Default arguments need to be specified in the class declaration and cannot be redeclared.
https://bugs.gentoo.org/show_bug.cgi?id=570102

--- iulib-0.4/imglib/imgops.cc
+++ iulib-0.4/imglib/imgops.cc
@@ -133,7 +133,7 @@
 
     template<class T,class V>
     void addscaled(narray<T> &dest,narray<T> &src,
-            V scale=1,int dx=0,int dy=0) {
+            V scale,int dx,int dy) {
         for (int i=0; i<dest.dim(0); i++)
             for (int j=0; j<dest.dim(1); j++)
                 dest.unsafe_at(i,j) += (T)(scale*xref(src,i+dx,j+dy));
--- iulib-0.4/imglib/imgops.h
+++ iulib-0.4/imglib/imgops.h
@@ -67,7 +67,7 @@
     }
 
     template<class T, class V>
-    void addscaled(colib::narray<T> &, colib::narray<T> &, V, int, int);
+    void addscaled(colib::narray<T> &dest, colib::narray<T> &src, V scale=1, int dx=0, int dy=0);
     template<class T>
     void tighten(colib::narray<T> &image);
     template<class T>