resolved tickets #904 (nicer floating-point formatting) and #816 (reduce with CV_REDUCE_AVG on 8-bit arrays)

modules/core/src/matrix.cpp

@@ -1873,6 +1873,7 @@ void cv::reduce(const InputArray& _src, OutputArray _dst, int dim, int op, int d
         op = CV_REDUCE_SUM;
         if( sdepth < CV_32S && ddepth < CV_32S )
             temp.create(dst.rows, dst.cols, CV_32SC(cn));
+        ddepth = CV_32S;
     }
 
     ReduceFunc func = 0;

modules/core/src/persistence.cpp

@@ -775,7 +775,7 @@ icvDoubleToString( char* buf, double value )
             sprintf( buf, "%d.", ivalue );
         else
         {
-            static const char* fmt = "%.16e";
+            static const char* fmt = "%.17g";
             char* ptr = buf;
             sprintf( buf, fmt, value );
             if( *ptr == '+' || *ptr == '-' )
@@ -814,7 +814,7 @@ icvFloatToString( char* buf, float value )
             sprintf( buf, "%d.", ivalue );
         else
         {
-            static const char* fmt = "%.8e";
+            static const char* fmt = "%.9g";
             char* ptr = buf;
             sprintf( buf, fmt, value );
             if( *ptr == '+' || *ptr == '-' )