The nu parameter of the shape_predictor must be in the range (0,1].

I changed the spec to reflect this.

dlib/image_processing/shape_predictor.h

@@ -461,7 +461,7 @@ namespace dlib
             double nu
         )
         {
-            DLIB_CASSERT(nu > 0,
+            DLIB_CASSERT(0 < nu <= 1,
                 "\t void shape_predictor_trainer::set_nu()"
                 << "\n\t Invalid inputs were given to this function. "
                 << "\n\t nu:  " << nu 

dlib/image_processing/shape_predictor_abstract.h

@@ -185,7 +185,7 @@ namespace dlib
         );
         /*!
             requires
-                - nu > 0
+                - 0 < nu <= 1
             ensures
                 - #get_nu() == nu
         !*/