| Re: Large Image Handling FAQ? [message #14531 is a reply to message #14528] |
Wed, 10 March 1999 00:00   |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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"Andy Sowter" <asowter@synopticsga.freeserve.co.uk> writes:
>
> I'm dealing with many large (130MB is a typical size - 16-bit integers)
> images at a time, doing matching and 3-d reconstruction. Are there any FAQs
> or links out there relating to tips to do this - I don't like just ramping
> up my RAM or virtual memory every time because, well, it doesn't feel right
> (and it only seems to cause problems in the long run).?
>
I don't have a FAQ, but I do handle large files. In typical X-ray
astronomy applications we don't have big images, but we do have large
database files.
Chunking is your friend. That is, operating on a "large" but not "too
large" subsegments of your data at once. That way you can take
advantage of IDL's vector processing, but not overflow the memory. In
X-ray astronomy this is facilitated by the fact that data elements are
independent.
Below is a summary of the techniques I use for batch processing, in
pseudocode. The technique depends on the situation.
Pixel-by-pixel
--------------
FOR I = 0, N_PIXELS-1 DO BEGIN
X = READ_PIXEL(I)
Y = OPERATE_ON(X)
WRITE_PIXEL, Y, I
END
Advantage: low memory usage
Disadvantage: loop-intensive, not vectorized
Entirely vectorized
-------------------
IMG = DBLARR(NX_PIXELS, NY_PIXELS)
READU, UNIT1, IMG
NEWIMG = OPERATE_ON(IMG)
WRITEU, UNIT2, IMG
Advantage: entirely vectorized
Disadvantage: may take too much memory, slows down with swapping
Chunked
-------
N = N_PIXELS
WHILE N GT 0 DO BEGIN
BUFSIZE = N < BUFMAX ;; BUFMAX is the chunk size
X = READ_PIXELS() ;; Read one chunk at a time
Y = OPERATE_ON(X)
WRITE_PIXELS, Y
N = N - BUFSIZE
END
Advantage: low memory footprint, fast because it is vectorized
Disadvantages: more complicated code, doesn't work if chunks must
interact with each other.
You may also want to look in to associated variables. I don't use
them, but they may provide some virtual-memory-like features. There
is also a virtual memory array package by Eric J. Korpela which may do
the trick, but that requires some compiling and LINK_IMAGEing.
Good luck,
Craig
--
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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@astrog.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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