| Re: Maximum value array resampling [message #45018 is a reply to message #45017] |
Fri, 05 August 2005 16:09   |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Fri, 05 Aug 2005 10:55:46 -0700, rechoncho@yahoo.com wrote:
> I'm trying to figure out an IDL-efficient way to resample a series of
> images. I know how to do this is a ruinously laborious fashion using loops
> but I know there's any easier way.
>
> Consider the following 4x4 array:
>
> x = [$
> [0,3,4,5],$
> [1,2,7,0],$
> [3,2,9,0],$
> [7,0,5,6]]
>
> I want to resample this to y, a 2x2 array. Each element would contain the
> maximum value of the corresponding 4 pixels. y would then look like
>
> [$
> [3,7],$
> [7,9]]
>
> So element [0,0] in y is max(x[0:1,0:1]) etc. As I understand it,
> rebin/congrid won't do this. Each image is about 5000x2000 and there are
> several hundred to process.
For arbitrary images with both dimensions even:
d=size(x,/DIMENSIONS) & nx=d[0]/2 & ny=d[1]/2
y=transpose(max(reform(transpose(reform(x,2,nx,2*ny),[0,2,1] ), $
4,ny,nx),DIMENSION=1))
How does it work? It juggles dimensions so that the indices of all the
2x2 sub-arrays are next to each other in memory, and then uses
max(/DIMENSION) to collapse over them. The inner call to REFORM puts
them adjacent to each other, but in the wrong dimension, then TRANSPOSE
makes them adjacent in the fast-changing dimension. The rest is
straightforward.
There may be a quicker way with only one call to TRANSPOSE, but I
couldn't find it (anyone?). Also, if you don't care to keep X, throw a
couple of /OVERWRITE keywords for both REFORM statements, to save some
memory and time.
JD
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