Conversion from pstricks coordinates to postscript coordinates

Question

I am making a custom graphics library on top of pstricks and I need to convert from pstricks coordinates to postscript coordinates. I have a "working" example, cooked up via painful trial-and-error process, but I am sure it can be done properly. Reading the pstricks.tex file, I was left with the impression that pstricks uses somehow \pst@coor or - some other macro - perhaps you know better - to convert from pstricks coordinates to postscript coordinates. I have not been able to decode how that works. So here is my working solution.

My concrete question to you is: what are the native pstricks conversion functions I should use in place of \fcConvertPSXUnit and \fcConvertPSYUnit in the code below? [Edit] Question was answered below by Herbert. Including his answer in this post.

If done properly, the code below should display a perfect square cross (the top part of the cross is drawn using native pstricks, and the bottom part is drawn via the conversion functions).

\documentclass{article}
\usepackage{auto-pst-pdf} 
%to convert points to numbers, macro copied from stackexchange
\makeatletter
\begingroup
\catcode `P=12  % digits and punct. catcode
\catcode `T=12  % digits and punct. catcode
\lowercase{%
\def\x{\def\rem@pt##1.##2PT{##1\ifnum##2>\z@.##2\fi}}}
\expandafter\endgroup\x%
\newcommand{\stripPoints}[1]{\expandafter\rem@pt\the#1}
\newcommand{\fcConvertPSXUnit}{\stripPoints{\psxunit} 72.27 div 8000 mul mul\space %
\pst@number\pst@dima\space %3 sub 
72.27 div 8000 mul sub  %
}%
\newcommand{\fcConvertPSYUnit}{\stripPoints{\psyunit} 72.27 div -8000 mul mul\space %
\pst@number\pst@dimb\space 72.27 div -8000 mul sub %
}%
\makeatother

\begin{document}
\psset{xunit=1.2cm, yunit=1.2cm}
\begin{pspicture}(-2.3,-2.7)(2.1,2.55)%
\psline (0,0)(0,1)%
\psline (0,0)(1,0)%
\pstVerb{ %
100 setlinewidth %
newpath %
0 \fcConvertPSXUnit 0 \fcConvertPSYUnit moveto %
-1 \fcConvertPSXUnit 0 \fcConvertPSYUnit lineto %
0 \fcConvertPSXUnit 0 \fcConvertPSYUnit moveto %
0 \fcConvertPSXUnit -1 \fcConvertPSYUnit lineto %
stroke %
}%
\end{pspicture}
\end{document}

Thanks for your advice!

The code from Herbert that answers my question:

\documentclass{article}
\usepackage{auto-pst-pdf}
\usepackage{pstricks} %that should had been included

\begin{document}

\psset{xunit=1.2cm, yunit=1.2cm}
\begin{pspicture}(-2.3,-2.7)(2.1,2.55)%
\makeatletter
\psline (0,0)(0,1)%
\psline (0,0)(1,0)%
\pscustom{ %
\code{ %
100 setlinewidth %
newpath %
0  0 \tx@ScreenCoor\space moveto %
-1 0 \tx@ScreenCoor\space lineto %
0 0 \tx@ScreenCoor\space moveto %
0 -1 \tx@ScreenCoor\space lineto %
stroke %
}%
}%
\makeatother

\end{pspicture}
\end{document}

Answer 1

\documentclass{article}
\usepackage{pstricks} 

\begin{document}
\psset{xunit=1.2cm, yunit=1.2cm}
\begin{pspicture}[showgrid](-2.3,-2.7)(2.1,2.55)
\psline (0,0)(0,1)
\psline (0,0)(1,0)
\pscustom{
  \moveto(0,0) 
  \lineto(-1,0) 
  \moveto(0,0) 
  \lineto(0,-1) 
  \stroke[linewidth=2pt,linecolor=red] 
}
\end{pspicture}
\end{document}

if you do not want the basic macros \moveto et al then use for example:

\code{0 0 moveto -1 0 \tx@ScreenCoor\space lineto}

inside \pscustom and with a \makeatletter before the pspicture environment:

\documentclass{article}
\usepackage{auto-pst-pdf}
\usepackage{pstricks}
\begin{document}

\makeatletter
\psset{xunit=1.2cm, yunit=1.2cm}
\begin{pspicture}(-2.3,-2.7)(2.1,2.55)
\psline (0,0)(0,1)
\psline (0,0)(1,0)
\pscustom{% 
\code{
newpath 
2 setlinewidth 
0  0 \tx@ScreenCoor\space moveto 
-1 0 \tx@ScreenCoor\space lineto 
0 0 \tx@ScreenCoor\space moveto 
0 -1 \tx@ScreenCoor\space lineto 
stroke 
}}%
\end{pspicture}
\makeatother
\end{document}