[texworks] problem with \magstep1

[Philip Taylor]

Ohayou gozaimasu, Akira-san --

> %Sample file
> % %
> \magnification\magstep3
> \pdfpagewidth=210 true mm
> \pdfpageheight=297 true mm
> \pdfhorigin=1 true in
> \pdfvorigin=1 true in

Can you explain to me the difference(s) between \hoffset and
\pdfhorigin, \voffset and \pdfvorigin ?  I appreciate that
the default values differ by 1 true in (the *offsets default
to 0 true in, the *origins to 1 true in), and I also
appreciate that changing the *offsets does not affect the
*origins (and vice versa), so my question is in essence
"Is there any reason for electing to manipulate one pair
rather than the other" ?

I have always manipulated the *offsets (because my use of TeX
goes way back to when there was no alternative) but if I were
to switch to manipulating the *origins, would I gain anything ?
Sample preamble from one of my documents (A4 landscape, non-standard
symmetric vertical margins), with *offsets manipulated :

% !TeX Program = XeTeX

\magnification \magstep 5

\newdimen \hmargin
\newdimen \vmargin
\newdimen \origin

\hmargin = 1 true in
\vmargin = 2 true cm
\origin = 1 true in

\hsize = 297 true mm
\vsize = 210 true mm
\pdfpagewidth = \hsize
\pdfpageheight = \vsize
\advance \hsize by -2 \hmargin
\advance \vsize by -2 \vmargin
\advance \hoffset by -\origin
\advance \voffset by -\origin
\advance \hoffset by \hmargin
\advance \voffset by \vmargin

Philip Taylor