| about label_region [message #25610] |
Sun, 01 July 2001 15:02  |
julia65201
Messages: 9
Registered: July 2001
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Junior Member |
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hi, there,
Yesterday, I post a message. Since it has the same subject of the
previous one. It got confused. So I bother to post it again. Wish I
can get good results.
The ones who used label_region in IDL must be suprised at the
efficieny of this function. But how can I find out what algorithm is
used in LABEL_REGION? Since I do not see this function in the library,
I assume it is written in C somewhere.
Julia
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| Re: about label_region [message #25682 is a reply to message #25610] |
Wed, 11 July 2001 04:25  |
Martin Downing
Messages: 136
Registered: September 1998
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Senior Member |
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Julia,
The algorithm will be written in C as it requires a raster search through
the image for connected blobs. I cant find a reference right now but it goes
something like this psuedo code (considering the raster order to be left to
right working through rows down to up):
-------------------------------------
label _image, IMAGE
create a long image LAB_IM for output
for each pixel:
;check IMAGE to see if background pixel
if background then continue
; else check LAB_IMAGE for connectivity and labelling
if connected to labelled pixel BELOW or LEFT (also BELOW-LEFT,
BELOW-RIGHT if 8 connected) then begin
if connected to more than one label add label pairs to an equivalent
list, ie [1,5] means LABEL 1 = = LABEL 5
assign (first) label to pixel in lab_image
else assign new label to pixel in lab_image
endfor
use equivalent list to create a lookup table reducing all used labels to a
unique set starting from 1
now run through label image replacing each label with its reduced value
----------------------------------------------------
hope this make some sense
Martin
--
----------------------------------------
Mr. Martin Downing,
Clinical Research Physicist,
Orthopaedic RSA Research Centre,
Woodend Hospital,
Aberdeen, AB15 6LS.
"Julia" <julia65201@usa.net> wrote in message
news:f5bebc4d.0107100654.5c89f298@posting.google.com...
>> Amar Nayegandhi said,
>> [
>> label_region uses a 'connected components' algorithm that is described
>> in any good Image Processing text book. It has the option of using 4
>> neighbors or 8 neighbors. I have used labe_region often, and I have a
>> strong feeling i get the same results using the 4 or 8-neighbor
>> approach. I dont think the 8-neighbor approach gives any better results
>> than the 4-neighbor approach.
>> - amar
>> ]
>
> Thanks for your information!
>
> I just wonder where IDL put all these functions in. Is it written in
> IDL or C? I tried to realized it in IDL. As you know, since
> connectivity algorithm is not a parallel algorithm, it should be done
> on pixels one by one. And to keep the equivalent region information, I
> assign a very big matrix even it is very sparse. So the algorithm is
> very slow. Can you give me some hint to improve it, such as how to
> keep the equivalent region information efficiently?
>
> Julia
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| Re: about label_region [message #25696 is a reply to message #25636] |
Tue, 10 July 2001 07:54 |
julia65201
Messages: 9
Registered: July 2001
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Junior Member |
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> Amar Nayegandhi said,
> [
> label_region uses a 'connected components' algorithm that is described
> in any good Image Processing text book. It has the option of using 4
> neighbors or 8 neighbors. I have used labe_region often, and I have a
> strong feeling i get the same results using the 4 or 8-neighbor
> approach. I dont think the 8-neighbor approach gives any better results
> than the 4-neighbor approach.
> - amar
> ]
Thanks for your information!
I just wonder where IDL put all these functions in. Is it written in
IDL or C? I tried to realized it in IDL. As you know, since
connectivity algorithm is not a parallel algorithm, it should be done
on pixels one by one. And to keep the equivalent region information, I
assign a very big matrix even it is very sparse. So the algorithm is
very slow. Can you give me some hint to improve it, such as how to
keep the equivalent region information efficiently?
Julia
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