[Therion] Hiding small objects at large scales

[Martin Budaj]

On Wed, Feb 3, 2010 at 10:43 AM, Bruce <dangle at tomo.co.nz> wrote:
> I have just noticed Thomas Holders  metapost sample for hiding small rocks
> at large scale reductions.
>
>   def l_rockborder (expr p) =
>
>     if abs(llcorner p - urcorner p) > u:
>
>       l_rockborder_UIS(p);
>
>     fi;
>
>   enddef;
>
> What would be the chances of this working in the form presented above for
> point objects?

You would need to use something like

if abs(U) > 3pt:
    p_symbol_UIS(p);
fi;

where abs(U) means (approximate) distance from the centre of the point
symbol to the upper-right corner. The U variable is defined for all
(or almost all) point symbols directly in the symbol definition. It
means that you would need to implement the test after the U is defined
(so you need to change the definition of the original symbol; it would
be not enough just to define wrapper symbol like in l_rockborder
example).

Another possibility is create wrapper symbol which 1) draws the
desired symbol in the temporary picture 2) measures the dimensions of
temporary picture 3) decides whether to draw the symbol. But this
would be quite slow.

> Or for labels as below?

The easiest way would be to enhance p_label symbol itself:

vardef p_label@#(expr txt,pos,rot,mode) =
  if (mode=1) or (mode=7): interim labeloffset:=(u/8) fi;
  lab:=thelabel@#(txt, pos);
  if abs(llcorner bbox lab - urcorner bbox lab) > u:    % here is the size test
    if mode>1: pickup PenD fi;
    if mode=1:
      pickup pencircle scaled (u/6);
      drawdot(pos);
      process_label(pos,0);
    elseif mode=2: process_uplabel;
    elseif mode=3: process_downlabel;
    elseif mode=4: process_updownlabel;
    elseif mode=5: process_circledlabel;
    elseif mode=6: process_boxedlabel;
    elseif mode=7: process_label(pos,rot);  % station name
    elseif mode=8: process_filledlabel(pos, rot);
    else: process_label(pos,rot); fi;
  fi;    % end of size test
enddef;

> Or modified to show or hide point objects depending on the value of the
> –scale property?

You can test even for user-defined attributes.

martin