Polar Plot, determining Angle

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D-Dog - 07 Apr 2007 17:01 GMT

Hi,
I have a polar plot graph plotted on a JPanel, and I would like to
determine the angle selected around that graph, given the x,y position
of the mouse. I know this is probably more of a math (trig) question,
but I figured I would check here first, as I'm sure someone has dealt
with this before. I know how to retrieve the x,y position given the
radius, center_point, and angle, just not how to do the reverse of
that.

Thanks,

Dennis

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Patricia Shanahan - 07 Apr 2007 17:08 GMT

> Hi,
> I have a polar plot graph plotted on a JPanel, and I would like to
[quoted text clipped - 4 lines]
> radius, center_point, and angle, just not how to do the reverse of
> that.

I've never done this in Java graphics, so I don't know if there is any
graphics-specific method. On general trig principles, if the center
point is at (x1,y1), I would use Math.atan2(y-y1,x-x1).

Patricia

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Lew - 07 Apr 2007 17:18 GMT

>> Hi,
>> I have a polar plot graph plotted on a JPanel, and I would like to
[quoted text clipped - 8 lines]
> graphics-specific method. On general trig principles, if the center
> point is at (x1,y1), I would use Math.atan2(y-y1,x-x1).

In the graphics libraries, e.g.,
<http://java.sun.com/javase/6/docs/api/java/awt/Graphics2D.html>
, the sign of y is reversed.

> This default transform maps the user space coordinate system to screen and printer device coordinates such that the origin maps to the upper left hand corner of the target region of the device with increasing X coordinates extending to the right and increasing Y coordinates extending downward.

Signature

Lew

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Patricia Shanahan - 07 Apr 2007 17:57 GMT

>>> Hi,
>>> I have a polar plot graph plotted on a JPanel, and I would like to
[quoted text clipped - 18 lines]
>> coordinates extending to the right and increasing Y coordinates
>> extending downward.

Very good point. If the OP uses something like -r*Math.sin(theta) to get
the y coordinate, the y coordinate sign also needs to be flipped in the
atan2 call.

Patricia

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D-Dog - 08 Apr 2007 01:02 GMT

Thank you! I already ran across the Y coordinate inverse problem when
I was orginally determined the x,y. Your information will help a
great deal.

Thanks again.

Dennis

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Polar Plot, determining Angle

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