[tex-k] Where does conversion of PostScript pts to TeX pts occur?

[Tomas Rokicki]

The epsf macro reads the bounding box and interprets it using
TeX truept units.

On Sat, Jan 26, 2019 at 7:10 AM Doug McKenna <doug at mathemaesthetics.com>
wrote:

> All -
>
> When a Tex source file wants to place an illustration in PDF or other
> supported graphic file format, it needs to know the bounding box of that
> illustration, which for PDF is recorded inside that PDF file in PostScript
> points (72 per inch).  But TeX operates in TeX points, which are 72.27/inch.
>
> So some high-level macro in LaTeX (or whatever format or library) such as
> \includegraphics[]{} gets invoked and it presumably relies on some
> low-level primitive extension command implemented in pdftex (or equivalent)
> that reaches out to some directory and examines the PDF file requested to
> get that PDF's bounding box info.
>
> My question is this: deep down at the primitive level, whose
> responsibility is it to convert from PostScript points to TeX points in
> order know what the equivalent bounding box will be on the TeX page?
>
> Does some back-end implementation of a primitive do it on behalf of the
> primitive command, so that the primitive command delivers values in TeX
> points, or does it deliver values in PostScript points, and some TeX source
> code surrounding the invocation of that primitive then adjust the numbers
> to TeX points (and perhaps then scale them according to the argument to
> \includegraphics[]{}?
>
> I'm guessing the former, but I'm not familiar with the source code enough
> to know what's going on.  And I don't want to guess.
>
>
> Doug McKenna
>
>
>

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