| Unexpected rebin behavior [message #36271] |
Wed, 03 September 2003 09:20  |
mchinand
Messages: 66
Registered: September 1996
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Performing rebin on 2-d (or any higher dimension) integer arrays sometimes
give results I didn't expect. Here's a trivial example array:
IDL> help, b
B INT = Array[2, 2]
IDL> print, b
8 9
10 13
Minifying this to a 1x1 array with rebin gives:
IDL> print, rebin(b,1,1)
9
But,
IDL> print, fix(total(b)/4)
10
which is what I would have expected as the result from rebin. I think
what rebin is doing is first performing a rebin in one direction and then
performing it in the other direction on the new, intermediate rebinned
array. So the above is equivalent to:
IDL> print, rebin(rebin(b,1,2),1,1)
9
Switching the order of the rebinning yields a different result:
IDL> print, rebin(rebin(b,2,1),1,1)
10
Or equivalently:
IDL> print, rebin(transpose(b),1,1)
10
I was using rebin to reduce the size of an image and expected it to do
neighborhood averaging for the new array. The integer truncation after
rebinning along each direction can lead to different results than
averaging the neighborhood in a single step. I guess this isn't really a
bug (the manual only describes how rebin works in one dimension), but was
unexpected, at least for me. I simple work-around is to cast the array to
float and then back to integer (or just keep as float) after the rebin:
IDL> print, fix(rebin(float(b),1,1))
10
--Mike Chinander
--
Michael Chinander
m-chinander@uchicago.edu
Department of Radiology
University of Chicago
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