What's arcto good for?

[luser droog]

What's 'arcto' good for? It arose mysteriously after the original red book
apparently to help make rounded corners for screen windows and boxes.
But I could never figure out how to use it. Does anybody use it?

It seems like it might be applicable to my issue in Julian's thread about
the stylized '9' where I have the spans from one side of a strokepath but
no joins.

[edspike...@gmail.com]

%!
% draw rectangle with rounded corner : x y width height radius
/rectar {
4 index 3 index 2 div add % x y w h r (x + w/2)
4 index % x y w h r x y
moveto % x y w h r

4 index 3 index add 4 index % x y w h r x2 y2
1 index 1 index 5 index neg add % x y w h r x2 y2 x3 y3
4 index % x y w h r x2 y2 x3 y3 r
arcto 4 {pop} repeat % x y w h r

4 index 3 index add % x y w h r x3
4 index 3 index neg add % x y w h r x3 y3
6 index 1 index % x y w h r x3 y3 x4 y4
4 index % x y w h r x3 y3 x4 y4 r
arcto 4 {pop} repeat % x y w h r

4 index 4 index 3 index neg add % x y w h r x4 y4
6 index 6 index % x y w h r x4 y4 x y
4 index
arcto 4 {pop} repeat % x y w h r

4 index 4 index % x y w h r x y
1 index 5 index add 1 index % x y w h r x y x2 y2
4 index
arcto 4 {pop} repeat % x y w h r

closepath
5 {pop} repeat % clean stack
stroke
} def

/wid 10 def
wid setlinewidth
/x 400 def
/y 400 def
/dx 100 def
/dy 50 def
/rad wid def
x y dx dy rad rectar
showpage

[Mark Carroll]

On 25 Jun 2019, luser droog wrote:

> What's 'arcto' good for? It arose mysteriously after the original red book
> apparently to help make rounded corners for screen windows and boxes.
> But I could never figure out how to use it. Does anybody use it?

A glance through some code suggests that I do not; for that kind of
thing I tend to use curveto. I do notice arcto cropping up in some Xfig
diagrams converted by fig2dev though.

-- Mark

[John Reiser]

On 6/25/19 22:29 UTC, luser droog wrote:
> What's 'arcto' good for?

The arc is a portion of a circle, which is easier to draw
than a general bezier curve. The same DDA (digital difference
analyzer) that draws a 2D line on a grid of square pixels,
can be trivially modified to draw up to 1/8 of a circle.
The NEC 7220 graphics chip (circa 1981: 38 years ago)
had this enhancement. Among other things this was used
to draw the symbols for logic gates (NAND, NOR) at
varying scales into the frame buffer for a CRT display.