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<h4 class="subsection" id="Output-Language-Compatibility-1">6.1.4 Output Language Compatibility</h4>
<p>The intermediate output language of <abbr class="acronym">AT&T</abbr> <code class="code">troff</code> was
first documented in <cite class="cite">A Typesetter-independent TROFF</cite>, by Brian
Kernighan, and by 1992 the <abbr class="acronym">AT&T</abbr> <code class="code">troff</code> manual was
updated to incorprate a description of it.
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<p>The GNU <code class="code">troff</code> intermediate output format is compatible with this
specification except for the following features.
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<ul class="itemize mark-bullet">
<li>The classical quasi-device independence is not yet implemented.
</li><li>The old hardware was very different from what we use today. So the
<code class="code">groff</code> devices are also fundamentally different from the ones
in <abbr class="acronym">AT&T</abbr> <code class="code">troff</code>. For example, the <abbr class="acronym">AT&T</abbr>
PostScript device is called <code class="code">post</code> and has a resolution of only 720
units per inch, suitable for printers 20 years ago, while <code class="code">groff</code>’s
<code class="code">ps</code> device has a resolution of 72000 units per inch. Maybe, by
implementing some rescaling mechanism similar to the classical
quasi-device independence, <code class="code">groff</code> could emulate <abbr class="acronym">AT&T</abbr>’s
<code class="code">post</code> device.
</li><li>The B-spline command ‘<samp class="samp">D~</samp>’ is correctly handled by the intermediate
output parser, but the drawing routines aren’t implemented in some of
the postprocessor programs.
</li><li>The argument of the commands ‘<samp class="samp">s</samp>’ and ‘<samp class="samp">x H</samp>’<!-- /@w --> has the
implicit unit scaled point ‘<samp class="samp">z</samp>’ in <code class="code">gtroff</code>, while
<abbr class="acronym">AT&T</abbr> <code class="code">troff</code> has point (‘<samp class="samp">p</samp>’). This isn’t an
incompatibility but a compatible extension, for both units coincide for
all devices without a <code class="code">sizescale</code> parameter in the <samp class="file">DESC</samp>
file, including all postprocessors from <abbr class="acronym">AT&T</abbr> and
<code class="code">groff</code>’s text devices. The few <code class="code">groff</code> devices with a
<code class="code">sizescale</code> parameter either do not exist for <abbr class="acronym">AT&T</abbr>
<code class="code">troff</code>, have a different name, or seem to have a different
resolution. So conflicts are very unlikely.
</li><li>The position changing after the commands ‘<samp class="samp">Dp</samp>’, ‘<samp class="samp">DP</samp>’, and
‘<samp class="samp">Dt</samp>’ is illogical, but as old versions of <code class="code">gtroff</code> used this
feature it is kept for compatibility reasons.
</li></ul>
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