[NTG-context] prevgraf change if page break occurs in a paragraph

Joseph Canedo josephcanedo at gmail.com
Mon Mar 5 17:23:48 CET 2018

```BTW, I could write a MWE for the case when \prevgraf aggregates number of lines for 2 paragraphs (sometimes).

\starttext

\dorecurse{15}{
One line par.\par\nobreak A\\ B\\ C\\ D\\ E\\ F\\ G\\ H\\ I\\ J\\ K\\ L \\M\\ N.

\the\prevgraf

}

\stoptext

In some cases when « One line par » appears at first line of page (except the first one in this MWE output at least), you might see « 15 » after N. line.

De : Joseph Canedo
Envoyé le :lundi 5 mars 2018 16:32
À : mailing list for ConTeXt users
Objet :RE: [NTG-context] prevgraf change if page break occurs in a paragraph

I see. So is it reliable to simply use lua function tex.getcount(‘realpageno’) in vertical mode to determine that page has changed ? Or do I need some other mechanism please ? If page has changed I agree that there is no need to deal with hangindent.

I use grid typesetting so normally all lines have same height ?

De : Hans Hagen
Envoyé le :lundi 5 mars 2018 16:06
À : ntg-context at ntg.nl
Objet :Re: [NTG-context] prevgraf change if page break occurs in a paragraph

On 3/5/2018 3:51 PM, Joseph Canedo wrote:
> If I have a callback called just after paragraph has been split into
> lines, would not be enough to count the number of hboxes in the vbox the
> paragraph itself is contained ? Cannot say it’s ideal (easier to simply
> read tex.prevgraf for sure), but might be possible, unless I missed
> something (totally possible).
>
> In a couple of cases I see \prevgraf being larger than number of lines
> of previous paragraph, including lines for paragraph before than one as
> well. Could not create a MWE unfortunately to reproduce this case so far.
>
> I’ll try to amend my dropcaps/lettrine logic then. What are the reasons
> behind this change in \prevgraf TeX primitive which AFAIK exists since
> TeX has been written ? I thought that one of the strong points in TeX
> was overall stability of its behaviour among versions. Here with this
> semantic change in \prevgraf in recent luatex, this stability is a bit
> broken IMO.

it's *not changed* in luatex .. context *itself* sets the prevgraf at
the next page as indicator of the left-over lines

say that you have:

XXX ------------------
XXX ------------------
----------------------
----------------------

- a page break between the first and second line is a bug in your code
- a page break after the second line is harmless and as we're on a new
page then there is nothing to be done i.e. prevgraf then is set to 2 (as
we have two lines if we look back)

say that you have:

XXX ------------------
XXX

- if you get a page break here, again there is nothing to be done as
you're on a next page
- if you need to pick up in the next pagagraph (on the same page) then
prevgraf is 1

so, what in your case does it mean that when your next paragraph is on
the next page you look at prevgraf? there is no need to hangindent as
your hanging stuff is on the previous page

also, in general prevgraf is kind of useless when you have lines higher
than lineheight so if you use a callback, you also should calculate the
real accumulated drop so far

> Joseph
>
> *De : *Hans Hagen <mailto:pragma at wxs.nl>
> *Envoyé le :*lundi 5 mars 2018 10:15
> *À : *ntg-context at ntg.nl <mailto:ntg-context at ntg.nl>
> *Objet :*Re: [NTG-context] prevgraf change if page break occurs in a
> paragraph
>
> On 3/5/2018 9:43 AM, Joseph Canedo wrote:
>
>  > I see. I use prevgraf to get number of lines in most recent paragraph to
>
>  > compute eventually hangindent if there is a lettrine in paragraph (in
>
>  > case first paragraph has fewer lines than the lettrine itself, of course
>
>  > next paragraph needs to setup hangindent properly).
>
> but .. that one is then still on the same page; in fact, you should
>
> check for realpage being changed then
>
>  > I guess as an alternative, I could use the callback after paragraph is
>
>  > split in N lines to count the number of lines, or is this number can be
>
>  > accessed via some other existing variable in tex lua « table » ?
>
>  > Currently I access prevgraf value Inside lua. Probably better than
>
>  > making things more complicated in luatex itself.
>
> prevgraf is still ok after a 7 line paragraph but ... when you cross a
>
> page, and have say 4 lines moved to the next one, then a 7 line
>
> hangafter would be wrong (you can check if a par is on the same page)
>
> with callbacks after breaking in lines you still don't know ...
>
> Hans
>
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