Shadows/Highlights: Use iterated boxblur for radius > 40 when Sharp mask is disabled, fixes #3730

rtengine/shmap.cc

@@ -85,12 +85,21 @@ void SHMap::update (Imagefloat* img, double radius, double lumi[3], bool hq, int
     if (!hq) {
         fillLuminance( img, map, lumi);
 
+        float *buffer = nullptr;
+
+        if(radius > 40.) {
+            // When we pass another buffer to gaussianBlur, it will use iterated boxblur which is less prone to artifacts
+            buffer = new float[W * H];
+        }
+
 #ifdef _OPENMP
         #pragma omp parallel
 #endif
         {
-            gaussianBlur (map, map, W, H, radius);
+            gaussianBlur (map, map, W, H, radius, buffer);
         }
+
+        delete [] buffer;
     }
 
     else {