Comments on: A structuring element decomposition “gotcha”

By: Steve

Steve — Mon, 01 Jun 2009 13:43:40 +0000

Alex—The appropriate structuring element shape depends on your application and data set, just like choosing between a lowpass filter and a highpass filter. You might want to get a good textbook on image processing.

By: Alex

Alex — Sun, 31 May 2009 17:37:22 +0000

Hi.. I cannot understand what is written in matlab help. Could you give me a brief description on how to select the appropriate structuring element parameter? wheter is disc or rectangle or wad.

By: Steve

Steve — Fri, 19 Dec 2008 21:08:29 +0000

Ken—I get similar results. Clearly, we have a lot of room to improve our dilation implementation! We’re looking into it.

By: Steve

Steve — Sat, 13 Dec 2008 16:59:08 +0000

Oliver—I didn't say bwlabel allows arbitrary strels, just arbitrary connectivities. I would be interested to hear more about your application, if you care to share it. For example, when would it make sense to say that the two 1-valued pixels below are connected?

1 0 0 0 0 0 0 0 1

Anyway, some of the material in my series on connected component labeling algorithms might be useful to you.

By: Oliver Woodford

Oliver Woodford — Sat, 13 Dec 2008 16:35:47 +0000

Thanks, Steve. However, the CONN matrix doesn’t allow arbitrary strel neighbourhoods. In particular, the neighbourhoods are limited to 3×3 (for 2d) and must be symmetric. For the problem I have (with a 9×9 neighbourhood), BWLABELN can’t be used.

By: Steve

Steve — Sat, 13 Dec 2008 16:02:08 +0000

Oliver—bwlabeln supports labeling operations with an arbitrary connectivity definition. See the doc regarding the CONN input argument. You can use bwlabeln on two-dimensional inputs.

By: Oliver Woodford

Oliver Woodford — Sat, 13 Dec 2008 14:33:22 +0000

Hi Steve

On the topic of structural elements, why can they be used for IMDILATE, but not for BWLABEL? Currently I need to use them in the latter (see http://www.mathworks.com/matlabcentral/newsreader/view_thread/240944), but I can’t see how to achieve this using the IPT. Is there a way?

Many thanks,
Oliver

By: Ken Eaton

Ken Eaton — Wed, 10 Dec 2008 20:09:15 +0000

Steve,

I’ve found your posts about dilation and structure elements very interesting, since I’ve been doing some things with it recently. I came across something strange that I wasn’t expecting, and I was hoping you might explain it for me…

I had some operations were I wanted to dilate points with a disk shape, and found that using IMFILTER instead of IMDILATE was actually much faster:

  a = zeros(9);
  a(5,5) = 1;

  % using imdilate:

  disk = strel('disk',5);
  tic;
  for iCount = 1:100,
    b_dilate = imdilate(a,disk);
  end
  toc;

  % using imfilter:

  disk = fspecial('disk',4);
  tic;
  for iCount = 1:100,
    b_filter = double(imfilter(a,disk,'replicate') > 0);
  end
  toc;

  % using a thresholded imfilter:

  disk = double(fspecial('disk',4) > 0);
  tic;
  for iCount = 1:100,
    b_thresh = imfilter(a,disk,'replicate');
  end
  toc;

Using MATLAB version 7.1, I get the following from the above code, and all the results are equal:

Elapsed time is 0.341824 seconds.
Elapsed time is 0.089447 seconds.
Elapsed time is 0.082622 seconds.

Why is this happening? Is this the same for newer MATLAB versions?

Thanks,
Ken