[NTG-context] Disable reuse of figures

[Aditya Mahajan]

On Sat, 19 Feb 2022, Hans Hagen via ntg-context wrote:

> On 2/19/2022 7:30 AM, Aditya Mahajan via ntg-context wrote:
> > Hi,
> > 
> > This is related to a bug report for the filter module:
> > https://github.com/adityam/filter/issues/60
> > 
> > Normally, when the same figure is included twice, context only includes the
> figure once and refers to the included figure the second time. The option
> `object=no` to externalfigure is supposed to prevent that, but it doesn't
> work. For example, consider the following example the bug-report uses graphviz
> as the external program, but I have changed that to context to keep things
> self contained)
> > 
> > \usemodule[filter]
> > 
> > \defineexternalfilter
> >    [graphviz]
> >    [
> >      filter=context --purge,
> >      output=\externalfilterbasefile.pdf,
> >      readcommand=\ReadPDF,
> >    ]
> > 
> > \def\ReadPDF#1%
> >      {\externalfigure[#1][object=no]}
> > 
> > \starttext
> > 
> >    Circle:
> >    \startgraphviz
> >      \startMPpage
> >        draw fullcircle scaled 1cm;
> >      \stopMPpage
> >    \stopgraphviz
> > 
> >    Square:
> >    \startgraphviz
> >      \startMPpage
> >        draw fullsquare scaled 1cm;
> >      \stopMPpage
> >    \stopgraphviz
> > 
> > \stoptext
> > 
> > Behind the scenes, the first \startgraphviz ... \stopgraphviz environment
> writes the content to an external file (\jobname-temp-graphviz.tmp), runs
> `context --purge \jobname-temp-graphviz.tmp`, which creates
> `\jobname-temp-graphviz.pdf`. The `\ReadPDF` macro then includees the file
> (with object=no).
> > 
> > At the next \startgraphviz .. \stopgraphviz, the same process is repeated.
> The `\jobname-temp-graphviz.pdf` file is overwritten and the new file should
> have been included. But it is not. The output contains the first image twice.
> > 
> > I thought that object=no should have prevented the reuse. In fact, if I
> compile the file with luatex, I get an error message:
> > 
> > ! error:  (pdf inclusion): file has changed
> '12-two-outputs-temp-graphviz.pdf'
> > mtx-context     | fatal error: return code: 256
> > 
> > Any idea why object=no is not working?
> the pdf file is kept open, for various reasons:
> 
> - performance
> - the might be different pages to fetch
> - analysis happens when the image is placed, inclusion happens later
> 
> so you try to reopen an already open file (which has not been completely 
> processed)

I see. So, for the filter module this means that one always has to use `cache=yes` (so that each PDF file has a different name). I'll add that to the documentation. 

Thanks,
Aditya