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17. Interface To [gt]roff

The output of pic is [gt]roff drawing commands. The GNU gpic(1) command warns that it relies on drawing extensions present in groff(1) that are not present in troff(1).

17.1. Scaling Arguments

The DWB pic(1) program will accept one or two arguments to .PS, which is interpreted as a width and height in inches to which the results of pic(1) should be scaled (width and height scale independently). If there is only one argument, it is interpreted as a width to scale the picture to, and height will be scaled by the same proportion.

GNU gpic is less general; it will accept a single width to scale to, or a zero width and a maximum height to scale to. With two non-zero arguments, it will scale to the maximum height.

17.2. How Scaling is Handled

When pic processes a picture description on input, it passes .PS and .PE through to the postprocessor. The .PS gets decorated with two numeric arguments which are the X and Y dimensions of the picture in inches. The post-processor can use these to reserve space for the picture and center it.

The mgs macros, for example, include the following definitions:

.de PS
.sp \\n[DD]u
.ie \\n[.$]<2 .@error bad arguments to PS (not preprocessed with pic?)
.el \{\
.    ds@need (u;\\$1)+1v
.    in +(u;\\n[.l]-\\n[.i]-\\$2/2>?0)
.de PE
.sp \\n[DD]u+.5m

Equivalent definition will be supplied by GNU pic(1) if you use the −mpic option; this should make it usable with macro pages other than ms(1).

If .PF is used instead of .PE, the troff position is restored to what it was at the picture start (Kernighan notes that the F stands for “flyback”).

The invocation

.PS <file

causes the contents of file to replace the .PS line. This feature is deprecated; use ‘copy file’ instead).

By default, input lines that begin with a period are passed to the postprocessor, embedded at the corresponding point in the output. Messing with horizontal or vertical spacing is an obvious recipe for bugs, but point size and font changes will usually be safe.

Point sizes and font changes are also safe within text strings, as long as they are undone before the end of string.

The state of [gt]roff’s fill mode is preserved across pictures.

The Kernighan paper notes that there is a subtle problem with complicated equations inside pic pictures; they come out wrong if eqn(1) has to leave extra vertical space for the equation. If your equation involves more than subscripts and superscripts, you must add to the beginning of each equation the extra information space 0. He gives the following example:

box "$space 0 {H( omega )} over {1 - H( omega )}$"

Image grohtml-2197045.png

Figure 17-1: Equations within pictures

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