Hello, luatex includes the MetaPost library MPlib, so MetaPost code is called and then injected into the output stream. As a user, it is useful to know that MP is in fact called in two passes, thus twice, and this can have some unexpected consequences when not programmed cleanly. Global variable storage, for example, can reflect the multiple passes. Take the following (silly) example:
\starttext \startMPcode if unknown n : n := 0 ; fi n := n + 1 ; draw textext(decimal n) ; \stopMPcode \startMPcode n := n + 1 ; draw textext(decimal n) ; \stopMPcode \stoptext yielding: 1 3 (perhaps Hans or Taco can explain why one does not get 2 4) This side effect is somewhat handled by the fact that each call to MP is treated as a group : begingroup ... endgroup so \startMPcode save n ; n := 0 ; ... \stopMPcode works as expected, but n is then local and becomes unknown (or keeps its global value) for subsequent calls to MP. What should be understood is that calls to MP returns a bounding box to TeX, and space is reserved for this bounding box. Likewise, calls within MP to TeX, for example textext() or label(), or btex ... etex returns a bounding box to MP, and this (picture) object is placed in the graphic. Mechanisms exist allowing the passing of values of variables via lua (as strings, in fact). This is a more advanced subject. Now, there are a number of ways of calling MP from TeX, directly as above, but also as unique MP graphics or as reusable MP graphics, ... This is another subject all together. Hopefully this helps some. Alan On Fri, 30 Mar 2018 13:31:30 +0200 Gerhard Kugler <g.w.kug...@posteo.de> wrote: > Hello, > > my problem is that in other programming langguages there is a main > procedure which calls others. In metapost in the context of ConteXt I > can use \starttext ... \stoptext or \startMPpage ... \stopMPpage or > \beginfig ... \endfig > > And it seems not to be transparent how procedures are called. > > Gerhard > > ___________________________________________________________________________________ If your question is of interest to others as well, please add an entry to the Wiki! maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context webpage : http://www.pragma-ade.nl / http://context.aanhet.net archive : https://bitbucket.org/phg/context-mirror/commits/ wiki : http://contextgarden.net ___________________________________________________________________________________
