[luatex] handling resolution of jpg files

[David Carlisle]

The jpg file clip-1-2.jpg available from

https://github.com/latex3/graphics-def/tree/master/testfiles

has a resolution of 120 as reported by image magic identify

$ identify -verbose clip-1-2.jpg | head -8
Image: clip-1-2.jpg
  Format: JPEG (Joint Photographic Experts Group JFIF format)
  Mime type: image/jpeg
  Class: DirectClass
  Geometry: 243x214+0+0
  Resolution: 120x120
  Print size: 2.025x1.78333
  Units: PixelsPerInch


pdftex picks this up and reports the natural image width  as
146.34676pt=2.025in agreeing with image magic

luatex (at least how I am using it) doesn't pick up the resolution and
reports the width as 243.91125pt=243bp agreeing with the image geometry at
1bp per pixel.


To see the effect in latex  compare

pdflatex t-clip-1.lvt

where the image is clipped to the circled 1 and 2, with

 lualatex t-clip-1.lvt

where the initial image is larger so the clipping coordinates are off.

(You could also compare with xelatex, which does the wrong thing entirely,
but leave that for another list....)

To narrow it down, here is a tex primitive example without the latex
interfaces...


%%%%%%%%%%%%%%%%%

\ifx\saveimageresource\undefined\else
\let\pdfximage       \saveimageresource
\let\pdflastximage   \lastsavedimageresourceindex
\let\pdfrefximage    \useimageresource
\fi

\setbox0\hbox{%
  \immediate\pdfximage{clip-1-2.jpg}%
  \pdfrefximage \pdflastximage}

\showthe\wd0

\box0

\end
%%%%%%%%%%%%%%%%%%

with pdftex I get


> 146.34676pt.
l.12 \showthe\wd0

and with luatex :

> 243.91125pt.
l.12 \showthe\wd0



As usual, a two part question

a) is it possible to make luatex act in a more pdftex compatible way here?

and

b) if not, am I missing something in the tex or lua interfaces that would
let me get hold of that 120 and so adjust the sizes in the image inclusion
macros?

David
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