| Re: rebin question [message #29914 is a reply to message #11091] |
Fri, 22 March 2002 10:53   |
hradilv.nospam
Messages: 19
Registered: November 2001
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Junior Member |
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Sorry. I see the hassle now.
I just did a quick search for 'rebin' at
http://www.astro.washington.edu/deutsch/idl/htmlhelp/index.h tml and
maybe you should take a look at 'boxave'?
On Fri, 22 Mar 2002 13:40:32 -0500, Jonathan Joseph <jj21@cornell.edu>
wrote:
>
> It looks nice doesn't it, and I did write a procedure for the simple
> case of averaging, but it's not as clean cut as you indicate:
>
> 1. first one needs to get the type of the incoming image - you don't
> want to round the result of a floating point type image - that
> would give you the wrong result.
>
> 2. conversion should be done to double precision floating point
> (not float) otherwise large long integers will lose precision.
> loss of precision for large L64 integers will occur even with
> conversion to double, so they can't be handled properly at all.
>
> 3. need to convert back to the proper type, so your solution
> should be wrapped by a fix(..., type=type)
>
> 4. instead of a rebin, there is now a rebin, two type conversions
> and a round, which will slow things down and use more memory.
>
> So, it is a hassle.
>
> But yes, it's still not difficult to write a function to handle the
> SIMPLE case of averaging for CERTAIN data types. But that does not
> help with the problem of writing a more general function that handles
> downsampling using median or downsampling using a mean excluding
> outliers (pixels with values far from the mean) or downsampling using
> your favorite method. Doing this quickly in IDL means doing it
> w/o loops, so while conceptually the problem is not difficult, it
> does seem somewhat more difficult to do it properly in IDL.
>
> Anyone out there thought about this problem before?
>
> -Jonathan
>
> Vince wrote:
>>
>> print, round(rebin(float([5,5,5,5,4]),1))
>>
>> Hassle?
>>
>> Maybe you could write a function. Which leads me to a new question:
>>
>> Is it possible to define a function or procedure in IDL that can take
>> an arbitrary number of arguments, e.g.:
>>
>> function my_rebin, a, arg1, arg2, ...
>>
>> return, round( rebin( float(a), arg1, arg2, ... ) )
>> end
>>
>> On Fri, 22 Mar 2002 11:58:41 -0500, Jonathan Joseph <jj21@cornell.edu>
>> wrote:
>>
>>> I figured I would use rebin to downsample an image by averaging the
>>> pixels in blocks of specified size. What I discovered, was that for
>>> integer type images, rebin averages the pixels, but then instead of
>>> rounding to the nearest integer, simply takes the integer part of
>>> the average. Hence:
>>>
>>> print, rebin([5,5,5,5,4], 1)
>>>
>>> gives the result of 4, not 5 which is what I would like. I suppose
>>> this is done for speed - to work around the problem, I need to convert
>>> to a floating point type, do the rebin, then round, then convert back
>>> to the proper integer type - a hassle.
>>>
>>> But, I would really like a more generic way of doing downsampling
>>> of this sort, without the high overhead of a loop. Apart from
>>> taking the mean of a block of pixels, I would also like the option
>>> of downsampling using the median of a block of pixels, or using the
>>> mean of a block of pixels disregarding the farthest outlier (or
>>> n outliers).
>>>
>>> Has anyone written IDL code to do downsampling in a more generalized
>>> way than rebin, or have any clever ideas about how to do it quickly?
>>>
>>> Thanks
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