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hi!
how can i do polar plot in python with negative axes
that is the value of the magnitude is negative

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  • Steve Holden at Aug 13, 2007 at 1:49 pm

    yadin wrote:
    hi!
    how can i do polar plot in python with negative axes
    that is the value of the magnitude is negative
    Surely a negative magnitude doesn't make sense in polar coordinates,
    since the magnitude is supposed to represent the distance from the center.

    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?

    regards
    Steve
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  • Erik Max Francis at Aug 13, 2007 at 5:58 pm

    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.

    --
    Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
    San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
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  • John K Masters at Aug 13, 2007 at 8:54 pm

    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)

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postedAug 13, '07 at 10:07a
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