Which internal TeX macro stores the MediaBox string, and how can I read it?

Question

Background: All PDF has a MediaBox that describes the page size, for each page. This is found in the PDF as (for example, US letter) the text string /MediaBox [0 0 612 792] where the numbers are "big points."

It is easy to calculate what the MediaBox should be, based on page dimensions. MWE:

% Must work with pdflatex. Specifically pdflatex.  
\documentclass[letterpaper]{memoir} % All pages same size.  
\usepackage{pgf}  
\newcommand{\whatTexUsesForMBwidth}{That is the width question!}  
\newcommand{\whatTexUsesForMBheight}{That is the height question!}  
% First is where the result is stored, second is the input.  
% Input TeX pt, Result in bp to 0.1 no units.  
\newcommand{\getbigpoints}[2]{  
   \pgfmathsetmacro#1{round(0.99626401*(#2+0.005))}  
}  
\newcommand\getMediaBoxWidth{  
    \getbigpoints{\myMBW}{\stockwidth}  
}
\newcommand\getMediaBoxHeight{  
    \getbigpoints{\myMBH}{\stockheight}  
}  
\getMediaBoxWidth  
\getMediaBoxHeight  
\begin{document}  
My calculated MediaBox width = \myMBW\par  
My calculated MediaBox height = \myMBH\par  
TeX internal MediaBox width = \whatTexUsesForMBwidth\par  
TeX internal MediaBox height = \whatTexUsesForMBheight\par  
\end{document}  

OUTPUT:

My calculated MediaBox width = 612.0  
My calculated MediaBox height = 792.0  
TeX internal MediaBox width = That is the width question!  
TeX internal MediaBox height = That is the height question!  

Now, I can always inspect the PDF in an editor, and read the MediaBox directly. But what I want to do is learn the MediaBox string while TeX is running, so that I can use the string within the TeX document.

Why I want to do this: For compliance purposes, my documents require /TrimBox, which in my case is exactly /MediaBox. I can calculate the values and write the string into the PDF without problem using TeX with the correct packages (and it is valid, according to Adobe Acrobat Pro 9). But that's because I am currently using a standard page size. If I ever need to use an exotic page size, there is the risk that my calculation will differ from the TeX internal values by a fraction of a point. That could cause an undetected error. However, if I knew the actual /MediaBox string, I could merely copy it to /TrimBox without need of calculation.

I have already set up a mock landing strip and prayed for Cargo, without result.

Answer 1

I'm not sure you want the values rounded to integers, because in case of A4 paper, we find

/MediaBox[0 0 595.276 841.89]

However, this depends on the setting of \pdfdecimaldigits whose default value is 3. (Thanks to Heiko Oberdiek for remarking it.)

The parameters to use are \pdfpagewidth and \pdfpageheight:

\documentclass[letterpaper]{memoir} % All pages same size.
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\getMediaBox}{O{\MediaBox}}
 {
  \cs_gset:Npx #1
   {
    /MediaBox[0~0~
      \robta_get_in_bp:n {\pdfpagewidth}~
      \robta_get_in_bp:n {\pdfpageheight}]
   }
 }
\cs_new:Nn \robta_get_in_bp:n
 {
  \fp_eval:n
   {
    round ( \dim_to_decimal_in_bp:n { #1 } , \pdfdecimaldigits )
   }
 }
\ExplSyntaxOff

\begin{document}

\getMediaBox

\MediaBox

\end{document} 

Here's the output; the \MediaBox so obtained is a string that you can use in different ways. Other strings can be produced if you prefer.

Here's the output for A4 paper:

If you set \pdfdecimaldigits to 0 at the start of your document, with

\pdfdecimaldigits=0

then with A4 paper we get

/MediaBox[0 0 595 842]