v.to.rast: Use verbose for no area message, not warning (OSGeo#2459)

When there are no areas in the vector map, v.to.rast would give warning, 'No areas selected'. However, no areas is a perfectly fine state when converting a vector map with points or lines only, so the warning is misleading.

To cover the case where a user may need to know about the number of areas, a verbose message is now added reporting the number replacing both the warning and a debug message with the number.

This removes the false alarm, warning inconsistency in behavior between types, and warning inconsistency with other modules. Empty selection is generally a valid result, so here it also gives no warning.

The alternative of always giving warning when no features of a specific type are present, generates a lot of warnings. Another alternative would be to completely rework of how the type option works, e.g., the default would be empty and warnings would be present if the type was explicit, but that still would be somewhat inconsistent with other modules and does not follow that an empty selection is generally a valid result.

vector/v.to.rast/vect2rast.c

@@ -138,11 +138,9 @@ int vect_to_rast(const char *vector_map, const char *raster_map, const char *fie
     Points = Vect_new_line_struct();
 
     if (use != USE_Z && use != USE_D && (ftype & GV_AREA)) {
-	if ((nareas = sort_areas(&Map, Points, field, cat_list)) == 0)
-	    G_warning(_("No areas selected from vector map <%s>"),
-			  vector_map);
-
-	G_debug(1, "%d areas sorted", nareas);
+        nareas = sort_areas(&Map, Points, field, cat_list);
+        G_verbose_message(_("Number of areas selected from vector map <%s>: %d"),
+                          vector_map, nareas);
     }
     if (nareas > 0 && dense) {
 	G_warning(_("Area conversion and line densification are mutually exclusive, disabling line densification."));