Steve on Image Processing with MATLAB

Image processing concepts, algorithms, and MATLAB

Note

Steve on Image Processing with MATLAB has been archived and will not be updated.

Tracing George

Note

See the 17-Jun-2021 post for new or updated information about this topic.

Last time, in my spatial transformations series, I introduced the head-and-shoulders outline that I like to call George (George P. Burdell). Today I want to diverge briefly from my main topic and show how I recently "recovered" George.

George appears in section 5.11 of Digital Image Processing Using MATLAB. While working on the book, I tried to keep all data files and M-files necessary for regenerating the book's figures. I found, however, that I could not locate the data for George's curve that appears in Figure 5.12 and Table 5.3.

After a few minutes of head scratching, I decided to get the curve data by taking a digital snapshot of the figure in the book. Here's a cropped version:

url = 'https://blogs.mathworks.com/images/steve/36/george.jpg';
I = imread(url);
imshow(I)

A little fuzzy and dark, but maybe workable. The first step is to threshold the image. The Image Processing Toolbox function graythresh computes binarization thresholds automatically. It works well for a variety of images.

threshold = graythresh(I);
bw = im2bw(I, threshold);
imshow(bw)

Now we have a nice fat line. Toolbox function bwmorph has a thinning option, but it works on white (foreground) pixels instead of black pixels. So complement the image:

bw2 = ~bw;

and then thin it. Specify the number of iterations to be Inf so that bwmorph will keep thinning until the lines are a single pixel wide.

bw3 = bwmorph(bw2, 'thin', inf);
imshow(bw3)

Now trace the line using toolbox function bwboundaries.

boundaries = bwboundaries(bw3);

boundaries is a cell array containing one P-by-2 matrix for each object boundary. This image contains only one object, so boundaries contains only one P-by-2 matrix. The matrix has twice as many rows as we need, because bwboundaries traces all the way around the object.

b = boundaries{1};
b = b(1:floor(end/2), :);

Finally we are ready to plot the curve.

x = b(:,2);
y = b(:,1);
plot(x,y)
axis ij     % Place the origin at upper left, with y values increasing from
            % top to bottom.
axis equal  % Set the aspect ratio so that the data units are the same in
            % every direction.
title('George P. Burdell')




Published with MATLAB® 7.1

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