Re[4]: arrowheads too long in PostScript terminal

[Alan G I. <ai...@am...>]

> 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.

On Tue, 21 Oct 2003 11:20:41 -0700 Ethan Merritt <merritt@u.washington.edu> wrote:
> Gnuplot does that now.  What we are disagreeing about is the
> meaning of the word "precisely".

Actually I think we are disagreeing on the more basic words
'start' and 'end'.  I have not been able to understand your
apparent suggestion (?) that the user will expect ink past
ptB (in the direction of the arrow).

On Tue, 21 Oct 2003 11:20:41 -0700 Ethan Merritt <merritt@u.washington.edu> wrote:
> You may not like the behavior, but that doesn't mean it
> is ambiguous.

I think you are giving a programmer's perspective rather 
than a user's perspective on ambiguity here.  Of course,
the code says what it says.  But a user has no simple
way to tell where an arrow will end. (I.e., where the
ink will end.)

On Tue, 21 Oct 2003 11:20:41 -0700 Ethan Merritt <merritt@u.washington.edu> wrote:
> 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.

I accept that if I draw several independent (i.e., unjoined)
line segments end to end that round caps will produce a
better looking line.  But this is not how people use arrows.
Or rather, I don't know anyone using arrows this way.
And those who do have a round caps option available to them.

> I personally think that rounded ends are best.
> But it should be left up to the user, as it is now.

No, it isn't.  Not for arrows with heads.  Selecting butt
caps does not cause arrowhead ink to terminate at the
specified end point.


> But they don't.   They extend exactly to the specified
> 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.

Stroke the tail to where, exactly?  (That is the 
user interface issue!)

A picture is worth a thousand words.  How can the sript
below be considered to produce desirable output for these
script commands?  I still say the key issue is: what will
users expect, and what makes it easiest for them to achieve
precisely what they desire in a predictable fashion?  When
the user wants an arrow from ptA to ptB, that really ends at
ptB (i.e., no ink past ptB), s/he should not have to make
length adjustments everytime s/he changes the linewidth or
arrowhead angle.  Yet that is the current situation.  I
cannot see that this behavior (in the sample script) can
meet the goal of being predictable for the user.  And I add
to that a personal query: thinking back over your own use of
arrows in gnuplot, can you really claim that your *intent*
when you first think of drawing an arrow from ptA to ptB
is for the ink to extend past ptB?

Alan Isaac

##############################################################
set term post eps color solid butt
set output 'c:\temp3.eps'
set xrange [0:4]
set yrange [0:5]
set style arrow 1 front head size 0.2,15 nofilled lt -1
set style arrow 2 front nohead lt -1
set style arrow 3 front nohead lw 5 lt -1
set style arrow 4 front head size 0.2,15 nofilled lw 5 lt -1
set arrow from  1,1 to 3,1 as 1
set arrow from  1,2 to 3,2 as 2
set arrow from  1,3 to 3,3 as 3
set arrow from  1,4 to 3,4 as 4
plot '-' with points ps 1 pt 7 lt 1
3 1
3 2
3 3
e
##############################################################