Re: Re[2]: arrowheads too long in PostScript terminal

[Ethan M. <merritt@u.washington.edu>]

On Monday 20 October 2003 23:14, Alan G Isaac wrote:
> Basic user interface issue:
> set arrow from ptA to ptB
> will be expected to do precisely that:
> start at ptA and end at ptB.

Gnuplot does that now.  What we are disagreeing about is the
meaning of the word "precisely".

> Basic implementation issue:
> there should be no ambiguity about what "from" and "to" mean.
> Additionally, for filled arrows (and on capable terminals
> *all* heads can be drawn as filled) this keeps the drawing
> computations as simple a possible.  (Related to this:
> an arrow without a head should start and stop and the
> same precise location as an arrow with a head!)

But all of this is true right now, so far as I know.
You may not like the behavior, but that doesn't mean it
is ambiguous.

> Finally, on the interpretation of where an arrow goes, I am
> not proposing an idiosyncratic interpretation.  To the
> contrary, I believe it is very standard.  (Compare e.g. the
> LaTeX picture environment.) =20

But TeX conceives of arrows as characters, and as such
they are entirely enclosed within a conceptual character
cell.  If you want to embed TeX/LaTeX arrows in your
gnuplot figure, you can do so using 'set term post enhanced'

By contrast think of line segments in gnuplot as a
connect-the-dots process:  place a dot at the "from"
position, place another at the "to" position, then connect
them.

> And contrary to Ethan's claim,
> I argue that this provide a good match for how lines are
> handled. Lines are generally expected *not* to project
> beyond the path endpoint.  Using PostScript terminology, the
> natural presumption is a butt cap, and this is the natural
> default.=20

People's expectations obviously vary, and this leads to
complaints from time to time. In particular, the PostScript and X11=20
implementation of butt caps causes the line segment to
terminate along a line perpendicular to the direction of the line
segment itself. This means that in general it overshoots the
x-coordinate, the y-coordinate, or both, of the "to" specification.
There was, for example, a recent complaint here that line segments=20
should never extend beyond the x-coordinate specified;
i.e. the line segment fill should always be clipped against
a vertical boundary at the endpoints.

Furthermore, butt caps cause very ugly line joins.
See previous discussions about the use of polylines=20
rather than individual line segments in X11.

> Ethan seems to be suggesting that choosing a
> projecting square cap should be the default.  I cannot
> recognize this as a standard practice.

No. I personally think that rounded ends are best.
But it should be left up to the user, as it is now. =20

> But after all is said and done, they key issue is: what will
> users expect, and what makes it easiest for them to achieve
> precisely what they desire in a predictable fashion?  I was
> personally startled to find that arrows project past their
> assigned end point

But they don't.   They extend exactly to the specified=20
endpoint *** to the resolution of the pen used ***.

> What is more, with
> this behavior it is essentially impossible for me to make an
> arrow terminate precisely where I wish it to.

I don't see why.  Can't you just specifiy a very thin
pen width and a filled arrowhead?  Stroke the tail of
the arrow separately if you want that part thicker.

--=20
Ethan A Merritt       merritt@u.washington.edu
Biomolecular Structure Center Box 357742
University of Washington, Seattle, WA 98195