Output Language Compatibility (The GNU Troff Manual)

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6.1.4 Output Language Compatibility

+ +

The intermediate output language of AT&T troff was +first documented in A Typesetter-independent TROFF, by Brian +Kernighan, and by 1992 the AT&T troff manual was +updated to incorprate a description of it. +

The GNU troff intermediate output format is compatible with this +specification except for the following features. +

The classical quasi-device independence is not yet implemented. + +
The old hardware was very different from what we use today. So the +groff devices are also fundamentally different from the ones +in AT&T troff. For example, the AT&T +PostScript device is called post and has a resolution of only 720 +units per inch, suitable for printers 20 years ago, while groff’s +ps device has a resolution of 72000 units per inch. Maybe, by +implementing some rescaling mechanism similar to the classical +quasi-device independence, groff could emulate AT&T’s +post device. + +
The B-spline command ‘D~’ is correctly handled by the intermediate +output parser, but the drawing routines aren’t implemented in some of +the postprocessor programs. + +
The argument of the commands ‘s’ and ‘x H’ has the +implicit unit scaled point ‘z’ in gtroff, while +AT&T troff has point (‘p’). This isn’t an +incompatibility but a compatible extension, for both units coincide for +all devices without a sizescale parameter in the DESC +file, including all postprocessors from AT&T and +groff’s text devices. The few groff devices with a +sizescale parameter either do not exist for AT&T +troff, have a different name, or seem to have a different +resolution. So conflicts are very unlikely. + +
The position changing after the commands ‘Dp’, ‘DP’, and +‘Dt’ is illogical, but as old versions of gtroff used this +feature it is kept for compatibility reasons. + + +

+ + + +