On 10:58 Mon 13 Aug , Erik Max Francis wrote:

Steve Holden wrote:

About the best interpretation I can think of is to add 180 degrees to

the angle and reverse the sign of the magnitude, but this would be a

hack. Where are those coordinates coming from?

Well, sometimes in polar coordinates (r, theta), r is allowed to be

negative. The usual translation from polar to Cartesian coordinates

makes this meaningful, albeit weird, so in effect the resulting

positions are just reflections around the origin.

Which I suppose is what the original poster was asking about, but it's

still not clear.

Many years ago when I started programming machine tools (on punched

paper tape) if you wished to specify a cutter path around a radius as

being more than 180 degrees you programmed it as a negative r value.

There are 2 possible paths from x1y1 to x2y2 along a radius r and going

in the same direction; that less than 180 deg and that more than 180

deg, unless the radius is exactly 180. But this was rarely used, the

other method of specifying the end point as an incremental value in

relation to the radius centre is less error prone when the arcs are

close to 180.

Sorry about the slight diversion but I'm getting nostalgic.

Regards, John

--

War is God's way of teaching Americans geography

Ambrose Bierce (1842 - 1914)