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Re: Quick question regarding "where" function [message #45825]

Wed, 12 October 2005 14:41

K. Bowman
Messages: 330
Registered: May 2000

Senior Member

In article <1129151161.183821.264480@g14g2000cwa.googlegroups.com>,
"ChiChiRuiz@gmail.com" <ChiChiRuiz@gmail.com> wrote:

> I have an image of pixels (xsize by ysize), each pixel is associated
> with a value. Say I'd like to know which pixels have value over 20. I
> need to book-keep the values as well as the location of the pixel (i.e.
> x, y values). I started with
>
> indices = where (image GT 20)
>
> now I have a list of indices, what do i do next? How do I go backwards
> to retrieve the values and the locations of the pixels that pass the
> test?
>
> thanks for your help.

values = image[indices]
locations = ARRAY_INDICES[image, indices]

But never do a WHERE without a count and subsequent IF

indices = WHERE(image GT 20, count)
IF (count GT 0) THEN ...

Cheers, Ken Bowman

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Re: Quick question regarding "where" function [message #45826 is a reply to message #45825]

Wed, 12 October 2005 14:40

Michael Wallace
Messages: 409
Registered: December 2003

Senior Member

ChiChiRuiz@gmail.com wrote:
> I have an image of pixels (xsize by ysize), each pixel is associated
> with a value. Say I'd like to know which pixels have value over 20. I
> need to book-keep the values as well as the location of the pixel (i.e.
> x, y values). I started with
>
> indices = where (image GT 20)
>
> now I have a list of indices, what do i do next? How do I go backwards
> to retrieve the values and the locations of the pixels that pass the
> test?

The locations of the pixels are the numbers in your indexes variable.
Remember that the where function (and others) index into your array as
if it were one-dimensional rather than two-dimensional, so will only see
these 1-d indexes rather than pairs of x,y indexes.

The values greater than 20 can be returned by indexing into your
original array with the array of indexes you just got from the where
function: values = image[indexes]

-Mike

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Re: Quick question regarding "where" function [message #45923 is a reply to message #45825]

Wed, 19 October 2005 15:09

ChiChiRuiz@gmail.com is currently offline

ChiChiRuiz@gmail.com
Messages: 32
Registered: October 2005

Member

Thank you very much!

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