Re: [Groff] Problems with arcs and angles

[Deri James]

On Tue 09 May 2017 13:55:38 John Gardner wrote:
> Looks like there's a mistake in groff_out(5). It describes its arc-drawing
> command as:
> 
> *Draw arc from current position to (h1, v1)+(h2, v2) with center at (h1,
> 
> > v1);*
> 
> However, gropdf.pl tells a different story. To quote line #2791:
> <http://git.savannah.gnu.org/cgit/groff.git/tree/src/devices/gropdf/gropdf.p
> l?id=453a8aa7c8f8dd0c78160466301f81be8c40df2e#n2791>
> 
> *Documentation is wrong. Groff does not use Dh1,Dv1 as centre of the
> circle!*
> 
> I noticed something odd when drawing an arc which wasn't an evenly-sized
> quarter-circle. For instance, the following Pic code:
> 
> .PS
> 
> > move down
> > A: box
> > arc at A.n
> > line right
> > .PE
> 
> ... generates the following output:
> > …
> 
> V84000
> 
> H99000
> 
> > Da 0 -36000 18000 54000
> 
> Which looks like this in gropdf and my own canvas code, respectively:
> 

In gropdf I adjust the circle centre using code I found in the geometry.cpp 
library used by the grops driver, so that I could duplicate the output 
produced by grops.