Steve on Image Processing

Today I want to demonstrate a useful technique to produce a false-color visualization of different sets of binary image objects. Here's the sample image that we'll use:

url = 'http://blogs.mathworks.com/images/steve/2008/segmented_rice.png';
bw = imread(url);
imshow(bw)

Let's look at two sets of objects: Those that touch the image border, and those that do not.

notouch = imclearborder(bw);
imshow(notouch)

The objects that do touch the border can be computed using logical operators:

touch = bw & ~notouch;
imshow(touch)

Here's one way to turn these two sets of objects into a single, false-color, indexed image. First, initialize the index matrix:

X = zeros(size(bw), 'uint8'); % Did you know about
                              % this way to call zeros?

Now assign 1 to the elements of X corresponding to the border-touching set, and assign 2 to the elements corresponding to the interior set.

X(touch) = 1;  % Logical indexing!
X(notouch) = 2;

Now we just need to pick some colors for the color map. I'll make the background white:

map(1,:) = [1 1 1];

Make the touching objects be purple-ish.

map(2,:) = [0.7 0.3 0.8];

And use a green shade for the removed objects.

map(3,:) = [0.4 0.8 0.7];

Now we can display the resulting indexed image.

imshow(X,map)

Get the MATLAB code

Published with MATLAB® 7.6