Label of float8 data shows to many digits

[artemp]

In quantumnik, I use a postgis layer and set its label to a float8 attribute. QGIS shows one of the labels as "24.6", but when quantumnik is turned on this label displays "24.60000000000001".

[artemp]

[springmeyer] hardcoded right here:

http://trac.mapnik.org/browser/trunk/include/mapnik/value.hpp#L553

[artemp]

[springmeyer] this (http://173.255.217.246:8000/mapnik_trac/attachment/ticket/430/control_precision.patch) patch implements flexibility needed to control the precision when converting doubles to unicode text. If this is a decent approach we could then expose as a text_symbolizer param:

(should also likely consider exposing fixed vs scientific formatting, and apply same fix to to_string())

#!diff
Index: src/agg/process_text_symbolizer.cpp
===================================================================
--- src/agg/process_text_symbolizer.cpp (revision 2046)
+++ src/agg/process_text_symbolizer.cpp (working copy)
@@ -45,7 +45,7 @@
     expression_ptr name_expr = sym.get_name();
     if (!name_expr) return;
     value_type result = boost::apply_visitor(evaluate<Feature,value_type>(feature),*name_expr);
-    UnicodeString text = result.to_unicode();
+    UnicodeString text = result.to_unicode(sym.get_precision());

     if ( sym.get_text_convert() == TOUPPER)
     {

[strk]

The pull requests #1631 (for 2.1 branch) and #1632 (for master) fix the precision used by to_string(), effectively fixing the issue reported in the title of this ticket.

Maybe there should be another ticket for "Controlling floating point formatting in label symbolizers" and this one should be closed as fixed ?

[artemp]

@strk - thanks, will check it out asap!

[springmeyer]

thanks @strk, you are right, so closing this as fixed and opened a new one for user control during symbolization: #1695