Re: \hpos in math - Half solved

[Gavin via ntg-context <ntg-context@ntg.nl>]

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Hi List,

I solved my original problem 2, allowing me to reuse the same \hpos labels in different equations without interference. I added an equation counter and set up my custom math version of \hpos to use the count as part of the label internally. I tried to do this with a regular ConTeXt counter, but had all sorts of mysterious, wrong behavior. When I switched to using a Lua counter, everything worked immediately. So, while there is nothing brilliant about this solution, I’ll share it for anyone who might want to do something similar in the future.

The code I added is below, with some comments. The complete example file is attached. The result is two equations.

[-- Attachment #2: hposQuestionEquations.pdf --]
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Each equation was written using the same labels (X,Y,Z), but internally they have separate sets, (1:X and 2:X, etc.).

The moral of the story for me: Use more Lua.

Gavin


% Lua counts the spin formulas 
\ctxlua{sfnum = 0} % Count starts at zero
\def\sfID{\ctxlua{context(sfnum)}} % ConTeXt can call the count from Lua to use as an ID for each equation.

% Spin formulas increase the spin formula count and typeset the formula as usual.
\define\startspinformula{
  \ctxlua{sfnum = sfnum + 1}
  \startformula
}
\define\stopspinformula{\stopformula}

% My own \mpos uses the spin formula ID as part of the box label, then sets the contents as math.
\define[2]\mpos{\hpos{\sfID:#1}{$#2$}}

% Two commands for requesting contractions (the connections between spinors), using the spin formula ID.
\define[2]\contract{%
  \startpositionoverlay{graphics}%
    \setMPpositiongraphic{\sfID:#1}{mypos:contract}{to=\sfID:#2}%
  \stoppositionoverlay%
}
\define[2]\contractbelow{%
  \startpositionoverlay{graphics}%
    \setMPpositiongraphic{\sfID:#1}{mypos:contractbelow}{to=\sfID:#2}%
  \stoppositionoverlay%
}



[-- Attachment #4: hposQuestion.tex --]
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% !TEX useAlternatePath
% !TEX useConTeXtSyncParser

% Setting up the overlay for \hpos
\defineoverlay [graphics] [\positionoverlay{graphics}]
\setupbackgrounds
  [page]
  [background={foreground,graphics}]

% The MetaPost code for drawing the contractions.
\startMPpositiongraphic{mypos:contract}
     path pa, pb, pab ; numeric na, nb ; string ta, tb ;
     ta := mpvars("from") ;
     tb := mpvars("to") ;
     na := positionpage(ta) ; % page number of a
     nb := positionpage(tb) ; % page number of b
     pa := positionbox(ta) ;  % box of a
     pb := positionbox(tb) ;  % box of b
     z1 = 0.5[ulcorner pa,urcorner pa] ;
     z2 = 0.5[ulcorner pb,urcorner pb] ;
     y3 = max(y1,y2) + 3pt ; 
     if na = nb : % Check that positions are on the same page
       pab := z1 -- (x1,y3) -- (x2,y3) -- z2{down} ;
       pickup pencircle scaled 0.6pt ;
       draw pab yshifted 1pt ;
       positioninregion ;
     fi ;
\stopMPpositiongraphic

\startMPpositiongraphic{mypos:contractbelow}
     path pa, pb, pab ; numeric na, nb ; string ta, tb ;
     ta := mpvars("from") ;
     tb := mpvars("to") ;
     na := positionpage(ta) ; % page number of a
     nb := positionpage(tb) ; % page number of b
     pa := positionbox(ta) ;  % box of a
     pb := positionbox(tb) ;  % box of b
     z1 = 0.5[llcorner pa,lrcorner pa] ;
     z2 = 0.5[llcorner pb,lrcorner pb] ;
     y3 = min(y1,y2) - 3pt ; 
     if na = nb : % Check that positions are on the same page
       pab := z1 -- (x1,y3) -- (x2,y3) -- z2{down} ;
       pickup pencircle scaled 0.6pt ;
       draw pab yshifted -1pt ;
       positioninregion ;
     fi ;
\stopMPpositiongraphic

% Lua counts the spin formulas 
\ctxlua{sfnum = 0} % Count starts at zero
\def\sfID{\ctxlua{context(sfnum)}} % ConTeXt can call the count from Lua to use as an ID for each equation.

% Spin formulas increase the spin formula count and typeset the formula as usual.
\define\startspinformula{
  \ctxlua{sfnum = sfnum + 1}
  \startformula
}
\define\stopspinformula{\stopformula}

% My own \mpos uses the spin formula ID as part of the box label, then sets the contents as math.
\define[2]\mpos{\hpos{\sfID:#1}{$#2$}}

% Two commands for requesting contractions (the connections between spinors), using the spin formula ID.
\define[2]\contract{%
  \startpositionoverlay{graphics}%
    \setMPpositiongraphic{\sfID:#1}{mypos:contract}{to=\sfID:#2}%
  \stoppositionoverlay%
}
\define[2]\contractbelow{%
  \startpositionoverlay{graphics}%
    \setMPpositiongraphic{\sfID:#1}{mypos:contractbelow}{to=\sfID:#2}%
  \stoppositionoverlay%
}

% A textstyle fraction to make the normalization factor look nice.
\define[2]\textfrac{{\textstyle\frac{#1}{#2}}}


\starttext

\startspinformula
  \mpos{X}{e}_\alpha \mpos{Y}{e}^\mu \mpos{Z}{e}_{\dot\beta}
    = \textfrac{1}{\sqrt{2}}\sigma_{\alpha\dot\beta}^\mu
  \contract{X}{Y}
  \contractbelow{Y}{Z}
\stopformula

\startspinformula
  \mpos{X}{e}^{\dot\alpha} \mpos{Y}{e}_\mu \mpos{Z}{e}^{\beta}
    = \textfrac{1}{\sqrt{2}}\bar\sigma^{\dot\alpha\beta}_\mu
  \contractbelow{X}{Y}
  \contract{Y}{Z}
\stopformula

%\startformula
%  % Each \connectboxanchors works alone, but both together make trouble:
%    %\connectboxanchors[top][top]{X}{Y}
%    \connectboxanchors[bottom][bottom]{Y}{Z}
%  \mathboxanchored{X}{e}_\alpha \mathboxanchored{Y}{e}^\mu \mathboxanchored{Z}{e}_{\dot\beta}
%    = \textfrac{1}{\sqrt{2}}\sigma_{\alpha\dot\beta}^\mu
%\stopformula
%\blank[8mm]
%
%\startformula
%  % Each \connectboxanchors works alone, but both together make trouble:
%    \connectboxanchors[top][top]{X}{Y}
%    %\connectboxanchors[bottom][bottom]{Y}{Z}
%  \mathboxanchored{X}{e}_\alpha \mathboxanchored{Y}{e}^\mu \mathboxanchored{Z}{e}_{\dot\beta}
%    = \textfrac{1}{\sqrt{2}}\sigma_{\alpha\dot\beta}^\mu
%\stopformula

\stoptext

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