How to draw a line between two coordinates of an image permanently in Matlab?

Question

I have a set of points that I want to connect sequentially. Suppose the points are (A1,A2,A3,...A9); I want to connect A1 to A2, A2 to A3 and so on and finally connect A9 to A1.

All I need is to know a function that would help me connect A1 to A2, I could do the rest using for loops.

I know connecting two points is a question that has been asked here several times before but I couldn't find the answer I required. Several of the solutions suggest using "plot" and "line" but these functions overlay the results on the image and don't actually make any changes to the original image.

I did try them out and managed to save the resulting figure using the "saveas" and "print" functions but the image doesn't get saved in the proper format and there are a lot of problems using the parameters for these functions. Besides, I don't really want to save the image, it's just an unnecessary overhead I was willing to add if I could get the desired image with the lines. I've also tried "imline" to draw lines but it seems to be interactive.

This particular question reflects my problem perfectly but when I ran the code snippets given as solutions, all of them gave a set of dots in the resulting image.

I tried the above mentioned codes in the link with this image that I found here.

A dotted line was an output for all three code snippets in the link above. For example, I ran the first code like this:

I = imread('berries_copy.png'); 
grayImage=rgb2gray(I); 
img =false(size(grayImage,1), size(grayImage,2)); 

I wrote the above piece of code just to get a black image for the following operations:

x = [500 470];        % x coordinates 
y = [300 310];        % 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

This is the resulting image for the above code as well as the other two, which as you can see, is just a few points on a black background which isn't the desired output.

I changed the code and used the "plot" function for the same to get this output which is what I want. Is there anyway I can change the dotted output into a solid line?

Or if could anyone suggest a simple function or a method that would draw a line from A1 to A2 and would actually make a change in the input image, I'd be grateful. (I really hope this is just me being a novice rather than Matlab not having a simple function to draw a line in an image.)

P.S. I don't have the Computer Vision toolbox and if possible, I'd like to find a solution that doesn't involve it.

Answer 1

Your first problem is that you are creating a blank image the same size as the image you load with this line:

img =false(size(grayImage,1), size(grayImage,2));

When you add the line to it, you get a black image with a white line on it, as expected.

Your second problem is that you are trying to apply a solution for grayscale intensity images to an RGB (Truecolor) image, which requires you to modify the data at the given indices for all three color planes (red, green, and blue). Here's how you can modify the grayscale solution from my other answer:

img = imread('berries_copy.png');  % Load image
[R, C, D] = size(img);             % Get dimension sizes, D should be 3
x = [500 470];                     % x coordinates
y = [300 310];                     % 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([R C], rIndex, cIndex);         % Linear indices
img(index) = 255;                  % Modify red plane
img(index+R*C) = 255;              % Modify green plane
img(index+2*R*C) = 255;            % Modify blue plane
imshow(img);                       % Display image
imwrite(img, 'berries_line.png');  % Save image, if desired

And the resulting image (note the white line above the berry in the bottom right corner):

Answer 2

You can use the Bresenham Algorithm. Of course it has been implemented and you can find it here: Bresenham optimized for Matlab. This algorithm selects the pixels approximating a line.

A simple example, using your variable name could be:

I = rgb2gray(imread('peppers.png'));
A1 = [1 1];
A2 = [40 40];
[x y] = bresenham(A1(1),A1(2),A2(1),A2(2));
ind = sub2ind(size(I),x,y);
I(ind) = 255;
imshow(I) 

Answer 3

You can use imshow to display a image and then use plot to plot lines and save the figure. Check the below code:

I = imread('peppers.png') ;
imshow(I)
hold on
[m,n,p] = size(I) ;
%% Get random points A1, A2,..A10
N = 9 ;
x = (n-1)*rand(1,N)+1 ;
y = (m-1)*rand(1,N)+1 ;
P = [x; y]; % coordinates / points 
c = mean(P,2); % mean/ central point 
d = P-c ; % vectors connecting the central point and the given points 
th = atan2(d(2,:),d(1,:)); % angle above x axis
[th, idx] = sort(th);   % sorting the angles 
P = P(:,idx); % sorting the given points
P = [P P(:,1)]; % add the first at the end to close the polygon 
plot( P(1,:), P(2,:), 'k');
saveas(gcf,'image.png')