[NTG-context] Downsampling images (again)

[Henning Hraban Ramm]

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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