + j];
-      filter_sum += current_data;
-      //logFile << "@" << i << "," << j << " -> " << current_data << std::endl;
+      filter_sum += abs(current_data); // support for negativ weights using abs()
+      logFile << "@" << i << "," << j << " -> " << current_data << std::endl;
     }
   }  
   
@@ -515,6 +515,9 @@ void ImageViewer::applyBasicFilter() {
       //std::cout << "old lightness: " << l << ", filter_sum: " << filter_sum << ", intensity_sum:" << intensity_sum;
       // divide by sum of weights
       l = qRound((intensity_sum*1.0) / filter_sum);
+      //optional clamping for exotic stuff like negative values
+      if(l > 255) l = 255;
+      if(l < 0) l = 0;
       //std::cout << " --> new l: " << l << std::endl;
       color.setHsl(h, s, l);
       // and set it.