| Index of a sector [message #24564] |
Wed, 04 April 2001 16:18  |
Pavel A. Romashkin
Messages: 531
Registered: November 2000
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Senior Member |
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Hi,
I tried to solve this yesterday, but it gave me a headache. I tried
again today, but the headache is back and I did not get any closer to
the solution. It is very simple. Well after all, what can you ask from a
Mac user.
If you have a (square) 2D array, let's say, DIST(200, 200), how to
obtain the index of points enclosed by a sector of a given radius, drawn
from the corner of the array (point [0, 0])? Foe instance, radius 50?
So, I will have
0,1,2,...50
101,102,...,149 (or so)
201,202,...
...
501,...?
I thought to try to cut all corners and use IDLanROI, but for some
reason, I think that JD or Craig have the answer ready. Besides, David
tried anROI... how can I dream of succeeding where the Titan (read:
Coyote) failed?
Thanks,
Pavel
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| Was: Index... Now: Vectorize, huh? [message #24611 is a reply to message #24564] |
Thu, 05 April 2001 12:24  |
Pavel A. Romashkin
Messages: 531
Registered: November 2000
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Senior Member |
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Craig and Med,
I appreciate it! This is exactly what I needed. My problem is the lack
of matrix operations knowledge. Craig's generic solution is exactly what
I expected to see from Craig :-)
By the way. We all are big on vectorizing things in IDL. But look at this:
IDL> a = test(2000)
1.4968959
IDL> a = test(2000, /v)
3.2976190
where TEST is below. I don't even mention that /VEC causes extremely
high memory usage and gets totally out of hand on my system if S > 5000
or so.
;******************************
pro test, s, vec=vec
start = systime(1)
x = findgen(s)
a = fltarr(s, s)
if keyword_set(vec) then begin
a = sqrt(transpose(rebin(x, s, s))^2 + rebin(x, s, s)^2)
endif else begin
for i = 0, s-1 do begin
a[0, i] = sqrt(x^2 + i^2.)
endfor
endelse
print, systime(1) - start
;return, a
end
;******************************
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| Re: Index of a sector [message #24614 is a reply to message #24564] |
Thu, 05 April 2001 11:42 |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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"Pavel A. Romashkin" <pavel.romashkin@noaa.gov> writes:
> Hi,
>
> I tried to solve this yesterday, but it gave me a headache. I tried
> again today, but the headache is back and I did not get any closer to
> the solution. It is very simple. Well after all, what can you ask from a
> Mac user.
>
> If you have a (square) 2D array, let's say, DIST(200, 200), how to
> obtain the index of points enclosed by a sector of a given radius, drawn
> from the corner of the array (point [0, 0])? Foe instance, radius 50?
> So, I will have
>
> 0,1,2,...50
> 101,102,...,149 (or so)
> 201,202,...
> ...
> 501,...?
Complex geometric selections can be easy if you do the right thing.
Pavel, I'm sure that Med's suggestion can work for you but I thought I
would expand on it a little more for the general case.
You start by labelling your pixels in X and Y. You might have
physical labels attached to each pixel, or they could just be pixel
numbers.
x = findgen(200) & y = findgen(200) ;; Just an example here!
Now expand these vectors into matrices along their respective axes:
xx = x # (fltarr(200)+1) ;; Now XX labels every pixel in image
yy = (fltarr(200)+1) # y ;; as does YY
Now compute anything you want in this coordinate system, say radius:
rr = sqrt(xx^2 + yy^2) ;; could compute theta as, th = atan(yy, xx)
And then do your selection:
wh = where(rr LE 50 AND xx GE 0 AND yy GE 0)
You are done. As you can see you can form very complex expressions
involving the radius and angle, and if you have multiple centers this
can be accomodated too. The above expansions of X into XX and Y into
YY are also the basis of fitting images in MPFIT2DFUN.
Good luck,
Craig
--
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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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