[NTG-context] Re: Downsampling images (again)

[Alan Braslau via ntg-context <ntg-context@ntg.nl>]

Hraban,

I would first "fix" my image sources, individually, to contain missing
resolution information.

Alan

On Sat, 21 Sep 2024 13:21:08 +0200
Henning Hraban Ramm <texml@fiee.net> wrote:

> This is lying around for years, let’s try to get it finally working.
> The code below works, but doesn’t do the right thing.
> 
> Since the original image doesn’t contain a resolution setting, gm 
> doesn’t do anything:
> “gm convert: image does not contain resolution”
> 
> It would be more reliable to use “gm convert -resize”, but for that
> we need to know the final pixel size.
> 
> How can a converter function access …
> – the final (scaled) size of a placed image
> – the original pixel size of an image
> ?
> With that information it would be easy to calculate the target pixel
> size.
> 
> And then it would be nice to hook this function into 
> \setupexternalfigure[conversion=]
> 
> 
> """
> \startluacode
> local function downsampler(oldname, newname, resolution)
>      if not resolution or resolution == "" then
>          resolution = 72
>      end
>      local cmd = string.format(
>        [[gm convert -resample %ix%i "%s" "%s"]],
>        resolution, resolution, oldname, newname)
>      logs.report("downsample", cmd)
>      os.execute(cmd)
> end
> 
> -- Set the PDF and default JPEG converters to the above function.
> figures.converters.jpg.pdf = downsampler
> figures.converters.jpg.default = downsampler
> \stopluacode
> 
> \setupexternalfigure[
>    %conversion=downsampler,% not used
>    resolution=10,
> ]
> 
> \starttext
> 
> \externalfigure[https://upload.wikimedia.org/wikipedia/commons/d/dd/Hermann_Zapf_signing.jpg][width=.5\textwidth]
> 
> \stoptext
> """
> 
> Hraban
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