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Re: Points in a rectangle with an angle [message #48226 is a reply to message #48222]

Wed, 05 April 2006 09:53

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

JJMeyers2@gmail.com writes:
> Hello,
>
> I have a problem in IDL that I was wondering if anyone has any idea how
> it can be done.
> I have a set of coordinates and I am trying to figure out how many of
> the coordinates are inside a rectangle. I know the coordinates of the 4
> edges of the rectangle but the problem is that the rectangle is at an
> angle in the x,y axis ( I know the slope). I can not just say
> coords=where((x LE xmax) AND (x GT xmin) AND (y LE ymin) AND (y GT
> ymax))
> because that will give me coordinates of a rectangle without an angle
> (parallel to the y axis).
> Is there any function in IDL that might be doing that?

Not really, but it's pretty easy to do yourself.

Assume that the rectangle is centered at (XCENT,YCENT) and has a WIDTH
and HEIGHT in its body coordinate system. Also, assume that the
rectangle is rotated THETA degrees counterclockwise from the "world" X axis.

;; Translate to center-of-box coordinates (xp,yp)
xp = x - xcent
yp = y - ycent
;; Rotate into box coordinates (u,v)
u = cos(theta*!dtor)*xp - sin(theta*!dtor)*yp
v = sin(theta*!dtor)*xp + cos(theta*!dtor)*yp
;; Do the selection
wh = where(abs(u) LT width/2 AND abs(v) LT height/2)

This takes advantage of the fact that in center-of-box coordinates,
half of the box is left and right of the origin, and above and below
the origin. The only tricky part is getting the signs right, which I
think I did.

Craig

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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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[Message index]

		Re: Points in a rectangle with an angle By: Maarten[1] on Thu, 06 April 2006 04:37
		Re: Points in a rectangle with an angle By: Craig Markwardt on Wed, 05 April 2006 09:53
		Re: Points in a rectangle with an angle By: JJMeyers2 on Wed, 05 April 2006 10:08
		Re: Points in a rectangle with an angle By: K. Bowman on Wed, 05 April 2006 09:43
		Re: Points in a rectangle with an angle By: Rick Towler on Wed, 05 April 2006 09:58
		Re: Points in a rectangle with an angle By: David Fanning on Wed, 05 April 2006 09:48

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