[Mapserver-users] altering $layer->{symbolscale} before draw()

[Sean Gillies]

Hi Nikolai,

Do you have an error in your code?  I'm going to insert a line into 
your code below.

On Feb 27, 2004, at 8:05 AM, Nikolai Vladychevski wrote:

> Hi,
> I have a perl script that uses mapscript to create a map with many 
> layers. I want to dynamicaly alter SYMBOLSCALE of the layer depending 
> on the scale of the map, because when user zooms too much, the symbol 
> is too big. I can not lower SYMBOLSCALE and leave it fixed in mapfile 
> becasue when user is browsing at high scales, the symbols is very 
> small, almost invisible.
> So I have this code, but it isn't giving expected results, nothing 
> happends:
> sub drawLayers {
>   my $m=shift;  #map
>   my $i=shift;  #image
> for ($lidx=0;$lidx<$m->{numlayers};$lidx++) {
>       $l=$m->getLayer($lidx);
>       if ($m->{scale}<32000 && ($l->{name} eq "emplazamientos")) {
>           $l->{symbolscale}=320;  #<- this doesn't make any difference

Try symbolscale = -1 to turn off scaling when you are zoomed in?

>       }
>       $l->draw($m,$i);
>   }
> here are some lines in mapfile:
> LAYER
>       NAME emplazamientos
>       DATA emplazamientos.shp
>       CLASSITEM 'ID'
>       LABELITEM 'ID'
>       SYMBOLSCALE 32000
>       CLASS
>           NAME 'emplazamientosactivos'
>           COLOR 0 0 255
>           SYMBOL 'emplazamiento'
>           SIZE 10
>           TEMPLATE "emplazamiento.html"
>       END
>       .....
>  SYMBOL
>       NAME "emplazamiento"
>       TYPE TRUETYPE
>       FONT transmision
>       FILLED true
>       ANTIALIAS true
>       CHARACTER "F"
>  END
> .......
> How do I have to change the {symbolscale} variable in order to achieve 
> changes in the display, as I do it above, setting it to different 
> values doesn't make any difference
> Thanks in advance
> Nikolai

The dynamic scaling in mapserver is a bit limited because what we 
*really* need in
many cases is not linear scaling, but *log* scaling of feature line 
widths.

If a feature has size0 = 10 at scale0 = 32000 (your case), it would be 
nice to
have the feature size scale like

    size = size0*log(scale0)/(log(scale)

So that symbol sizes would scale gently.  This might not be hard to 
implement
at all, requiring only a SYMBOLLOGSCALE property and a bit of new code 
in
mapserver.

cheers,
Sean