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Thanks, Dave.
I tried to cover some of this material in an earlier commit.
commit 1228acc430b2b9638c598a223496ceebfed778af Author: G. Branden Robinson <g.branden.robinson@gmail.com> Date: Thu Feb 3 04:05:14 2022 +1100 [docs]: Present several fundamental concepts. * doc/groff.texi (Page Geometry): * man/roff.7.man (Concepts): Add new node/section defining concepts. - device resolution - basic units - drawing position - text baseline - page offset - vertical spacing - vee - page break
comment #4:
> 1) the escape sequence '\v' is for local motions (are contained within a > line, see CSTR #54, paragraph 11.1). > The absolute position indicator '|' does not make sense there.
The groff manual says that \v can take the | modifier; it describes \v as: "Move vertically; usually from the current location on the page (if no absolute position operator '|' is used)."
> 2) The absolute position '0' (zero) is the first usable printable line, > not the distance from the edge of the paper. > The edge can't be a printable line.
Thank you, this is useful information, though it seems to apply more to bug #60820, which seeks to clarify how | works, than to this bug, which requests a reliable way (separate from |, which has an entrenched troff history) to measure from the page boundary.
Some comments:
1) the escape sequence '\v' is for local motions (are contained within a line, see CSTR #54, paragraph 11.1). The absolute position indicator '|' does not make sense there.
2) The absolute position '0' (zero) is the first usable printable line, not the distance from the edge of the paper. The edge can't be a printable line. Traps at position '0' are thus always sprung.
A test case
.pl 5v .vs 10p .ps 12 Word .sp |0c Second
comment #2:
> Rather conceitedly, I like to think of myself as somewhat above novice level, but I still find the vertical "|" semantics unintelligible.
Fair point--I also feel my grasp on them is not quite solid. Once I've done my experiments and written my planned "Drawing Position" Texinfo node, maybe that will change. Maybe I should have said "non-masters". :D
Rather conceitedly, I like to think of myself as somewhat above novice level, but I still find the vertical "|" semantics unintelligible.
Thanks for logging this, Dave.
Marking as "Need Info" since there is still some design work to do. I have some ideas coming together regarding a "Drawing Position" section for Chapter 5 of our Texinfo manual. Possibly, once I actually get that composed and pushed, it can help continue the discussion from a common basis of understanding (since I'm sure people will point out any erroneous claims I might make), and even if it doesn't do that, a presentation of how the page geometry is discretized might make the weird semantics of "|" more intelligible to novices.
The | modifier for absolute vertical positioning is less than ideal.
One drawback, pointed out in bug #60820, is that it is not quite the absolute-position indicator it purports to be, when used with, e.g., the \v escape.
Another -- pointed out in an October email thread (http://lists.gnu.org/r/groff/2021-10/msg00016.html) and, as of Sunday's commit f7bbcd8f, documented in Texinfo (though alluded to for many years before that, by the existence of the example "y-from-top-down" macro in the .sp section) -- is that when used in an .sp request, the motion unavoidably goes 1v further down than requested. (There is a related issue with .mk/.rt.)
Brainstorming took place in the aforementioned thread, but whether the new mechanism should be a new request, a new distance modifier akin to |, or something else, has not been decided.
(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)
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