Re[3]: arrowheads too long in PostScript terminal

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

> On Fri, 31 Oct 2003, Alan G Isaac wrote:
>> I found two widespread conventions:
>> i. The arrowhead ink ends right at the point the arrow is
>> drawn to.  This is the convention that I have been pushing
>> for.
>> ii. The arrowhead ink ends right at the edge of a filled
>> circle, the center of which is the point that the arrow
>> is drawn "to".  I find this odd conceptually, but I confess
>> it does produce some nice looking graphics.

On Sat, 01 Nov 2003, () Hans-Bernhard Broeker apparently wrote:
> These two conventions aren't necessarily all that different as they
> appear.  Think of the first as a special case of the second, with the
> curve radius of the tip reduced to zero --- or more precisely to some
> value too small to be visible as a round tip, so it appears sharp.
> Obviously, this also requires to interpret the tip radius as a feature
> independent of the arrow linewidth.

You appear to be describing a 3rd case, that I didn't come
across.  Here is what I hear you saying:
iii. arrowhead has a rounded rather than sharp tip.

If (iii) is what you meant, I would still expect the ink not
to pass the point the arrow is drawn "to" (in the direction
of the arrow).

What I meant by (ii) is perhaps better captured by:
(ii') a filled circle with radius 'r' centered on the "to"
point 'ptB' is drawn, and then the arrow from ptA to ptB is
drawn so that its very tip ends at the edge of the filled
circle.  Thus the arrow length will be 'r' less than the
distance from ptA to ptB.

As I said, I personally find the practice a bit odd, bit it
appears reasonably common.  (I also came across this
practice for headless arrows.  E.g., construct a line
segment between two points.)

It is still the case that (i) is a special case of (ii')
with r=0.

> Summing it up, the core problem all around is that arrows
> shouldn't ever have been drawn using wide lines in the
> first place.

I agree with this principle, but of course on
terminals that do not support filling one can
still draw thick lines but then shorten the arrow
by the miter length.

One odd thing about the current PostScript terminal is that
it draws a filled head and then "outlines" it with a thick
line, and it is only the superfluous outline that causes
ink to extend past the "to" point.  If this is true of
all terminals supporting filling, then filled arrowheads
at least will be simple to fix.  (Of course, as you have
indicated, on such terminals *all* arrowheads could be
drawn as filled outlines.)

Cheers,
Alan

PS One last thing I just want to mention again since I do not
recall any discussion of it: the 'vectors' plotting style
will only produce sensible results if the ink for each
vector starts and stops at exactly the specified points.
I suppose this is currently addressed to a certain extent
by refusing to honor 'linewidth' for plots in the
'vectors' style.  (But shouldn't 'linetype' be allowed
for color selection?)