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Re: Catch the area outside a contour [message #44099]

Sat, 21 May 2005 06:33

Chris Lee
Messages: 101
Registered: August 2003

Senior Member

James wrote:
> Hi Folks !
>
> Can anyone help me with the following problem. For example, there is an
> area
> of the image withing some contour level. How to determine and get
> parameters for a region otside that contour level withing the next N
> pixels ?
> Is there any IDL procedure/function for that ?
>
> Thanks in advance,
> James

You can get the X/Y coordinates of a contour levels with the PATH_INFO
keyword (and a few others). If you then want a fixed area outside this
you need to either

1. Use an IDL built in function (whose name I can't remember) which
'expands' an area by a fixed amount (then subtract the original)

or.

2. Draw the new path yourself. This would involve something like finding
the local normal at each point along the path. Calculate the path length
of the contour, expand along the the local normal to get the desired area.

Chris.

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Re: Catch the area outside a contour [message #44100 is a reply to message #44099]

Sat, 21 May 2005 00:36

Adam[1]
Messages: 2
Registered: May 2005

Junior Member

I'm sorry, I don't seem to understand the problem. Are you trying to
extract how many pixels are outside a given contour level? And by
"Fixed Size" do you mean that you want every pixel to represent the
same value of area? I may be able to help with some elaboration on the
topic.

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Re: Catch the area outside a contour [message #44112 is a reply to message #44100]

Fri, 20 May 2005 07:16

James[1]
Messages: 9
Registered: August 2004

Junior Member

> contourLevel = .5
> outside = where(array LT contourLevel)
>
> here "outside" will hold an index for every pixel that is below the
> contour level.

Thanks for the answer, but this is not exactly what is needed. The point
is to get a *fixed size* area outside the contour in pixels, but not
in units of contour levels.

Imagine, there is a smooth image and I'm going to extract some region
oustide the contour. Since the gradient is negligible, operating with s
contour
level you can give a region of 1000.. pixels wide (depends on the
gradient) ...

James

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Re: Catch the area outside a contour [message #44115 is a reply to message #44112]

Fri, 20 May 2005 07:08

Adam[1]
Messages: 2
Registered: May 2005

Junior Member

Im not sure this is what you are asking for but if you want all
elements that are below or above the contour then you can use the
function where().

;array is the array containing your contour data.
;contourLevel = the value of the contour line you are interested in.

contourLevel = .5
outside = where(array LT contourLevel)

here "outside" will hold an index for every pixel that is below the
contour level.

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Re: Catch the area outside a contour [message #44190 is a reply to message #44100]

Mon, 23 May 2005 04:20

James[1]
Messages: 9
Registered: August 2004

Junior Member

> I'm sorry, I don't seem to understand the problem. Are you trying to
> extract how many pixels are outside a given contour level? And by
> "Fixed Size" do you mean that you want every pixel to represent the
> same value of area? I may be able to help with some elaboration on the
> topic.

Chris Lee gave the right idea ! The question now, what is the name
of that function, which allows one to expand the area ... Anybody
can help with that ? I'm trying to find someting in IDL's help, but
still without a success

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