[NTG-context] Negative width vrule ignored in LuaMETATeX

[Hans Hagen]

On 4/24/2020 7:44 PM, Ao Shen wrote:
> Thanks for your timely reply!
> 
> Hans Hagen <j.hagen at xs4all.nl> 于2020年4月24日周五 下午11:38写道：
>>
>> no, but i must admit that it's kind of a border case ... i'll fix it but
>> also wonder if *no* visual rule showing up is intended (i would then
>> also expect an engine to show a rule .. could be an interesting new
>> (luametatex) option as it saves kerns in some cases)
> 
> My use cases of negative vrule is a little tricky. I'm trying to comply with
> W3C's [Requirements for Japanese Text Layout](https://www.w3.org/TR/jlreq/).
> It states that
> 
>    In principle, closing brackets (cl-02), commas (cl-07) or full stops (cl-06)
>    at the line end have a half em space after them (see Fig. 3.20). This half em
>    space can be deleted for line adjustment. However, the possibilities are only
>    half em space or solid. Other spaces, such as a quarter em space should not be
>    used.
> 
> The tricky part is that I also want the opening fullwidth bracket at the
> beginning of the line to be solid, i.e., without half em sidebearing on the left
> to make the left margin visually more aligned (this is one but not the only way
> of line begining handling shown in jlreq).
> 
> To achieve this, in the scripts handler I use the following injector between
> 'full_width_close' and 'full_width_open':
> 
> \hskip -0.5em \penalty 100 \hskip 0.5em minus 0.5em \vrule width
> -0.5em height 0pt
> ^^^(a)        ^^^(b)
> 
> If line is broken here, the only broken points are (a) before the glue or (b) at
> the penalty point. With the vrule the sidebearing of the opening bracket at
> beginning of next line is eliminated.
> 
> In this particular case, a kern is not applicable because it won't survive the
> line break. And here a vrule showing nothing is intended.
How about something ...

\startluacode

local classes = fonts.protrusions.classes
local vectors = fonts.protrusions.vectors

classes.myown = {
     vector = 'myown',
     factor = 1,
}

vectors.myown = table.merged (vectors.quality, {

     [0x0028] = { 1, 0 }, -- (
     [0x0029] = { 0, 1 }, -- )

})

\stopluacode

\starttext

\showframe

\definefontfeature[whatever][default][protrusion=myown]

\definedfont[Serif*whatever] \setupalign[hanging]

\dorecurse{100}{(#1) }

\stoptext

of course you need to define the cjk fences etc.

Hans

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