[NTG-context] Re: MetaPost behaviour differs between standalone and \startreusableMPgraphic

19 Feb 2025

      Hi Pavneet,

I recommend avoiding the mod function. Just draw the two grids (major and minor) each with their own spacing and pen.

I whole heatedly support converting TikZ to MetaPost!

- Gavin

\startreusableMPgraphic{graph}
 minor=2mm;     % Minor lines, each every 2mm
 major=8mm;     % Major lines, each every 4mm

 z1 = (0,0); % lower left corner
 % z2 = (2.75in,4.25in)-z1;  % upper right corner
 z2 = (2.75in,4.25in)-z1;% upper right corner

  path v, h;
  v := z1--(x1,y2); % vertical line
  h := z1--(x2,y1); % horizontal line

  % Use light gray
  drawoptions(withcolor .4white);

  % Draw the minor grid with the very thin pen
  pickup pencircle scaled 0.1;
  for i=minor step minor until (x2-x1):
     draw v xshifted i;
  endfor;
  for i=minor step minor until (y2-y1):
     draw h yshifted i;
  endfor;

  % Draw the major grid with the thicker pen
  pickup pencircle scaled 0.3;
  for i=major step major until (x2-x1):
     draw v xshifted i;
  endfor;
  for i=major step major until (y2-y1):
     draw h yshifted i;
  endfor;

\stopreusableMPgraphic
...
On Feb 18, 2025, at 3:28 PM, Pavneet Arora via ntg-context  wrote:
I am in the process of converting a bunch of old TikZ notepaper 
templates to native ConTeXt and MetaPost.  This is a part of a project 
to generate notebooks with documentation data and then blank pages appended to
fill out full sheets, and then collated in a particular order for trimming and
binding.
As such I rely on Imposition.  I am having issues with the various 
techniques of trying to duplicate a MP graphic across the pages using 
\[start|stop]MPcode \[start|stop]useMPgraphic, etc.  But that is a 
discussion for another time.
The method that I found to work was to use 
\[start|stop]reusableMPgraphic along with overlays
I am still struggling with some MetaPost code that works fine in 
standalone but not when included in a ConTeXt document.
With standalone MP, we get thicker lines on major increments as 
expected.  When the same code is included in a ConTeXt document the 
thicker lines are only drawn up a certain number and then stop --- 
presumably the mod function returning a non-zero result when it really 
shouldn't?
Here are the MWEs:
...
...
...
ConTeXt
% Paper size and layout {{{2
\definepapersize
|   [PocketPage]
|   [width=2.75in,height=4.25in]
\setuppapersize[PocketPage][letter,landscape]
% }}}2
% MetaPost template for graph paper for pocket sized notebook (2.75" W x
% 4.25in H). {{{2
\startreusableMPgraphic{graph}
 minor=2mm;|  |   % Minor lines, each every 2mm
 major=8mm;|  |   % Major lines, each every 4mm
  ┆   ┆   ┆   ┆ % major must be a multiple of minor
z1 = (0,0);| % lower left corner
 % z2 = (2.75in,4.25in)-z1;|  % upper right corner
 z2 = (2.75in,4.25in)-z1;|% upper right corner
% Use the very thin pen
  pickup pencircle scaled 0.1;
% Draw the vertical lines
  for i=minor step minor until (y2-y1):
  ┆   if (i mod major)=0:
  ┆   ┆   pickup pencircle scaled 0.3;
  ┆   fi;
  ┆   draw (z1 shifted (0,i))--((x2,y1) shifted (0,i)) withcolor .4white;
  ┆   if (i mod major)=0:
  ┆   ┆   pickup pencircle scaled 0.1;
  ┆   fi;
  endfor;
% Draw the horizonal lines
  for i=minor step minor until (x2-x1):
  ┆   if (i mod major)=0:
  ┆   ┆   pickup pencircle scaled 0.3;
  ┆   fi;
  ┆   draw ( z1--(x1,y2) ) shifted (i,0) withcolor .4white;
  ┆   if (i mod major)=0:
  ┆   ┆   pickup pencircle scaled 0.1;
  ┆   fi;
  endfor;
\stopreusableMPgraphic
\defineoverlay[OLgraph][\useMPgraphic{graph}]
\setupbackgrounds[page][background=OLgraph]
% }}}2
\starttext
hello.
\stoptext
<<<
...
...
...
Standalone MP
beginfig(1)
margin=0mm;             % Margin of the graph paper
minor=2mm;              % Minor lines, each every 1mm
major=8mm;              % Major lines, each every 10mm
                      % major must be a multiple of minor
z1 = (margin,margin);   % lower left corner
z2 = (2.75in,4.25in)-z1;        % upper right corner
% % Use the thick pen first
% pickup pencircle scaled 0.5;
% % Draw the frame
% draw z1--(x1,y2)--z2--(x2,y1)--cycle withcolor .5white;
% Use the very thin pen
pickup pencircle scaled 0.1;
% Draw the vertical lines
for i=minor step minor until (y2-y1-minor):
      if (i mod major)=0:
              pickup pencircle scaled 0.3;
      fi;
      draw (z1 shifted (0,i))--((x2,y1) shifted (0,i)) withcolor .4white;
      if (i mod major)=0:
              pickup pencircle scaled 0.1;
      fi;
endfor;
% Draw the horizonal lines
for i=minor step minor until (x2-x1-minor):
      if (i mod major)=0:
              pickup pencircle scaled 0.3;
      fi;
      draw ( z1--(x1,y2) ) shifted (i,0) withcolor .4white;
      if (i mod major)=0:
              pickup pencircle scaled 0.1;
      fi;
endfor;
endfig;
end;
<<<
advTHANKance
-- 
Pavneet Arora           m: +1 647-406-6843
Waroc Informatik        e: pavneet_arora@waroc.com
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