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Re: "absolute" vertical position seems awfully relative

From: Dave Kemper
Subject: Re: "absolute" vertical position seems awfully relative
Date: Thu, 24 Jun 2021 20:06:17 -0500

On 6/22/21, Tadziu Hoffmann <> wrote:
> I think \v'|100000u' does not mean "space to an absolute vertical
> position of 100000 units".  It means "space vertically a distance
> corresponding to the difference between the current point and
> the absolute vertical position of 100000 units".
> Normally these would be equivalent in effect, except for the
> combination of initial pseudo-page transition and trap-invoked
> page header macro (which you don't have if you're using plain
> groff and have not set one).

Thank you for the very detailed explanation, Tadziu!

I'm still trying to wrap my head around it -- both the mechanism, and
the rationale behind it.  "Absolute position" seems like a useful
concept, whereas the complex thing actually being done seems less
useful.  Are there use cases for troff's behavior here?  Or was it
merely simpler to implement?

In any case, the groff Texinfo documentation on this is inadequate, as
it describes | only as indicating an absolute position.  I opened for this.

CSTR#54 hedges a little more: it does call | the "absolute position
indicator" but then goes on to walk that back a bit, saying "For
vertically-oriented requests and functions, |N becomes the distance in
basic units from the current vertical place on the page or in a
diversion... to the vertical place N."  This seems to more closely
reflect the behavior you described, though I feel like it's a little
too terse to really communicate it clearly.  So thank you for your
full analysis.

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