| Re: rebin question [message #29912 is a reply to message #11091] |
Fri, 22 March 2002 11:20   |
Jonathan Joseph
Messages: 69
Registered: September 1998
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Member |
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Thanks, that's seems more reasonable than what I was thinking.
It still uses a loop, but only over the number of elements in the
box. I think I can definitely adapt this strategy to my needs.
-Jonathan
Vince wrote:
>
> 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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