MATLAB: Drawing a line over a black and white image

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:

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.

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;

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

This algorithm offers one approach.