Dynamic point labeling

[Jim Bowlin]

"Paul G. Allen" wrote:
> 
> My application plots points on a map from data retrieved from a DB2
> database. No problem here, I can plot points 'til my heart's content.
> How do I label these points from data retreived from the same
> database? The mapfile layer definitions from my current mapfile for
> the points and labels are below. Note that the "DATA field" entry in
> the LAYER definitions is now meaningless as I am no longer using this
> to define the points and labels. I am still using MapServer v. 3.3.001
> (or whatever the first MapScript version was :)

Here is some "stripped down" Perl code I used to plot either plain points
or labeled points depending on the number of points on the map.

~~~~~~~~~~~~~~~~~~~~~~~
   my $map = new mapObj($mapfile) or
       cgi_err($mapscript::ms_error->{code}.": ".$mapscript::ms_error->{message});     
   my $img = $map->prepareImage() or
       cgi_err ("Unable to prepareImage $mapscript::ms_error->{message}");
   for my $i (0 .. ($map->{numlayers}-1)) {
       my $layer = $map->getLayer($i);
       $layer->draw($map, $img); # if $layer->{status};
   }
   my $array_refref = $db->selectall_arrayref($sql) || []; 
   my $count = @$array_refref;
   my $class = "class1";
   if ($use_labels && $count <= 25) {
       my $labs = "labels";
       my $layer = $map->getLayerByName($labs) or 
           cgi_err("Unable to getLayerByName($labs) $mapscript::ms_error->{message}");
       for my $row (@$array_refref) {
           my $point   = new pointObj();
           $point->{x} = $row->[1];
           $point->{y} = $row->[0];
           my $label   = $row->[2];
           $point->draw($map, $layer, $img, $class, $label);
       }
       $layer->draw($map, $img);
   }
   else  { 
       my $labs = "no_labels";
       my $layer = $map->getLayerByName($labs) or
           cgi_err("Unable to getLayerByName($labs) $mapscript::ms_error->{message}");
       for my $row (@$array_refref) {
           my $point   = new pointObj();
           $point->{x} = $row->[1];
           $point->{y} = $row->[0];
           $point->draw($map, $layer, $img, $class, undef);
       }
       $layer->draw($map, $img);
   }
   $map->drawLabelCache($img);
   mapscript::msSaveImage($img, $full_file, 0, 0);
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Below are the relevant layers from the .map file.  The key difference
is the "TYPE POINT" versus the "TYPE ANNOTATION".  I don't claim to
understand all of this.  I just sort of fusted with it until it did
what I wanted.  The $db thingy is a DBI object.  The SQL statement
selected on lat/lon and other criteria and returned three fields per
row: lat, lon and the text for the label.  I was writing the image to
a file for server-side caching but mostly because this was running
under FastCGI (which means the process was persistent) and I think
there was a bug somewhere that was closing STDOUT if I let mapscript
write directly to STDOUT.

HTH -- Jim Bowlin


LAYER
  NAME no_labels
  TYPE POINT
  STATUS DEFAULT
  PROJECTION
    geographic
  END
  MAXSCALE 100000000
  CLASS
    NAME "class1"
    SYMBOL 16
    COLOR 0 0 0
    LABEL
      TYPE TRUETYPE
      ANTIALIAS
      FONT arial-bold
      BUFFER 4
      SIZE 8
      POSITION AUTO
      COLOR 0 0 0
      BACKGROUNDCOLOR 255 255 204
    END
  END
END

LAYER
  NAME labels
  TYPE ANNOTATION
  STATUS DEFAULT
  PROJECTION
    geographic
  END
  MAXSCALE 100000000
  CLASS
    NAME "class1"
    SYMBOL 16 # red ball
    COLOR 0 0 0
    LABEL
      TYPE TRUETYPE
      ANTIALIAS
      FONT arial-bold
      BUFFER 4
      SIZE 8
      POSITION AUTO
      COLOR 220 0 0
      BACKGROUNDCOLOR 255 255 204
    END
  END
END