MATLAB: Drawing a line over a black and white image

Question

What is the best way to draw a line over a black and white (binary) image in MATLAB, provided the start and end coordinates are known?

Please note, I am not trying to add an annotation line. I would like the line to become part of the image.

Answer 1

You may want to look at my answer to an SO question about adding a line to an image matrix. Here's a similar example to the one I have in that answer, which will make a white line running from row and column index (10, 10) to (240, 120):

img = imread('cameraman.tif');  % Load a sample black and white image
x = [10 240];                   % x coordinates
y = [10 120];                   % y coordinates
nPoints = max(abs(diff(x)), abs(diff(y)))+1;    % Number of points in line
rIndex = round(linspace(y(1), y(2), nPoints));  % Row indices
cIndex = round(linspace(x(1), x(2), nPoints));  % Column indices
index = sub2ind(size(img), rIndex, cIndex);     % Linear indices
img(index) = 255;  % Set the line points to white
imshow(img);       % Display the image

And here's the resulting image:

Answer 2

If you are bothered by exceptional cases of other methods here's a bullet-proof method that results in a line:

• whose pixels always touch each other during the whole length of the line (pixels are 8-neighbors to each other),
• density of the line is not dependent on the additional parameter, but is determined flexibly to accommodate guarantee from the first point.

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Inputs (convenient for making function out of this code):

• img - matrix that contains image,
• x1, y1, x2, y2 - coordinates of the end points of the line to be drawn.

Code:

% distances according to both axes
xn = abs(x2-x1);
yn = abs(y2-y1);

% interpolate against axis with greater distance between points;
% this guarantees statement in the under the first point!
if (xn > yn)
    xc = x1 : sign(x2-x1) : x2;
    yc = round( interp1([x1 x2], [y1 y2], xc, 'linear') );
else
    yc = y1 : sign(y2-y1) : y2;
    xc = round( interp1([y1 y2], [x1 x2], yc, 'linear') );
end

% 2-D indexes of line are saved in (xc, yc), and
% 1-D indexes are calculated here:
ind = sub2ind( size(img), yc, xc );

% draw line on the image (change value of '255' to one that you need)
img(ind) = 255;

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Here's the example image with three lines drawn on it: