MetaPost behaviour differs between standalone and \startreusableMPgraphic

18 Feb 2025

      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

Pavneet Arora

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