Re: about label_region

["Martin Downing" <martin.downing(at)ntlworld.com>]

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
f5bebc4d.0107100654.5c89f298@posting.google.com">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