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

Tomas Rokicki rokicki at gmail.com
Sat Jan 26 17:47:58 CET 2019

I'm sorry; I mean bp (big point) units.

On Sat, Jan 26, 2019 at 8:46 AM Tomas Rokicki <rokicki at gmail.com> wrote:

> 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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