Detecting center point of cross using Matlab

Question

Hello, I have an image as shown above. Is it possible for me to detect the center point of the cro

Answer 1

I just encountered the same kind of problem, and I found other solutions that I would like to share:

Assume image file name is pict1.jpg.

1.Read input image, crop relevant part and covert to Gray-scale:

origI = imread('pict1.jpg'); %Read input image

I = origI(32:304, 83:532, :); %Crop relevant part
I = im2double(rgb2gray(I)); %Covert to Grayscale and to double (set pixel range [0, 1]).

2.Convert image to binary image in robust approach:

%Subtract from each pixel the median of its 21x21 neighbors
%Emphasize pixels that are deviated from surrounding neighbors
medD = abs(I - medfilt2(I, [21, 21], 'symmetric'));

%Set threshold to 5 sigma of medD
thresh = std2(medD(:))*5;

%Convert image to binary image using above threshold
BW = im2bw(medD, thresh);

BW Image:

3.Now I suggest two approaches for finding the center:

• Find find centroid (find center of mass of the white cluster)
• Find two lines using Hough transform, and find the intersection point

Both solutions return sub-pixel result.

3.1.Find cross center using regionprops (find centroid):

%Find centroid of the cross (centroid of the cluster)
s = regionprops(BW, 'centroid');
centroids = cat(1, s.Centroid);

figure;imshow(BW);
hold on, plot(centroids(:,1), centroids(:,2), 'b*', 'MarkerSize', 15), hold off

%Display cross center in original image
figure;imshow(origI), hold on, plot(82+centroids(:,1), 31+centroids(:,2), 'b*', 'MarkerSize', 15), hold off

Centroid result (BW image):

Centroid result (original image):

3.2 Find cross center by intersection of two lines (using Hough transform):

%Create the Hough transform using the binary image.
[H,T,R] = hough(BW);

%ind peaks in the Hough transform of the image.
P  = houghpeaks(H,2,'threshold',ceil(0.3*max(H(:))));
x = T(P(:,2)); y = R(P(:,1));

%Find lines and plot them.
lines = houghlines(BW,T,R,P,'FillGap',5,'MinLength',7);
figure, imshow(BW), hold on
L = cell(1, length(lines));
for k = 1:length(lines)
    xy = [lines(k).point1; lines(k).point2];
    plot(xy(:,1),xy(:,2),'LineWidth',2,'Color','green');

    % Plot beginnings and ends of lines
    plot(xy(1,1),xy(1,2),'x','LineWidth',2,'Color','yellow');
    plot(xy(2,1),xy(2,2),'x','LineWidth',2,'Color','red');

    %http://robotics.stanford.edu/~birch/projective/node4.html
    %Find lines in homogeneous coordinates (using cross product):
    L{k} = cross([xy(1,1); xy(1,2); 1], [xy(2,1); xy(2,2); 1]);
end

%https://en.wikipedia.org/wiki/Line%E2%80%93line_intersection
%Lines intersection in homogeneous coordinates (using cross product):
p = cross(L{1}, L{2});

%Convert from homogeneous coordinate to euclidean coordinate (divide by last element).
p = p./p(end);
plot(p(1), p(2), 'x', 'LineWidth', 1, 'Color', 'white', 'MarkerSize', 15)

Hough transform result: