| Re: Average over odd/even lines [message #51792] |
Fri, 08 December 2006 12:39 |
peter.eddy@shaw.ca
Messages: 19
Registered: March 2005
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Junior Member |
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Cool, I had never heard of the stride operator.
I will likely keep the code as you wrote it just in case the program is
used with another system with different dimentions.
Thanks for your help!
Pete
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| Re: Average over odd/even lines [message #51817 is a reply to message #51792] |
Thu, 07 December 2006 10:58  |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Wed, 06 Dec 2006 19:46:16 -0800, Pete wrote:
> Hi All,
>
> I am trying to write an IDL program for "smoothing" over lines of image
> data acquired with an aerial CCD system. This requires reading the odd
> lines, calculating the mean and placing it in the even. The images are
> a constant 640x480 pixels.
>
> i.e.)
> line 1 : 2 2 2 2...
> line 2 : x x x x...
> line 3 : 4 4 4 4...
>
> After processing,
>
> line 1 : 2 2 2 2...
> line 2 : 3 3 3 3...
> line 3 : 4 4 4 4...
>
> I can think of several ways to implement this but I thought the group
> may point me to the most efficient.
This is a good chance to use the mostly neglected stride operator for
IDL's range subscripts, which has the syntax [low:high:stride].
d=size(a,/DIMENSIONS)
x=indgen(d[0]) & y=findgen((d[1]-1)/2)+.5
a[*,1:d[1]-2:2]=interpolate(a[*,0:*:2],x,y,/GRID)
Note that the last line is unchanged for images with an even number of
lines (that comes from (d[1]-1)/2).
Since your dimensions never change, you can cache x & y and use them
over and over. For this reason as well, it might also be faster to
expand out [*,1:d[1]-2:2] etc. into index arrays and cache them,
rather than have IDL recompute them for each image. It would be
lovely if IDL provided a function to convert a given subscript syntax
into an array of indices, but I don't believe it has one.
JD
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| Re: Average over odd/even lines [message #51819 is a reply to message #51817] |
Thu, 07 December 2006 10:45 |
peter.eddy@shaw.ca
Messages: 19
Registered: March 2005
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Junior Member |
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Thanks Josiah,
This does work well, better than my original program with loops.
The last line will likely be clipped or maybe I can pad the image prior
to processing.
At any rate thanks for your suggestion,
Pete
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| Re: Average over odd/even lines [message #51836 is a reply to message #51819] |
Wed, 06 December 2006 20:54 |
jschwab@gmail.com
Messages: 30
Registered: December 2006
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Member |
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One caveat is that I'm not exactly sure how you want to deal with the
last even numbered line. But here's what I would probably do. (I'm sure
JD, David, or someone else can probably come up with something a bit
more clever.)
; picks off the odd rows
oddlines = oldimage[*, indgen(240) * 2]
; averages odd row with odd row below it by adding the odd lines
; to themselves shifted up a row
evenlines = (oddlines + shift(oddlines, -640)) * .5
; puts the even rows after the odd rows
; and then reshapes so they are below
new_image = reform([oddlines, evenlines], 640, 480)
The shift statement moves row 3 -> row 1, and row 1 -> row 479, that
means in the final image, row 480 is the mean of row 1 and row 479,
which is probably not how you want it.
Other than the last row, I think this is a decent way, though it
doesn't use histogram. :-)
Cheers,
Josiah
Pete wrote:
> Hi All,
>
> I am trying to write an IDL program for "smoothing" over lines of image
> data acquired with an aerial CCD system. This requires reading the odd
> lines, calculating the mean and placing it in the even. The images are
> a constant 640x480 pixels.
>
> i.e.)
> line 1 : 2 2 2 2...
> line 2 : x x x x...
> line 3 : 4 4 4 4...
>
> After processing,
>
> line 1 : 2 2 2 2...
> line 2 : 3 3 3 3...
> line 3 : 4 4 4 4...
>
> I can think of several ways to implement this but I thought the group
> may point me to the most efficient.
>
> Thanks,
> Pete
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