| Re: Dereferencing a Pointer Array [message #24967] |
Tue, 01 May 2001 12:28 |
John-David T. Smith
Messages: 384
Registered: January 2000
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Senior Member |
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Art Burden wrote:
> I can't figure out a way to dereference the pointers to all 12 pixel
> values from a given coordinate in one step.
As you can imagine, it would be a sordid business indeed if you could
dereference entire pointer arrays all at once:
parr=[ptr_new([1,2,3]),ptr_new([[5,6],[7,8]])]
print,total(*parr,1)
hmm.. what would we do with that? Since pointers can point to anything,
pointer arrays don't necessarily point to similarly dimensioned blocks
of data, or even similarly typed:
parr=[ptr_new([1,2,3]),ptr_new([[5,6],[7,8]]),ptr_new('Octop us')]
If you really know you will have N images of the same size, you can have
your cake and eat it too. Instead of storing them as a pointer array,
store them in a single pointer as an image cube:
pcube=ptr_new(randomu(sd,20,20))
*pcube=[[[*pcube],[[randomu(sd,20,20)]]]
now you can easily get the mean along the "depth" of the cube:
meanim=total(*pcube,3)/n_ims
This will be much faster than separately dereferencing/adding a long
list of pointed-to arrays. It is less flexible though (what if you
wanted to remove an array from the middle of the list?).
JD
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| Re: Dereferencing a Pointer Array [message #24969 is a reply to message #24967] |
Tue, 01 May 2001 11:58  |
Art Burden
Messages: 2
Registered: May 2001
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Junior Member |
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Thanks Liam, that worked beautifully. I knew I was making this complicated..
Sometimes it's easy to use the routines blindly without thinking about how
they are defined (such as, mean = sum/num)
"Liam E. Gumley" wrote:
> Art Burden wrote:
>> I have a structure that contains a pointer array that points to twelve
>> 512-by-512 images. I would like to find the mean image from the twelve
>> images in a simple and fast way. I understand that I can dereference
>> the pointer to an image or an individual element in an image by using,
>> for example
>>
>> img = (*info.images[0])
>> and
>> img_element = (*info.images[0])[240,240]
>>
>> but I can't figure out a way to dereference the pointers to all 12 pixel
>> values from a given coordinate in one step. At this stage, I pass the
>> structure into my averaging subroutine and I create a new array to
>> store the 12 images. I then fill the array by dereferencing the pointer
>> to each image in a loop. Finally, I loop through the rows and columns
>> to get each mean pixel value, as shown below. Can anyone think of a
>> better (mainly faster) way to do this?
>>
>> ;retrieve array of images
>> ffim = lonarr(12,512,512)
>> for num=0,11 do ffim[num,*,*] = *info.images[num]
>>
>> ;calculate mean of images
>> mean_ff = fltarr(512,512)
>> for ir = 0,511 do for ic = 0,511 do mean_im[ic,ir] = mean(ffim[*,ic,ir])
>
> Perhaps something like this (untested):
>
> sum = fltarr(512, 512)
> n = 12
> for i = 0L, n - 1L do sum = sum + *info.images[i]
> avg = sum / float(n)
>
> Cheers,
> Liam.
> http://cimss.ssec.wisc.edu/~gumley/
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| Re: Dereferencing a Pointer Array [message #24970 is a reply to message #24969] |
Tue, 01 May 2001 11:30 |
Liam E. Gumley
Messages: 378
Registered: January 2000
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Senior Member |
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Art Burden wrote:
> I have a structure that contains a pointer array that points to twelve
> 512-by-512 images. I would like to find the mean image from the twelve
> images in a simple and fast way. I understand that I can dereference
> the pointer to an image or an individual element in an image by using,
> for example
>
> img = (*info.images[0])
> and
> img_element = (*info.images[0])[240,240]
>
> but I can't figure out a way to dereference the pointers to all 12 pixel
> values from a given coordinate in one step. At this stage, I pass the
> structure into my averaging subroutine and I create a new array to
> store the 12 images. I then fill the array by dereferencing the pointer
> to each image in a loop. Finally, I loop through the rows and columns
> to get each mean pixel value, as shown below. Can anyone think of a
> better (mainly faster) way to do this?
>
> ;retrieve array of images
> ffim = lonarr(12,512,512)
> for num=0,11 do ffim[num,*,*] = *info.images[num]
>
> ;calculate mean of images
> mean_ff = fltarr(512,512)
> for ir = 0,511 do for ic = 0,511 do mean_im[ic,ir] = mean(ffim[*,ic,ir])
Perhaps something like this (untested):
sum = fltarr(512, 512)
n = 12
for i = 0L, n - 1L do sum = sum + *info.images[i]
avg = sum / float(n)
Cheers,
Liam.
http://cimss.ssec.wisc.edu/~gumley/
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