How do I remove/replace border pixels from my image?

Question

I am new to Matlab and will need some help. I have a segmented image in which I would want to remove background (or black) pixels (with value == 1) from the image borders. I have been able to obtain an image mask of the border pixels that I do not want. The interior_blackPixels are useful but I want to get rid of the outer_blackPixels, however, I am not sure how to retrieve the final image without the black pixel borders using the mask (outer_blackPixels) that I have. The code so far is shown below:

img =   [1 1 1 1 1 1 1 1
         1 1 1 1 2 2 2 1
         1 1 1 2 2 2 2 1
         1 1 1 2 2 2 2 1
         1 1 2 2 2 2 2 1
         1 1 2 2 2 2 2 1
         1 3 3 1 1 1 3 1
         1 3 3 1 1 1 3 1
         1 3 3 3 3 3 3 1
         1 1 1 1 1 1 1 1];

% Get the black pixels image array
blackPixels = (img == 1);

% Obtain the other pixels by negating the black pixels
otherPixels = ~blackPixels

% Get the border black pixels (or mask)
outer_blackPixels    = blackPixels & ~imclearborder(blackPixels)
interior_blackPixels = blackPixels & ~outer_blackPixels

Please note that I do not mind replacing the pixel values of the outer_blackPixels to '0' as this will not affect my analysis. Hence, I would expect my final image to be something like this:

img =   [0 0 0 0 0 0 0 0
         0 0 0 0 2 2 2 0
         0 0 0 2 2 2 2 0
         0 0 0 2 2 2 2 0
         0 0 2 2 2 2 2 0
         0 0 2 2 2 2 2 0
         0 3 3 1 1 1 3 0
         0 3 3 1 1 1 3 0
         0 3 3 3 3 3 3 0
         0 0 0 0 0 0 0 0];

Any help/suggestions would be appreciated. Thanks!

Answer 1

You can use bwlabel to differentiate between the 1's areas. Because this function treats only binary images, you can use logical to convert img to binary:

L=bwlabel(~logical(img-1),4)

And then just convert the borders to 0:

img(L==L(1))=0

img =

 0     0     0     0     0     0     0     0
 0     0     0     0     2     2     2     0
 0     0     0     2     2     2     2     0
 0     0     0     2     2     2     2     0
 0     0     2     2     2     2     2     0
 0     0     2     2     2     2     2     0
 0     3     3     1     1     1     3     0
 0     3     3     1     1     1     3     0
 0     3     3     3     3     3     3     0
 0     0     0     0     0     0     0     0

Answer 2

Since you are using the Image Processing Toolbox, I would suggest the imfill function:

img(~imfill(img ~= 1, 'holes')) = 0;

This first creates a mask for pixels in img that are not equal to one, fills any internal holes that contain ones using imfill , negates that to get a mask for the border ones, and indexes img with that mask, setting values to zero.

If you want to do something like this without using the Image Processing Toolbox, a solution of the type from Adiel using bwlabel is the way I would go.