use axes coordinate to generate an image without plotting it, in Matlab

Question

I have an image full of objects of the shape of an ellipse. I need to design an ellipse for each object that is the best fit for the object itself. I have found a code that helps me to plot the ellipses on the image here.

I have changed the final part saving x and y in a 3D matrix (one dimension for x, the other for y, and the 3rd for the number of objects). Because this code is in a for loop, I do not want to generate the figure plot ellipses on top of it, save it and upload with imread to pass it to the rest of the code.

Is there a way to convert this 3D matrix in a bw image full of the fitting ellipses in the correct position in the image?

vgru · Answer 1 · 2011-10-14T11:06:44.420

Ellipses are drawn on top of the existing figure because of the hold on statement after the image is shown using imshow. So, instead of this:

imshow(bw)
hold on

Simply create a new figure using the figure statement:

figure

[Edit]

Ok, first of all, storing only (x, y) gives you ellipse centers only. To draw an ellipse, you will also need to store its majos/minor axis size (a, b) and its orientation angle (theta).

I would simply reuse the loop that you already have, but replace plot with simply setting a bw image pixel to 1 for each coordinate:

% get image dimensions
dim = size(bw);

% preallocate a blank bw image
target = false(dim);

% for each ellipse
for k = 1:length(s)

    % this part remains the same:
    xbar = s(k).Centroid(1);
    ybar = s(k).Centroid(2);

    a = s(k).MajorAxisLength/2;
    b = s(k).MinorAxisLength/2;

    theta = pi*s(k).Orientation/180;
    R = [ cos(theta)   sin(theta)
         -sin(theta)   cos(theta)];

    xy = [a*cosphi; b*sinphi];
    xy = R*xy;

    x = xy(1,:) + xbar;
    y = xy(2,:) + ybar;

    % ----------
    % but replace plot(x,y) with this:

    % limit to image dimensions (1:256)
    x(x<1) = 1; x(x>dim(1))=dim(1);
    y(y<1) = 1; y(y>dim(2))=dim(2);    

    % set those pixels to 1
    target(sub2ind(dim, round(x),round(y))) = 1;

end

imshow(target);

Right now, there are ellipses which are half outside the image boundaries. That's why their x,y coordinates need to be limited to (1:256); otherwise you get an out-of-range error. You still need to rethink whether these ellipses should be removed entirely, or drawn partially as done here.

Thanks a lot!!! But I actually would like to eliminate both figure and imshow (btw, in my code I am using figure(); imagesc(bw), instead of imshow and it is not my problem). What I have is that bw is a 1024x1392 matrix in my case, where each element is a pixels. The code I toke from the web is actually generating two arrays:x and y, of 50 elements length (each element value is the ellipse coord). How can I pass this information in axes coordinates to the bw matrix? My final goal is to do what the code I referred to does, skipping the plotting part and just adding the ellipses to the bw image — user990253, Oct 14 '11 at 01:08
@user990253: ok, now I get it. I've updated the code with an example. — vgru, Oct 14 '11 at 11:07
thank you very much! It is a great improvement, but still on the road! I actually need that the ellipse perimeter is a line, because next step in my code is to fill the perimeter in! BTW, I used dim(1) for both x and y, in order to have a superposition of the original figure objects and the generated ellipses. With the out-of-range error, I lose a lot of objects in the original image! — user990253, Oct 14 '11 at 19:58

use axes coordinate to generate an image without plotting it, in Matlab

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