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Re: Multi-band sampling strategy [message #72171 is a reply to message #72170]

Wed, 18 August 2010 12:46

wita
Messages: 43
Registered: January 2005

Member

Dear Jess,

The procedure that you are now using is fetching a complete slice of
your image
for each sample location. I do not know how large your image is, but
if I take
a typical TM scene (6000x6000 pixels), then many of your 38,000 pixels
will
actually be positioned in the same slice. So you are probably fetching
the
same slice a couple of times from your image just to take out another
sample
location. The more sample locations you have, the worse the speed
penalty gets.

Instead, what you could do to speed this up is first sort out which
sample
locations fall on which slice and group them together. There is
actually a
very efficient way to accomplish this in IDL by using the Histogram()
command
in combination with the REVERSE_INDICES. If you have never heard of
this then
please have a look at the histogram tutorial on David Fannings site to
understand the trick.

So in pseudo code it could look like this, where I assume that
SampleLines,
SampleCols store the line/column numbers of your sample positions:

h = Histogram(SampleLines, MIN=0, REVERSE_INDICES=ri)
FOR j=0, N_Elements(h)-1 DO BEGIN
IF ri[j+1] GT ri[j] THEN BEGIN
SampleColsinLine = samplecols[ri[ri[j]:ri[j+1]-1]]
SampledSpectra = Get_Sample_Spectra(fid, j, SampleColsinLine)
ENDIF
ENDFOR

with the function Get_Sample_Spectra() looking like this:

FUNCTION get_sample_spectra, fid, line, samplecols
Slice = ENVI_Get_Slice(FID=fid,line=line, pos=bandsSelected)
RETURN, Slice[samplecols, *]
END

This has the additional advantage that you are progressing through the
file
in the order in which it is stored on disk. This automatically takes
advantage
of the read-ahead capabilities of operating systems and disk drives.

Creating point ROIs as Maxwell suggest is, in my experience, also
becoming
very slow for a large number of samples. The suggestion by Paulo is
inherently
limited by the size of your memory. Moreover, it is very inefficient
for small
sample sizes.

Hope this helps.

Allard

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[Message index]

		Re: Multi-band sampling strategy By: Maxwell Peck on Wed, 18 August 2010 14:11
		Re: Multi-band sampling strategy By: JessW on Wed, 18 August 2010 13:58
		Re: Multi-band sampling strategy By: wita on Wed, 18 August 2010 12:46
		Re: Multi-band sampling strategy By: penteado on Wed, 18 August 2010 11:09
		Re: Multi-band sampling strategy By: JessW on Wed, 18 August 2010 10:26
		Re: Multi-band sampling strategy By: Maxwell Peck on Tue, 17 August 2010 23:35
		Re: Multi-band sampling strategy By: JessW on Wed, 25 August 2010 11:13
		Re: Multi-band sampling strategy By: wita on Tue, 31 August 2010 02:08

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